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# What is a floating power supply output?

Posted by StevenLee Aug 9, 2018

### What is a floating power supply output?

First let me tell you that a floating power supply output is NOT what is shown below in Figure 1 (haha).

Now some background: "earth ground" is the voltage potential of the earth. To greatly reduce the risk of subjecting a person to an electrical shock, the outer covering (chassis) of most electrical devices is internally connected to a wire that is connected to earth ground. Most devices connect to earth ground through their power cord. The idea here is to ensure that all surfaces a person can touch are at the same voltage potential - earth ground.

As long as that is true, the person can freely touch things without the risk of getting shocked due to two of the things he touches at the same time being at different voltage potentials, or one of the things being at a high voltage potential with respect to the earth. If the voltage difference is high enough, the person could be shocked.
Earth grounding the chassis also protects the user if there is an internal problem with an electrical device causing its chassis to inadvertently come in contact with an internal high voltage wire. Since the chassis is earth grounded, an internal short to the chassis is really a short to ground and will blow a fuse or trip a circuit breaker to protect the user instead of putting the chassis at the high voltage. If you touched a chassis that had a high voltage with respect to ground on it, your body completes the path to ground and you get shocked!

So to protect the user (and for some other reasons), the chassis of Agilent power supplies are grounded internally through the ground wire (the third wire) in the AC input line cord. Additionally, most if not all of our Agilent power supplies have isolated (floating) outputs. That means that neither the positive output terminal nor the negative output terminal is connected to earth (chassis) ground. See Figure 2.

Figure 3 shows an example of non-floating outputs with the negative output terminal grounded.
For floating DC power supplies, the voltage potential appears from the positive output terminal to the negative output terminal. There is no voltage potential (at least, none with any power behind it) from either the positive terminal to earth ground or from the negative output terminal to earth ground. A power supply with a floating output is more flexible since, if desired, either the positive or negative terminal (or neither) can be connected to earth ground. Some devices under test (DUT) have a DC input with either the positive or negative input terminal connected to earth ground. If one of the power supply outputs was also internally connected to earth ground, when connected to the DUT, it could short out the power supply output. So power supplies with floating output terminals (no connections to earth ground) are more versatile.
If the outputs are floating from earth ground, we need to specify how far above or below earth ground you can float the output terminals. Our power supply documentation provides this information. For example, most Agilent power supply output terminals can float to +/-240 Vdc off of ground. You will frequently see the following in our documentation:
Also, some power supplies have different float ratings for the positive and negative output terminals. For example, for Agilent N5700 models rated for more than 60 Vdc, the following note in the manual means you can float the positive output terminal up to +/-600 Vdc from ground or the negative output terminal up to +/-400 Vdc from ground:
The output characteristic table may list this as “Output Terminal Isolation” as shown below which means the same thing as maximum float voltage:
Figure 4 shows an example of floating a power supply to 200 V above ground. The power supply output is set to 40 V.
You can see from the last example that you have to take the power supply output voltage into consideration when ensuring you are not violating the float voltage rating. If you exceed the float voltage rating of the power supply, you are potentially exceeding the voltage rating of internal parts that could cause the internal parts to fail or break down and present a shock hazard, so don’t violate the float voltage rating!

# Waveform modulation with your function generator

Posted by bernard ang Jul 12, 2018

Basic function generators generally can generate basic periodic waveforms such as sine wave, square wave, triangle wave, ramp wave, pulse wave and so on. Modern function generators nowadays have a very important feature called modulation. With the advent of radio wave telegraphy in the 19th century by Guglielmo Marconi and later the wide band FM radio in the 20th century by Edwin Howard Armstrong, radio waves are generally modulated by its carrier waves for efficient, long-range transmissions.

AM (amplitude modulation) and FM (frequency modulation) have been and are still very important electrical transmission techniques for radio communications. They have also evolved into sophisticated techniques such as QAM (quadrature amplitude modulation), FSK (frequency-shift keying) and more, that are used in short range wireless links such as Wi-Fi to long-range telecommunications networks.

### Advanced modulation features in modern function generators

Here in this blog, let us explore some advance modulation features on a modern function generator.

##### AM (Amplitude Modulation)

Amplitude modulation by a carrier sine wave is by far the most common in terms of usage. Figure 1 shows the graphical settings of a function generator modulated by sine wave with AM Depth of 100% and AM Frequency of 100 Hz. The original data sine wave has a frequency of 1 kHz.

Figure 1. AM function generator setup (left) using a Keysight 33600A and an oscilloscope display of AM (right).

Modern function generators can do more than just generate sine wave as carrier signal for waveform modulation. Figure 2 shows some examples of other types of signals that are used as carrier signals for waveform modulations (other than sine wave).

Figure 2. Various carrier signals used for waveform modulations (other than sine wave).

Modern function generators can be quite versatile. You can also choose the source of your AM modulation from another channel of your function generator without any external connections or from an external source. For example, you can use waveform from Channel 2 to modulate your waveform from Channel 1.

###### FM (Frequency Modulation)

Frequency modulation has been the best-known analog modulation method for radio broadcasting for more than half a century. Its applications have grown into video broadcasting, critical medical monitoring systems, radar and more.

Figure 3. FM function generator setup using a Keysight 33600A.

Here is an example of how to setup a function generator to simulate FM signal. Figure 3 above shows frequency modulation on a 1 kHz sine wave. The modulation method used is also a sine wave with FM frequency of 10 Hz. Its peak frequency deviation is 100 Hz.

###### PM (Phase Modulation)

Phase modulation is widely used in digital data transmissions through digital modulation techniques such as PSK (Phase-shift keying), BPSK (Binary phase-shift keying), QPSK (Quadrature phase-shift keying) and more. It is used in Wi-Fi, GSM and satellite broadcasting transmissions.

Figure 4. PM function generator setup using a Keysight 33600A

Here is an example of how to setup a function generator to simulate PM signal. Figure 4 above shows phase modulation on a 1 kHz sine wave. The modulation signal used is also a sine wave with PM frequency of 200 Hz. Its phase deviation is 180°.

###### FSK (Frequency-shift keying) / BPSK (Binary phase-shift keying)

There are many types of digital modulation techniques available today. The common ones are FSK (frequency-shift keying) and BPSK (binary phase-shift keying). These digital modulation techniques are used in many applications such as digital radio transmissions, RFID, Bluetooth and more.

Figure 5. FSK function generator setup using a Keysight 33600A

Here is an example of how to setup a function generator to simulate an FSK signal. Figure 5 above shows frequency-shift keying modulation on a 1 kHz carrier sine wave. The hop (or alternate) frequency is set at 100 Hz. The FSK rate is set at 10 Hz, which is the rate at which output frequency "shifts" between the carrier and hop frequency.

Figure 6. BPSK function generator setup using a Keysight 33600A

Here is an example of how to setup a function generator to simulate BPSK signal. Figure 6 above shows binary phase-shift keying modulation on a 1 kHz carrier sine wave. The BPSK rate is the rate at which the output phase "shifts" between the carrier and offset phase. The BPSK phase setting in this case is 180° (phase shift in degrees). Typically, you have the option to set the phase shift from 0° to 360°.

###### SUM (Sum modulation)

Sum modulation adds a modulating signal to any carrier waveform; it is typically used to add gaussian noise to a carrier. The modulating signal is added to the carrier as a percentage of carrier waveform amplitude.

Figure 7. Sum modulation function generator setup – adding noise into sine wave carrier signal.

Figure above shows how we can use sum modulation to add measured noise to the carrier signal. This figure shows noise with an amplitude of 30% (referencing to carrier amplitude) and a noise bandwidth of 100 kHz was added to the sine wave carrier signal.
The Keysight 33600A series function generator accepts both internal and external modulation sources. On a two-channel instrument you can modulate one channel with the other. When using internal source as modulation, we can select sine, square, triangle, up ramp, down ramp, noise, PRBS and arbitrary waveforms as source of modulation.

### Conclusion

We have just explored some advance modulation features and how intuitive the configuration setup can be using a Keysight 33600A.
If this blog helps provide you with more insight into generating modulated signals, please give it a ‘like’. If you have further questions on this topic, you can always reply to this blog.

# How to make simultaneous measurements with a digital multimeter

Posted by Seok Jin Wong Jul 2, 2018

Traditionally, digital multimeters (DMMs) have been single-measurement instruments. When engineers want to measure more than one parameter on their signal, they need to have two multimeters to measure two different measurements at the same time. This is not time and cost efficient.

With the right architecture and design, DMMs can make multiple measurements. This advanced feature can help you save cost and analysis time and finish your analysis faster!

### Two measurements in a single screen

Secondary measurements are defined as auxiliary measurements that augment information provided by a main primary measurement function. Depending on the function, you can measure complementary data that traditionally would have taken two different operations to acquire. The table below illustrates all secondary measurement capabilities of the Keysight’s Truevolt DMMs.

 Primary measurement function 34460A secondary measurement function 34465A/70A secondary measurement function DCV ACV ACV, peak, pre-math ACV Frequency DCV, frequency, pre-math 2-wire, 4-wire resistance - Pre-math DCI ACI ACI, peak, pre-math ACI Frequency DCI, frequency, pre-math Frequency Period Period, ACV, pre-math Period Frequency Frequency, ACV, pre-math Temperature Sensor Sensor, pre-math Ratio Input/Ref Input/ref, pre-math Capacitance - Pre-math Continuity - None Diode - None

An example of a common secondary measurement would be the ability to measure the frequency of an AC signal, as shown in Figure 1.

Figure 1. AC voltage with frequency.

The secondary measurement provides more information than is possible with other digital multimeters because of the advanced secondary features in Keysight’s 34465A and 34470A DMMs. As an example, Figure 2 shows the primary measurement of DC voltage (DCV) with a secondary measurement of AC voltage (ACV). This is an especially important measurement if your signal has both an AC and DC component.

Figure 2. DC voltage (primary) and AC voltage (secondary) measurements.

In DCV mode, there are two additional secondary measurements that can be made to provide insight into your signal: Peak and Pre-Math. The Peak measurement, as shown in Figure 3, keeps track of the minimum and maximum DCV readings read by the DMM.

Figure 3. Peak measurement of DCV.

The Pre-Math is a very valuable measurement because it allows you to see modified readings and raw readings in one screen (Figure 4). You can also modify your primary display by applying useful math functions to your data (e.g., a null value or scaling) or filtering your data (Figure 5). See Adding Math Enables Faster Analysis section for more information on applying a math function.

Once you have applied the desired math function, the secondary display will display the raw reading without the math. This is useful for determining if the applied math is correct and if the readings are within the expected range.

Figure 4. A DCV signal with dB scaling with the Pre-Math measurement

Figure 5. A with the null value applied with the raw measurement on the secondary display.

You have seen the benefits of the secondary measurement capability. Below you will see how to do this without changing the instrument’s configuration.

### Example 1

A test engineer wants to monitor the temperature inside of an environmental chamber and needs a high level of confidence that the measurements are accurate. A 34465A DMM is selected due to its ability to log data and provide simple trend charts. A 5-KΩ NTC thermistor is used to spot check for accuracy. The engineer notices that the thermistor has a temperature error of a few degrees. To understand the error, the secondary display on the Truevolt DMM is turned on and temperature and resistor readings are read at the same time. According to the datasheet for the thermistor, it should read 25 ºC at 5 KΩ. The engineer’s probe is put inside of a calibrated chamber set to 27 ºC, but the probe reads 25 ºC with a 5-KΩ resistance reading, a two-degree error. After a bit of characterization, the engineer decides that he can simply add an offset value to adjust for the offset of his thermistor.

### Example 2

A system designed to apply a linear force to a small structure can provide an oscillating force with an AC signal and a constant force with a DC signal. The system designer wants to keep track of both signals concurrently to characterize how much force is being applied. Using a Truevolt DMM with its ability to make secondary measurements, he can read both the DC and AC components of his control signal at the same time.

# How to Phase Synchronize Two Signals Together with a Function Generator

Posted by bernard ang May 30, 2018

When you hear the word “synchronize,” you might think of synchronized swimming. Or, perhaps, synchronizing your watches. Well, we are talking about the same thing related to electronic signaling phase synchronization.

For example, consider a group of synchronized swimmers that are swimming backstrokes at the same rate. The number of backstrokes per second is analogous to signal frequency. If their hand strokes are all perfectly timed, we can say they are phase synchronized. Clocks and watches are also phase aligned if their ‘second’ hands are in synch.

Today, we’ll look at what phase synchronization is from an electronic signaling perspective and how to easily generate phase synchronized electronic signals with a function generator. We’ll also look at some applications where you need two-phase synchronized signals.

Not all function generators are the same. Some function generators require many steps to phase synchronize multiple channels and may go out of synch easily when changes are made to the signals.

## So, what is phase synchronization, and how do you set it up with a function generator?

Let us take an example of two periodic sinewave signals. In a single periodic cycle, a sinewave will oscillate from 0° to 359°. After that, it will restart at 0°. See Figure 1.

Figure 1. Sinusoidal signal plotted in degree angle.

Figure 2 below shows two sinewaves (yellow and dark green waves) that are phase synchronized. The light green wave is 90° angle phase shifted from the dark green wave and yellow wave.

Figure 2. Two signals that are phase synchronized and another signal with 90° phase difference.

Let’s look at how to phase synchronize two waveforms. For this example, we will use Keysight’s 33600A Trueform function generator. During pre-setup, I have already selected 3kHz and 100mV sinewaves for both channels via the Waveforms and Parameters menu buttons. Like most function generators, setting up two outputs doesn’t mean they are phase synchronized.

To synchronize your signals, select “Phase” under the Parameters menu and then select “Sync Internal.” Now, both channel waveforms will automatically phase synchronize.

Figure 3. Select “Sync Internal” to automatically synchronize two channels.

When testing, you may want to intentionally set a phase difference between the two output waveforms. The Trueform function generators allows you to set the phase difference in degree angle, in radians or a phase offset in time. All you need to do is simply select “Phase” under the Parameters menu button, and enter your phase angle.

Another way to create two identical, phase synchronized waveforms is tracking mode. Tracking mode is a simple way to make both outputs identical. To set up tracking mode, you first choose your desired waveform. In this case, we’ll use the same square wave with 3 kHz frequency and 100 mV on Channel 1. Then, select Dual Channel operation by pressing the Channel 1 Output button, and turn on the Tracking mode (see Figure 4).

Figure 4. Identical phase synchronized channel output settings.

The output of both Channels from the Trueform function generator are now two identical phase synchronized square waves with 3 kHz frequency and 100 mV amplitude, as shown in Figure 5.

Figure 5. Two identical phase synchronized waveforms (Phase 0° highlighted in red box).

## What else can I do with my Trueform function generator?

#### 1) Select frequency or amplitude dual channel coupling

Trueform function generator allows you to change the frequency or amplitude of one of your waveform outputs without losing phase synchronization with your other waveform.

This helps reduce complexity of simulating mechanical gear systems with fixed ratios and always requires phase synchronization. AC inductive motors also require phase synchronization of sinusoidal waveforms.

#### 2) Create identical inverted signals

The example in Figure 4 shows how to create two identical phase synchronized waveforms. Additionally, you can also create two identical but complementary (opposite amplitude) waveforms by selecting the “Inverted” option.

Two identical but complementary (opposite amplitude) waveforms that are phase synchronized can be combined to simulate differential output signal.

#### 3) Create an intentional phase offset with arbitrary waveforms

Sometimes, you need to intentionally set a phase offset between your two waveforms for testing. Here is one example: If you have a custom pair of IQ signals, you will want to keep a 90° phase angle relationship between your I and Q channels. To accomplish this, you simply set the signals to start at the same time and then load both custom signals onto Channels 1 and 2, respectively. Finally, go to the parameters menu and press SYNC ARBS button.

## Phase synchronization of two waveforms

Now you know how to phase synchronize two waveforms together to enable you to quickly simulate signals and test your devices or systems.

Keysight’s waveform generators are the ideal solution for this type of analysis. It simplifies setup, leaving you more time to run actual testing. The signal generation process has never been simpler, quicker, and less frustrating.

To learn more about phase synchronizing two waveforms with a waveform generator, check out the “Effortlessly Couple or Synchronize Two Signals on a Waveform Generator” application brief.

# Does My Power Supply Have Multiple Range Output?

Posted by CS May 24, 2018

During the last couple of decades of technology evolution, we’ve seen how electrical and electronic design platforms have product cycles shortened to three to five years. Measurement instruments need to catch up to fast-moving innovation and technology advancements, or they will quickly become redundant.

Mobile device chargers have evolved from an output of 5 V to variable voltage outputs of 5 V, 9 V, or even 12 V for fast charging. For high-power application, electrification of vehicles no longer uses 12 V or 42 V. Voltages now range from tens of volts to power air-conditioning systems and other car electronics to a few hundred volts to drive the powertrain. These demands require a power supply that is equipped with multiple output ranges.

In this blog, we are going to discuss the types of power supply output ranges available in the market and why they are important. Let’s start the discussion by understanding a power supply characteristic.

## Power supply output characteristic

A power supply output characteristic shows the borders of an area containing all valid voltage and current combinations for that particular output. Any voltage-current combination that is inside the output characteristic is a valid operating point for that power supply.

There are three main types of power supply output characteristics: rectangular, multiple-range, and auto ranging. The rectangular output characteristic is the most common.

## Rectangular output characteristic

It’s not surprising to see a rectangle shape power supply output characteristic on a voltage-current graph (see Figure 1). Maximum power is produced at a single point coincident with the maximum voltage and maximum current values. For example, a 20 V, 5 A, 100 W power supply has a rectangular output characteristic. The voltage can be set to any value from 0 to 20 V, and the current can be set to any value from 0 to 5 A. Since 20 V x 5 A = 100 W, there is a singular maximum power point that occurs at the maximum voltage and current settings.

Figure 1. Rectangular output characteristic.

## Multiple-range output characteristic

When shown on a voltage-current graph, a multiple-range output characteristic looks like several overlapping rectangular output characteristics. Consequently, its maximum power point occurs at multiple voltage-current combinations. Figure 2 shows an example of a multiple-range output characteristic with two ranges, also known as a dual-range output characteristic. A power supply with this type of output characteristic has extended output range capabilities when compared to a power supply with a rectangular output characteristic. It can cover more voltage-current combinations without the additional expense, size, and weight of a power supply of higher power. So, even though you can set voltages up to Vmax and currents up to Imax, the combination Vmax/Imax is not a valid operating point. That point is beyond the power capability of the power supply, and it is outside the operating characteristic.

Figure 2. Dual-range output characteristic.

## Autorange output characteristic

When shown on a voltage-current graph, an autoranging output characteristic looks like an infinite number of overlapping rectangular output characteristics. A constant power curve (V = P / I = K / I, a hyperbola) connects Pmax occurring at (I1, Vmax) with Pmax occurring at (Imax, V1). See Figure 3.

Figure 3. Autoranging output characteristics.

An autoranger is a power supply that has an autoranging output characteristic. While an autoranger can produce voltage Vmax and current Imax, it cannot produce them at the same time. For example, Keysight N6755A has maximum ratings of 20 V, 50 A, 500 W. You can tell it does not have a rectangular output characteristic since Vmax x Imax (= 1000 W) is not equal to Pmax (500 W). So, you can’t get 20 V and 50 A out at the same time. You can’t tell just from the ratings if the output characteristic is multiple-range or autoranging, but a quick look at the documentation reveals that the N6755A is an autoranger. Figure 4 shows its output characteristic.

Figure 4. N6755A output characteristic.

For applications that require a large range of output voltages and currents without a corresponding increase in power, an autoranger is a great choice. Here are some example applications where using an autoranger provides an advantage:

• The device under test (DUT) requires a wide range of input voltages and currents, all at roughly the same power level. For example, at maximum power out, a DC/DC converter with a nominal input voltage of 24 V consumes a relatively constant power even though its input voltage can vary from 14 V to 40 V.

During testing, this wide range of input voltages creates a correspondingly wide range of input currents even though the power is not changing much.

• There are a variety of different DUTs of similar power consumption but different voltage and current requirements. Again, different DC/DC converters in the same power family can have nominal input voltages of 12 V, 24 V, or 48 V, resulting in input voltages as low as 9 V (requires a large current) and as high as 72 V (requires a small current). The large voltage and current are both needed, but not at the same time.

• A known change is coming for the DC input requirements without a corresponding change in input power. For example, the input voltage on automotive accessories could be changing from 12 V nominal to 42 V nominal, but the input power requirements will not necessarily change.

• Extra margin on input voltage and current is needed, especially if future test changes are anticipated, but the details are not presently known.

# How to Measure Inrush AC Current

Posted by bill griffith May 23, 2018

Measuring inrush current is always interesting. In some devices, inrush current can be surprisingly high (10x or higher than their steady current). Excessive inrush current can damage components and pc boards designed for lower steady-state currents. To avoid damage, many devices include a protection circuit to limit the inrush current. Typically, large inrush current lasts for a few cycles before returning to a steady-state current. Inrush current is measured as a peak current and is useful for sizing fuses or designing additional protection circuitry.

Figure 1. An inrush current of an inductive load.

It is also important to consider what phase the AC voltage is at when it is applied to the DUT. The turn-on phase can significantly affect the inrush current. A couple of voltage waveforms with different turn-on phases are provided below.

Figure 2. Various turn-on phases of an AC voltage. Left 0 degrees, middle 30 degrees, and right 180 degrees.

When using a mechanical switch, you have no control over the turn-on phase. The device’s inrush current will be completely unpredictable since it is dependent on the phase of the AC voltage. To control the voltage turn-on phase and thus predict the inrush current, you can use Keysight’s AC6801B AC source. A peak hold measurement determines the inrush current while a peak measurement is used for steady-state current.

Figure 3. Current waveform with peak hold and steady state peak current.

Figure 4. AC6801B AC source measurement panel displaying AC current peak, peak hold, and rms.

Determining the maximum inrush current

Most electronics contain switching power supplies as they are incredibly efficient. The test setup below tests an external switching power supply with an AC source and a 30 W load.

Figure 5. Test setup to measure the inrush current of the 12 VDC supply.

Figure 6. The steady state 2.26 A peak current drawn from the AC source.

A series of measurements are made to determine the maximum inrush current. The first measurement uses a 0-degrees turn-on phase, and the second uses 10 degrees. Each subsequent measurement uses a 10-degree higher turn-on phase. Configuring the AC source for inrush current measurements is a two-step process.

1) Setting the turn-on phase from the front panel of the AC source.

2) Clear the peak hold measurement from the front panel.

The 12 VDC power supply output will turn on 1.3 seconds after the AC power is applied. Only after turning on its output will it be drawing steady state current. In Figure 7, two different time scales are used to display the inrush current. The screen capture on the left shows the voltage phase and the inrush current. The screen capture on the right shows the inrush current and the 1.3 second delay before the steady-state current.

Figure 7. The voltage applied to a 12VDC power supply and the current it draws with two different time scales is shown. You can see the details of the inrush current spike on the left. On the right, after 1.3 seconds, the power supply turns on and pulls steady-state current.

Graphing the inrush measurements versus phase for the 12 VDC power supply reveals a trend. The inrush currents are lower when the voltage is turned on with a phase of 0 and 180 degrees. This is because at a phase of zero and 180 degrees, the voltage is turning on at zero volts.

Figure 8. Inrush current for a capacitive device, 12 VDC power supply vs. phase.

Devices with capacitive input will have low inrush currents when the voltage is turned on at zero volts. At zero volts a sinewave has its maximum rate of change, this change causes an inductive load to create their largest inrush current. An inductive load will have its maximum inrush current at zero and 180-degrees.

Figure 9. A simulated graph of inrush current vs. phase for an inductive load.

Limiting inrush current

Inrush current can be limited by designing a device with lower reactance. An example is lower capacitance or lower inductance. Another possibility is to turn on a small part of the device and synchronize the rest of the turn-on to the AC line, taking advantage of the phase with the lowest inrush. The 12 VDC power supply tested delayed the turn on of its output. A third possibility is adding a negative temperature coefficient (NTC) current limiting device to your design. The NTC device initially has a high impedance, which reduces the inrush current. As the NTC device warms up, its impedance is reduced. The steady-state current is not affected by the NTC. Knowing the steady-state current and maximum inrush current helps in selecting the right NTC.

Figure 10. Using an NTC to reduce the inrush current into capacitive device.

Summary

To accurately measure the maximum inrush current, it is essential to consider the turn on phase. The design of the device will affect the phase at which the maximum inrush current occurs. Some devices will have to be designed to limit the inrush current. Adding an NTC current limiting device to your design will limit inrush current. Several measurements need to be made to select the right NTC. An AC6801B AC source can be used to characterize the inrush current of a device quickly, helping you design a device with inrush current that you and your customers can trust.

# Digital Multimeter AC Measurement Considerations

Posted by Seok Jin Wong May 4, 2018

AC RMS is the most useful measurement for real-world waveforms because it does not depend on the shape of the signal. Most of the time, RMS measurement is described as a measure of equivalent heating value with a relationship to the amount of power dissipated by a resistive load driven by the equivalent DC value. For example, a 1Vpk sine wave will deliver the same power to a resistive load as a 0.707Vdc signal. A true RMS reading on a signal will give you a better idea of the effect the signal will have on your circuit.

If an AC RMS reading does not make sense, do not automatically assume there is something wrong with your circuit; the trouble might be with how you made the measurement. Study this list of five considerations that can affect your AC RMS measurement below:

1. Take note on the measurement scale

Most meters specify AC inputs down to 5 or 10 percent of full scale, some even lower. For maximum accuracy, you need to measure as close to full scale as you can. In some cases, you might need to override auto scaling. Make sure the peak of the signal does not overload and saturate the meter’s input circuitry.

1. Settling time consideration

RMS measurements require time-averaging over multiple periods of the lowest frequency being measured. Be sure to select your digital multimeter’s appropriate low frequency filter to allow for the fundamental to be captured. The lower the AC filter frequency is, the longer the settling time, and the longer it will take to make the measurement. Consequently, if you are not concerned about low frequencies in a measurement and your DMM has selectable averaging filters, switch to a faster filter.

1. AC and DC coupling
It is easy to overlook this simple issue when you are in a hurry. If your meter is AC coupled (or has selectable AC coupling), it inserts a capacitor in series with the input signal that blocks the DC component in your signal. Blocking the DC may not be desirable, depending on the signal and what you are trying to accomplish. If you are expecting to include the DC component, but the meter is AC coupled, the results can be dramatically wrong. As a side note, if you need to measure a small AC signal riding on a large DC offset but your meter doesn’t provide AC + DC directly, you can measure the AC component using AC coupling and measure the DC component separately, then square each
and take the square root of the sum, sqrt(Vac^2+ Vdc^2).
2. Low-level measurement errors

When measuring AC voltages less than 100 mV, be aware that these measurements are especially susceptible to errors introduced by extraneous noise sources. An exposed test lead will act as an antenna, and a properly functioning digital multimeter will measure the signals received. The entire measurement path, including the power line, acts as a loop antenna. Circulating currents in the loop will create error voltages across any impedances in series with the DMM’s input. For this reason, apply low-level AC voltages to the digital multimeter through shielded cables, and connect the shield to the input LO terminal. Connect the DMM and the AC source to the same electrical outlet whenever possible, and minimize the area of any ground loops that cannot be avoided.

1. Bandwidth errors

Signals that are rich in harmonics can produce low-reading measurements if the more significant of these components are not included in the measurement. Check the instrument’s data sheet to find the bandwidth of your multimeter. Then make sure your signals do not exceed it.

Summary

Making accurate, true RMS AC measurements with modern digital multimeters is simple and straightforward. However, you need to avoid common traps and pay attention to details. To get accurate results, know your meter and its measurement capabilities.

# How to Time Synchronize Multiple Function Generators Together

Posted by bernard ang Apr 29, 2018

Most basic bench function generators have only one or two output channels. In some cases, you may need more than two channels. Due to this need, many test engineers are forced to buy very expensive multichannel waveform generators. In this blog, we will look at how to easily time-synchronize multiple basic function generators together without the hassle of using external synchronizing trigger boxes or tedious programming. This mitigates the need to purchase expensive multichannel waveform generators.

There are many applications that require multichannel, time-synchronized waveforms. For example, some devices calibrate with synchronized pulses of varying pulse widths. You can easily simulate these signals using multiple basic function generators. Other applications could include simulating optocoupler decoder signals, multichannel Pulse Width Modulation (PWM) motor controllers, and so on.

# How to synchronize two function generators

Today, we’re going to look at how to display four time-synchronized output waveforms using two 2-channel function generators (Keysight 33612A). For our test, we want to generate four 3 kHz pulse trains at 1Vpp. And we want the pulse widths of the four channels to start off at 100 µs and increase by 10 µs for each channel. So, the fourth channel pulse width will be 130 µs.

The first thing to do is wire-configure the rear of the two function generators, as shown in Figure 1. Make one of the function generator’s 10 MHz frequency timebase be the reference for the other function generator. Connect the 10 MHz clock out of the reference function generator to the 10 MHz clock in of the other function generator. (Note that you can fan out the reference clock up to four Trueform function generators).

Figure 1. Rear wire configurations on timebase and external trigger synchronization between two function generators.

Next thing to do is to connect the "External Trigger out" of the reference function generator to the "Trigger in" of the other function generator. That’s all that is needed for external wire configuration.

# User interface system and waveform settings

### Reference timebase oscillator and trigger setup

The next step is to set the second function generator’s reference oscillator to external. This ensures that your function generators both operate off from the same timebase.

Button presses: Select menu button, "System" > "Instrument Setup" > "10 MHz Ref Osc" > "Source" > "External".

Once that is done, you will see a green "Ext Lock" sign on the top right-hand side of your function generator, as shown in Figure 2.

Figure 2. Second function generator’s reference oscillator and external trigger setup.

After setting up your reference oscillator, you will have to set up your trigger for both channels of your second function generator. This tells the second function generator to start at the same time as the first function generator.

Button presses: Select Channel 1 output button and then press the "Trigger" button. Select "Trigger Setup" > "Source" > "Ext" (External). On the same screen menu level, select zero seconds delay and trigger slope on the rising ↑ edge. See Figure 2 on the external trigger menu.

On the first reference function generator, just set up the trigger for both channels to manual and zero delays.

Button presses: Set Channel 1 as the source of your trigger out and on the rising ↑edge. Select "Trigger" on Channel 1 > "Trig Out Setup" > "Source" > "CH1". On the same menu level, select the trigger level and trigger out ↑edge.

### Set up your pulse waveform signals for all your 4 channels (2 channels per instrument)

As mentioned earlier, we want to set up four 1 Vpp pulse trains that are 3 kHz in frequency, and the pulse widths should start at 100 µs and increase by 10 µs increments for every channel. (See Table 1 and Figure 3.)

 Function Generator 1 2 Output Channel 1 2 1 2 Waveform Type Pulse Pulse Pulse Pulse Frequency 3 kHz 3 kHz 3 kHz 3 kHz Amplitude 1 Vpp 1 Vpp 1 Vpp 1 Vpp Offset 0.00 V 0.00 V 0.00 V 0.00 V Phase 0.00° 0.00° 0.00° 0.00° Pulse Width 100 µs 110 µs 120 µs 130 µs

Table 1. Pulse waveform signals set up for 4 channels.

Figure 3. Example pulse setup menu for all 4 channels.

### Set up burst mode to time synchronize all 4 channels

At this point, if you look at all 4 channels on an oscilloscope, you will see the 4 pulse trains, but they are not time synchronized (Figure 4). All channels run independently, hence they are not initially synchronized.

Figure 4. Multichannel outputs that are not time synchronized.

To time synchronize all 4 channels, use the Burst mode (Figure 5).

Button presses: Go to all 4 channels, Select "Burst" > "N Cycle" > "# Cycle" > "Infinite".

Figure 5. Burst mode setup menu on function generator.

Turn on Burst for all 4 channels. You will be prompted by a blinking light on the "Trigger" button of your Channel 1 on your first function generator. Press the button twice, and instantly, you will see 4 channels of time-synchronized pulse trains (Figure 6).

Figure 6. Time-synchronized, multichannel outputs.

You will also notice that each pulse width is 10 µs longer than the previous channel. You have just walked through the hardware and front panel configurations to time synchronize 4 channels. Since you can fan out the 10 MHz reference clock to up to four Keysight Trueform function generators, you can have up to 8 time-synchronized channels.

Knowing how to set up time-synchronized multiple waveforms does not mean that you cannot be creative with your waveforms. Figure 7 gives you an example of simulating a 3-phase AC generator with 3 sinusoidal waveforms, 120° apart from one another.

Figure 7. Simulated 3-phase AC generator 120° apart with multiple function generators.

To learn more about phase, frequency, and amplitude signal coupling, please read the Effortlessly Couple or Synchronize Two Signals on a Waveform Generator application brief. I invite you to like and share if you have found this blog helpful. Thanks for reading.

# 3 Things You Need to Know about Remote Sensing

Posted by CS Apr 27, 2018

Have you ever set your power supply output voltage to a value and found the voltage at your load was lower than you expected? Many of us have experienced that outcome, and that’s because remote sensing needs to be part of the setup. In this article we are going to share with you three things you need to know about remote sensing to help you get a value you can trust.

Figure 1 shows the power supply setup using remote sensing. The remote sense terminals are connected to the load at the points where you want the 5 V setting to be regulated. In this case, the power supply regulates 5 V at the load by adjusting its output voltage to 5.3 V to make up for the drops in the load leads. It does this by using the voltage across the sense leads as part of the feedback loop inside the power supply to adjust the voltage on the output terminals.
The purpose of the power supply is to keep the sense lead voltage constant at the setting; the power supply changes the output terminal voltage based on the sense terminal voltage. The input impedance of the sense terminals is high enough to prevent any significant current flow into the sense terminals – this makes any voltage drop on the sense leads themselves negligible.

Figure 1: Using remote sense to compensate for load lead voltage drop.

# 2. Use sense leads for overvoltage protection (OVP)

One of our military customers providing DC power to a very expensive device during test asked about the availability of a special option on one of our power supplies. They wanted the option that changed the location of the overvoltage protection (OVP) sensing terminals from the output terminals of the power supply to the sense terminals of the power supply. Since the device under test (DUT) is located quite a distance away from the power supply, they are using remote sensing to regulate the power supply voltage right at the device under test. And since the DUT is very expensive and sensitive to excessive voltage, it’s important to protect the input of the DUT from excessive voltage as measured right at the DUT input terminals.

The power supply used, Keysight N6752A installed in an N6700C mainframe, normally uses the output terminals as the sensing location for the overvoltage protection. OVP is used to prevent excessive voltage from being applied to sensitive devices. If the voltage at the output terminals exceeds the OVP setting, the output of the power supply shuts down.

Since this customer is very interested in preventing excessive voltage from being applied to the expensive DUT, sensing for an overvoltage condition right at the DUT is important. For the N6752A, Keysight offers a special option (J01) that adds the ability to perform OVP sensing with the sense leads. See Figure 2. with the J01 option added to the N6752A, the customer’s DUT is protected against excessive voltage.

Figure 2: OVP sensing is done right at the DUT using sense lead terminals in addition to the output terminals.

You may be wondering why the standard OVP would sense at the output terminals instead of at the sense terminals. Probably the biggest reason for sensing at the output terminals is because that approach provides more reliable protection than sensing at the sense leads even though it is less accurate. The output terminals are the power-producing terminals.

If the sense leads become inadvertently shorted, the voltage at the output terminals would rise uncontrolled beyond the maximum rated output of the power supply. This uncontrolled high voltage could easily damage any device connected to the power supply’s output leads. So, sensing for an overvoltage condition at the output terminals makes sense. It may not be the most accurate way to protect the DUT, but it is the most reliable given all of the things that can go wrong, such as a wiring error or an internal fault in the power supply.

# 3. Remote sensing can affect load regulation performance

The voltage load effect specification tells you the maximum amount you can expect the output voltage to change when you change the load current. In addition to the voltage load effect specification, some power supplies have an additional statement in the remote sensing capabilities section about changes to the voltage load effect spec when using remote sensing. These changes are sometimes referred to as load regulation degradation.

When you are choosing a power supply, if you want the output voltage to be well regulated at your load, be sure to consider all the specifications that will affect the voltage. Be aware that as your load current changes, the voltage can change as described by the load effect spec. Additionally, if you use remote sensing, the load effect could be more pronounced as described in the remote sensing capability section (or elsewhere). Be sure to choose a power supply that is fully specified so you are not surprised by these effects when they occur.

# Summary

Remote sense is used to regulate the set voltage at the DUT, compensating for any loss in your leads. Using remote sense will have an impact on regulation performance, which should be considered along with the benefit of compensating for the voltage drop in your leads. Overvoltage protection at the power supply outputs should be used in conjunction with remote sense to protect the DUT.

# How to detect stray voltages with a handheld multimeter

Posted by Seok Jin Wong Apr 25, 2018

Have you ever encountered a scenario in which an AC voltage signal is measured on an electrical circuit that has been completely disconnected? Isn’t it confusing when voltage is measured in the dummy circuits?

Stray voltage, sometimes referred to as ghost voltage, is a voltage that appears in an electrical conductor such as a wire, even though the wire is disconnected from an electrical circuit. You may spend hours troubleshooting this circuit and end up realizing that it’s a stray voltage, even though all wires are disconnected!

# Where do stray voltages come from?

It is very common for electricians and technicians to pull extra wire when facilities or buildings are built and wired. This is just like renovating your house - you will pull extra wire from the conduit for future usage. Normally, these wires are left unconnected. These are the areas where phantom voltage will appear in the circuits.

Wires left unconnected are most likely to be the areas where stray voltage will appear in your circuits.

# Why do stray voltages appear?

Stray voltage readings can be caused by capacitive coupling of energized conductors with nearby unused wire. This capacitance increases as the length of the conductor increases. The longer the wire, the more prevalent a stray voltage.

Current in an active circuit can also trigger a stray voltage reading; the higher the current in the active circuit, the higher the stray voltage. Stray voltage readings caused by active circuits can range from a few volts up to the voltage of the adjacent conductors. It should be noted that according to Underwriters Laboratories Inc. (UL), stray voltage is not real voltage and it cannot cause physical harm to a person. This is because, even though the voltages may be high, the amount of energy stored in the capacitive coupling is very low.

UL also states that care must be taken to ensure that the voltage reading is a stray voltage and not a result of a cable defect or improper installation; as such a situation may result in a shock hazard.

Here is an example that illustrates the overall situation. Imagine that you are installing low voltage lighting in a warehouse office, as shown in Figure 1. The warehouse is equipped with two wires running in parallel to the conduit. One is for light A, which is ON, and the other pair of wires will be used to install a new light using a new expansion cable that runs parallel with light A.

Figure 1: Installation of low voltage lighting in a warehouse office.

Before beginning the installation, you check the voltage on the wire using a normal handheld multimeter with high input impedance, and the measurement result shows as 40 volts even though the line is disconnected from the main switch. Now, you suspect that touching conductors has formed a short circuit, causing voltage to leak through the conductor’s insulation. You spend a lot of time troubleshooting and investigating. However, after a thorough investigation, you find that there is no short circuit to ground! The 40 volts displayed on the measurement reading is a phantom voltage reading formed by the unused wire. After all the hard work troubleshooting, you realize you have lost a lot of time troubleshooting a stray voltage.

From this example, we can conclude using a normal handheld digital multimeter to measure such circuit can make it difficult for you to differentiate ghost voltage reading from legitimate readings. Most handheld digital multimeters have high input impedance compared to the impedance of the circuit being measured. The handheld multimeters with high input impedance that is greater than 1 MΩ are designed to place very little load on the circuit under test. In this capacitive coupling situation, a phantom voltage reading is measured by this high input impedance multimeter.

In our example, if a low input impedance multimeter had been used to perform the AC voltage measurement, the electrician would have found virtually zero stray voltage. This is because stray voltage is a physical phenomenon involving very small values of capacitance; it cannot energize a load. Using a multimeter with low input impedance will short out the capacitive coupling effect, while using a high input impedance multimeter will not.

# The solution

Certain models of Keysight’s handheld multimeters, for example, the U1242C, have a unique feature: a ZLow function (Figure 2) that allows you to switch from high input impedance mode to low input impedance mode to check for the presence of stray voltages. This solution eliminates the need to carry both a low impedance meter and a high impedance meter.

Figure 2. U1242C handheld DMM with ZLow function

The ZLow function acts like a backup voltage indicator and eliminates the need to carry additional tools for troubleshooting. If a real voltage is measured using the ZLow function, the positive temperature coefficient (PTC) thermistor that is designed as an over current protection will ensure the multimeter always operates in high input impedance.

# Use a multimeter with flexible input impedance

Now you know how to detect stray voltages efficiently and effectively using a handheld multimeter with low impedance mode. Keysight offers different handheld multimeters that come with ZLow function that can remove stray voltages from your measurements by dissipating the coupling voltage. Use ZLow to reduce the possibility of false readings in areas where the presence of stray voltages is suspected.

# How to Create Differential Signals with a Function Generator

Posted by bernard ang Apr 23, 2018

Aside from wireless and fiber optic transmissions, in this modern digital and mixed-signal age, many of our data transmissions go through good old-fashioned conducting cables. The two most common types of cables are differential and single-ended. There are obviously pros and cons in choosing either type of cable, but differentials have many advantages over single-ended cables. Differential signaling is usually used in conjunction with tightly twisted pair wires to reduce or cancel out the generation of electromagnetic noise. Hence, it has superior signal-to-noise ratio and fewer timing errors.

There is a Low-Level Differential Signaling (LVDS) standard for electrical transmission and communication protocols that are used in very low voltage and sometimes high-speed data transmissions such as video, graphics, and digital data bus transfers. Other applications include transmitting sensitive analog signals with audio microphones and medical heart monitoring devices.

# What is a Differential Signal, and How Do I create a Differential Signal?

Figure 1 below shows two complementary signals (one signal inverted from the other) being used to create a differential output transmission signal. Here’s how to create differential signals for testing.

Figure 1. Differential output derived from two complementary signals (out+/out-).

Step 1: Use a function generator to create the first of your two complementary signals (out+/out-). For example, create the (out+) signal on Channel 1.

Step 2: Select Dual Channel Inverted Tracking mode (see Figure 2).

As a result, Channel 2 will output an identical, but inverted, signal of Channel 1 (out-). Both signals will be amplitude and phase synchronized.

Figure 2. Differential channel output function generator setup on a Keysight Trueform function generator.

Step 3: Combine the differential outputs. To physically combine the two channels into a single differential output, connect the two common connections (the connector shells) of both channels together. Use the middle signal pin of Channel 1 as the high signal path of the differential signal, and use the signal pin of Channel 2 as the inverse return path.

Then connect a twisted pair cable to signal pins of Channel 1 and 2, as shown in Figures 3 and 4.

Figure 3. Differential signal block diagram.

Figure 4. Function generator setup based on block diagram.

On the other end of the twisted pair cable, connect the signal pin of Channel 1 to the signal pin of the receiver’s BNC connector (Differential Signal Input) and the signal pin of Channel 2 to the ground connector shell of the receiver’s BNC (Figure 4).

# Why and When Do You Use Differential Cables?

Basically, a product or systems designer will choose differential over single-ended signaling if:

• The designer needs to reduce EMI or electromagnetic interference
• The designer needs to reduce crosstalk or interference from nearby cables
• The designer needs to transfer very low voltage signals, especially in millivolt range, to simulate bio-signals for medical applications. Low voltage signals are susceptible to noise interference
• The designer needs to transfer low digital voltage signals to save power
• The designer needs precise timings of digital signal crossover or digital switching

# What Type of Function Generator Should You Use to Simulate Differential Signals?

## Create a Balanced Pair of Signals

Use a function generator with “inverted tracking mode” to create an inverted mirror image of a signal. The signals on both channels will then be perfectly balanced pairs with synchronized amplitude, offset, and phase. Make sure the lengths of your differential twisted pair wires are matched too.

## Reproduce Actual Signals from Your Design

Consider using an arbitrary function generator and its ability to reproduce actual signals from your design. You can quickly recreate your design signal by capturing it with a modern oscilloscope, then saving the captured trace to a .csv file.

At this point, you can use a USB thumb drive to import the data in the Waveforms > Arbs menu of your function generator to recreate and playback the signal from your design. You can even change your signal’s frequency, amplitude, or offset.

After recreating your signal, you can use the Dual Channel Inverted Tracking mode to recreate the output as a differential signal.

## Create Arbitrary Waveform Signals

Differential signals are not limited to basic function generator signals such as periodic pulse, sine, square, and ramp signals. A lot of applications need complex waveforms, such as ECG bio-signals for medical applications, automotive CAN bus test signal simulations, telecommunication network test signals, and more.

Keysight’s Trueform function generators provide software tools such as BenchVue Waveform Builder (see Figure 5) and a built-in waveform editor to create your arbitrary waveforms. You can also use tools such as Excel or Matlab to create your arbitrary waveforms and transfer them into your function generator via .csv file format.

Figure 5. BenchVue function generator app to create arbitrary waveforms.

## Use a Function Generator with the Lowest Jitter

If timing of your signal is very critical, differential signaling is a better option than single-ended signaling since it eliminates the uncertainties of transition crossover points. This takes care of jitter noise from cable transmission.

How about jitter noise from the source of the signal? Not all function generators are built the same! Keysight’s Trueform function generators have superior jitter noise compared to the conventional DDS technology used by most function generators in the market. (See Figure 6.)

Figure 6. Trueform technology, shown on the left, has significantly better jitter performance compared to conventional DDS technology.

Sometimes, you need an ideal signal with low noise, low distortion, and high signal integrity for your tests. However, at the same time, you want to introduce a controlled imperfect signal with noise for your tests.

Make sure to use a function generator that allows you to add noise to your signals. Press the Modulate button, select Source as Internal/Shape as Noise, and turn on Modulation. (See Figure 7.)

Figure 7. Adding variable bandwidth noise to signal for testing.

# Conclusion

You can see that it’s easy to create differential signals if you have the right function generator. Choosing a function generator that has built-in differential capabilities will streamline your testing and ensure your signal’s phase and amplitude stay balanced.

To get better at using your function generator in the lab, download the Creating Differential Signals with a Waveform Generator application brief.

# Safeguard your Power-sensitive DUTs from an Over Power Condition

Posted by CS Apr 20, 2018

As the world continues to trend toward increased energy savings and green energy sources, more and more heavy machinery and vehicles are becoming electrified. Mechanical combustion engines are being replaced by electric motors as part of this technology trend. As these demands accelerate, higher expectations for reliable and safe power are spurring engineers to put all their brain power into coming up with the most efficient product designs. The last thing that R&D engineers want is to mess with the power supply reliability and create a potential safety issue.

## Over Voltage and Over Current Protection

Today’s system DC power supplies incorporate a variety of features to protect both the device under test (DUT) as well as the power supply itself from damage due to a fault condition or setting mishap. Over voltage protect (OVP) and over current protect (OCP) are two core protection features that are found on most system DC power supplies to help protect against power-related damage. But is that all that you need to know? In this blog, we will also discuss power protection on your devices that do not operate on fixed voltage and current levels.

Over voltage protect helps ensure the DUT is protected against power-related damage in the event the voltage rises above an acceptable range of operation. As over voltage damage is almost instantaneous, the OVP level is set at reasonable margin below this level to be effective; yet it is set suitably higher than the maximum expected DUT operating voltage so transient voltages do not cause false tripping. Causes of over voltage conditions are often external to the DUT.

Over current protect helps ensure the DUT is protected against power-related damage in the event it fails in some fashion, causing excess current, such as an internal short or some other type of failure. The DUT can also draw excess current by consuming excess power due to overloading or from an internal problem that causes inefficient operation and excessive internal power dissipation.

OVP and OCP are depicted in Figure 1 below in an example DUT that operates at a set voltage level of about 48V and uses about 450W of power. In this case the OVP and OCP levels are set at around 10% higher to safeguard the DUT.

Figure 1. OVP and OCP settings to safeguard an example DUT.

## Over Power Protection

However, not all DUTs operate over a limited range, as depicted in Figure 1. Consider, for example, that many (if not most) DC-to-DC converters operate over a wide voltage range while using relatively constant power. Similarly, many devices incorporate DC-to-DC converters to give them an extended range of input voltage operation. To illustrate with an example (see Figure 2), consider a DC-to-DC converter that operates from 24 to 48 volts and runs at 225 W. DC-to-DC converters operate very efficiently, so they dissipate a small amount of power and the rest is transferred to the load. If there is a problem with the DC-to-DC converter that causes it to run inefficiently, it could be quickly damaged due to overheating. While the fixed OCP level depicted here will also adequately protect it for over power at 24 volts, you can see that it does not work well to protect the DUT for over power at higher voltage levels.

Figure 2. Example DC-to-DC converter input V and I operating range.

A preferable alternative would be to have an over power protection limit, as depicted in Figure 3. This would provide an adequate safeguard regardless of the input voltage setting.

Figure 3. Example DC-to-DC converter input V and I operating range with over power protect.

Since an over power level setting is not a feature that is commonly found in system DC power supplies, this would then mean having to change the OCP level for each voltage setting change, which may not be convenient, desirable, or in some cases, practical to do. However, in the Keysight N6900A and N7900A advance power system DC power supplies, it is possible to continually sense the output power level in the configurable smart triggering system. This can then be used to create a logical expression to use the output power level to trigger an output protect shutdown. The N7906A software utility was used to graphically configure this logical expression, and then it was downloaded it into the advance power system DC power supply, as shown in Figure 4. Since the smart triggering system operates at hardware speeds within the instrument, it is fast-responding, an important consideration for implementing protection mechanisms.

Figure 4. N7906A software utility graphically configuring an over power protect shutdown.

A glitch delay was also added to prevent false triggers due to temporary peaks of power being drawn by the DUT during transient events. While the output power level is being used here to trigger a fault shutdown, it could just as easily be used to trigger a variety of other actions.

## Summary

We have discussed that advance system power supplies can provide over voltage and over current protection as well as protection for over power conditions. For more information on protection against power related damage, download the Protect Against Power-related DUT Damage During Test application note.

# Automate testing of DUT regardless of different local voltages and frequencies with Excel and Visual Basic

Posted by bill griffith Apr 20, 2018

Electronics are often designed to work anywhere in the world with local power. An AC source can create the various voltage-frequency combinations used in different countries and even measure the current and power consumption.

Figure 1. A power supply that accepts 100 Vac to 240 Vac.

You can automate the process using Visual Basic for Excel to program an AC6801B AC source. Excel lets you capture and share results in a neatly organized table. In this example, we chose eleven countries to demonstrate the wide variety of voltages and frequencies used around the world. Some countries use multiple voltages and plug types, and those displayed are selected to illustrate a broad range. The table is easily modifiable, and you can add more countries by increasing the rows as well as the country count in cell D6.

Figure 2. An example table of various voltages and frequencies. Row number and column letter shown in red.

The Visual Basic program reads the voltages and frequencies from the table and sets up the AC source to output them. The program pauses for the number of seconds provided in column F, allowing the test device to settle. The AC source then makes several measurements and adds them to each row of the table.

Figure 3. The AC source output is set to 100 Vac at 50 Hz. Three measurements are added to the table after the delay. The process is repeated for each country.

## Getting started with Visual Basic for Excel

If you are not currently using Visual Basic for Excel, you need to display the developer tab. It is included in Excel but is not shown by default. You can find instructions online for adding the tab to the version of Excel you use. The second step is adding Keysight IO Libraries Suite. In this example, all the commands are sent to the AC source using the Keysight IO library. With the software installed, open the developer tab and create the global variables.

Figure 4. Opening the VBA project and creating the global variables.

## Opening a connection to the AC6801B

The following subroutine creates a connection to communicate with an instrument based on its VISA address. The VISA address is read from the table, making it easy to update. You can use the Keysight Connection Expert, which is installed with the IO Suite to read the instrument address. Watch this YouTube video to see the steps to locate the instrument VISA address and verify the connection.

Figure 5. Subroutine to open a connection to the instrument.

You can insert a button onto the spreadsheet (Developer > Insert > Form Control > Button). You are then prompted to add the name of the subroutine that will run when the button is clicked.

Figure 6. The spreadsheet with the first two controls and the LXI address.

## Resetting and configuring the instrument

The subroutine initConfig_Click() has three primary roles: to clear the AC source, to configure DUT protection, and to select the upper range and turn on the AC source with a known voltage. Once the instrument is reset, it is set to output only AC voltage, and the voltage is limited by a min and max value. Obviously, overvoltage can cause damage to a device, but too low a voltage is just as harmful. In the case of a power supply, too low a voltage leads to higher current to meet the power needs of the load. In addition, a current limit is set to protect the DUT from damage due to excessive current. The time required to run the test is reduced by using a single range. The AC6801B is a dual range source and provides additional current in its low range. For our device, the upper range has enough current to test the device. Lastly, a voltage-frequency combination is selected, and the output is turned on. The primary goal of this program is to demonstrate some of the AC source’s capability. With the output turned on, the AC source displays measurements on the front panel.

Figure 7. Subroutine to put the AC source in a known state.

## Step through the list of countries

The createSequence_Click() subroutine populates the table with the power measurements for each country. It uses the number specified in cell D6 to determine the number of loops necessary to complete the table. In each loop, the voltage-frequency is pulled from the table, and the source is set to output the combination. The power measurements are made after the delay in column F, which allows the device to adjust to the new voltage and frequency. Three power measurements were selected from a choice of 17 different measurements. As each measurement is made, the results are added to the table.

Figure 8. Subroutine to apply the voltage-frequencies from the table and populate the measurements.

## Closing the instrument connection

Closing the instrument connection releases the resources. Once the connection closes, an error is generated if you attempt to send commands to the instrument.

Figure 9. Closing the instrument connection.

The read error subroutine is useful while creating or modifying a program. It pulls the error strings from an instrument one string at a time. You may need to run it multiple times to clear all the errors. It is designed to run independently of the rest of the program. It assigns resources, opens the IO, and then closes the IO.

Figure 10. A subroutine to query the error strings.

## Summary

The universal adapter test program is designed to demonstrate some of the AC6800 Series capabilities and a method to document the measurements using Excel with Visual Basic. Adding a main program to call each of the subroutines simplifies the program because it’s not necessary to connect each routine to a separate control button. The program ran many times over a two-week period using the AC6801B, and the results are repeatable. Often the results are identical when formatted to show a single decimal place. The program is easily modifiable to make additional measurements. A couple of real-world applications are to characterize a group of power supplies and statically determine the max power usage or verify results from a remote facility. Using a second AC source, it is easy to reproduce results with a production facility located on the other side of the globe.

# How to Characterize a Single Phase Synchronous Motor

Posted by bill griffith Apr 19, 2018

A search of the internet provides articles and videos showing the general operation of a synchronous motor. I thought it would be interesting to make some actual measurements and display some real-world voltage and current waveforms. I was fortunate to find a synchronous motor that had external wiring to its two coils, allowing independent characterization of each coil. The motor is small, but the characterization would scale to a larger motor.

## Synchronous Motor Basics

An AC signal powers a synchronous motor. It uses the positive/negative power cycle to create a changing magnetic field. Our motor uses two coils that are 90 degrees out of phase. The fields change such that the magnetic rotor turns to keep aligned with the fields. Synchronous 3-phase motors are popular due to the simplicity of using each phase to create a rotating field. You can view many good animations of how a synchronous motor works on the internet.

Figure 1. A drawing of a synchronous motor with two coils. The colors represent the colors of the motor leads.

The AC source causes the motor to rotate at a rate proportional to its frequency (typically measured in Hz). Our example motor is powered by a 60 Hz AC signal and rotates at 3600 RPMs. The synchronous motor can turn in either direction. It starts turning toward the attracting magnetic force, which could be either of the coils depending on its configuration. A minimum voltage is required to create a magnetic field strong enough to cause the rotor to spin. Once spinning, the AC source voltage does not affect the speed of rotation.

Figure 2. Changing the location of the AC source to cause a different direction of rotation.

## Electrical Connections to the Motor

I wanted to characterize the voltage and current through each coil. I used our PA2201A power analyzer to characterize each coil. Each channel of the power analyzer has an input for voltage and an input for current. It can make  measurements of each input or measurements that combine two inputs. While not shown in Figure 2, I added a switch to connect the AC source to coil A or coil B.

Figure 3. An electrical drawing of the motor with a power analyzer connected. The power analyzer connections are color-coded. Channel 1 is yellow and Channel 2 is green.

The AC6801B AC source supplies the 110 VAC input power. The AC source will characterize all the power parameters for the motor. We can also vary the frequency of the input voltage.

## Real-World Measurements

We will start with the AC source connected to the coil A as well as channel 1 of the power analyzer, shown in Figure 3. We expect to see the 110 V on channel 1 (top yellow waveform) of the power analyzer. Notice the coil B voltage shown on channel 2 (top green waveform) lags channel 1 almost perfectly by 90 degrees.

Figure 4. Screen capture from the PA2201A. The display is split into three parts: the top is voltage, the middle is current, and the bottom is power. Yellow waveforms represent channel 1, coil A, and green waveforms represent channel 2, coil B.

I do not work with inductors and capacitors every day and was surprised that coil B receives much more current and power. If you look at the equations for reactance, it makes sense. The capacitor reduces the reactance X in the branch with coil B, X = XL - XC. More current flows through this branch, which creates more voltage across coil B.

Figure 5. The diagram shows the reactance through each branch of the motor.

Figure 6. Measuring the power of channel 1, coil A, and channel 2, coil B, with PA2201A power panels.

The AC source is measuring the input to the motor, which is consuming 0.05 ARMS, 5.1 W, and 5.4 VA. Coil B uses most of the energy. I was also surprised to learn the coil's reactance is nearly 50% resistive, 50% inductance. A phase angle ɸ of 45% or power factor PF .707 would be precisely a 50% mix, and the measurements were only slightly more inductive. You can see the phase angle (the delay between voltage and current) in the Figure 6. Lastly, I had expected the current waveforms to be more sinusoidal. In reality, the motor acts as a transformer and sends energy between the coils. If I remove the capacitor, you can see 50 V across coil B; No current flows, as I left the branch open.

Figure 7. With the capacitor removed, the motor can turn in either direction. Energy in coil A creates a voltage on coil B. On the left, the motor is turning in a counterclockwise direction (normal), and on the right it is turning clockwise. Notice in the first case, the coil B voltage (shown in green) lags, and in the second it leads.

## Reversing the Motor Direction

Applying the source voltage to coil B instead of coil A causes the motor to run in the opposite direction.

Figure 8. Applying the source voltage to coil B, which causes the motor to turn clockwise.

Figure 9. Measurements of the two coils with the source connected to coil B.

The coils are not perfectly symmetrical, as the power through each branch is a bit lower than in the counterclockwise setup. Using the AC source, the overall power measurements are also lower, reduced to 4.8 and 5.1 VA. You will notice the voltage in coil B, green waveform, leads coil A. You can compare Figure 9 and Figure 6 to see the differences.

## Changing the Source Frequency

The AC source can vary its frequency from 40 Hz to 500 Hz. Changing the frequency of the source affects the speed of the motor. It also changes the impedance of the circuit, as the reactance of the capacitor and inductors are a function of frequency.

Figure 10. The waveforms on the left were captured with a source frequency of 40 Hz, and on the right the source frequency was increased to 80 Hz.

## Summary

I was impressed with the motor’s overall power factor. The AC source measured the PF to be 0.94. Earlier, we measured the current through each branch of the circuit. The total current consumed by the motor is a point-by-point summation of the two current waveforms (i.e., the sum of the current through each of the coils). A PF of 0.94 indicates that the summation has nearly identical phase to the input voltage. The power analyzer can sum the two current waveforms and display them overtop the input voltage.

Figure 11. The pink waveform represents the total current used by the motor. It is a mathematical summation of the current waveforms shown at the bottom of the screen.

As you can see, the system has a PF near 1.0, as the input current and voltage have nearly identical phase.

The capacitor used with our motor is considered a run or permanent split capacitor, as it maintains the 90-degree shift in the coil voltages. People sometimes generically refer to a capacitor used with a motor as a start capacitor. A start capacitor is disconnected by a switch once the motor starts spinning.

To overcome the difficulties in changing the speed of our single-phase motor, the manufacturer added a gearbox so it spins at 33.3 RPMs.

The AC6801B AC source made it easy to supply the required AC power (110 VAC at 60 Hz) while making power measurements. The measurements include current, real power, apparent power, reactive power, and power factor. The PA2001A was used to characterize the voltage and current through each coil.

# Tips for Preventing Damage to your Digital Multimeter

Posted by Seok Jin Wong Apr 18, 2018

Using a digital multimeter (DMM) to perform measurements is very common in today’s world. Technicians use multimeters for equipment servicing, engineers use multimeters to troubleshoot, students use multimeters for lab research, and so on. Since digital multimeters have many functions, you need to know how to properly use your multimeter. Most multimeter failures are caused by improper use.

In this blog, you will learn four tips to avoid damaging your digital multimeter:

• Read the warning labels and specifications

Before you begin taking measurements with your digital multimeter, you should read the warning labels and specifications. Do not exceed the values provided in the specifications guide or as indicated by the yellow warning labels on the instrument. Always refer to the specification guide for conditions required to meet the listed specification.

Figure 1. Example of the 34470A digital multimeter’s rear panel showing warnings on the maximum voltage input and maximum current input.

•  Ensure proper grounding

Always use the three-prong AC power cord supplied with the instrument. Proper grounding of the instrument will prevent a build-up of electrostatic charge that may be harmful to the instrument and you. Do not damage the earth-grounding protection by using an extension cable, power cable, or autotransformer without a protective ground conductor. It is good to check the AC power quality and polarity; Typical required AC voltage is 100 V, 120 V, 220 V ± 10%, or 240 V +5%/–10%. Typical expected grounding wire resistance is < 1 Ω; The voltage between neutral and ground line is < 1 V. If needed, you can install uninterruptible power supply [UPS] to power your meter.

• Avoid overpowering the digital multimeter

You can avoid damaging your digital multimeter by anticipating the signal level you’ll measure and presetting the proper signal range on the DMM. Overpowering the digital multimeter can damage the components inside the meter. For example, Figure 2 shows that the maximum voltage input of a 34461A digital multimeter is 1000VDC and 700 VAC. Before turning on or off the connected equipment or the DUT, reduce the signal level to the minimum safety level. This will prevent unexpected voltage or current swell or sag from affecting the input or output of your instrument.

Figure 2. Keysight’s 34461A digital multimeter front panel.

• Check for proper temperature and humidity

You need to keep your multimeter in a clean and dry environment. Typical temperature for storage condition is between – 40 and 75 °C; Typical humidity is < 95% RH. The DMM’s optimal operating temperature should be from -5°C to 23° You also need to ensure proper ventilation among racks so the temperature does not go up if all instruments are in use at same time. You should also frequently inspect and clean the cooling vents and fans.

Take good care of your instrument!

Using your instrument properly helps you and your organization save on maintenance costs. Therefore, it is always good to know the basic tips mentioned above to prevent damage to your multimeter.