2018

May 2018

# 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.