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Different instrument form factors are used for each of the various design and test phases in a project to increase efficiency. Many engineers believe their only choices for high performance measurements are large bench instruments, or modular choices. The USB platform is a popular option for anyone needing a small, portable device. However, most USB instruments available are not comparable to benchtop performance, and utilize a different user interface (UI). Having inconsistent measurements and automated coding between each platform makes the transition between each development phase more time intensive due to the learning curve and adjustments that must be made.

 

With the new Keysight Streamline Series, you can expect a common UI, measurement capabilities, and automated code between all form factors: USB, benchtop, and modular. This allows you to move between product development phases more efficiently and effectively since knowledge and data can easily be transferred among the various platforms.  With the same applications and accuracy as comparable benchtop instruments, the new Keysight Streamline Series is the perfect compact USB option. It is portable, easy to use, and there is zero compromise in performance.

 

The new Keysight Streamline Series consists of USB oscilloscopes, vector network analyzers (VNA), and an arbitrary waveform generator (AWG). The oscilloscopes range from 200 MHz to 1 GHz with 2 analog channels. They feature many of the capabilities you would find in an InfiniiVision 3000T X-Series benchtop or M924xA modular scopes, including zone triggering and 1,000,000 waveforms/s update rate. You will also find many of the same applications that you are used to from your benchtop instruments, like mask testing, frequency response analysis, built-in arbitrary WaveGen, and serial decoding. With a soft front panel that has the same UI as all InfiniiVision oscilloscopes, it makes it easy to transfer your skills between the multiple platforms.

 

 

The new USB vector network analyzers (VNA) have a wide frequency coverage that operates from 300 kHz up to 26.5 GHz, with two ports. The software has the same intuitive GUI as our benchtop VNAs, which again allows you to reduce the transition time between platforms. It utilizes the same measurements, automated code capabilities, calibration and metrology as our other trusted Keysight VNAs, so you can have consistent measurement results between form factors. You are also able to extend the number of ports available to increase your testing capabilities.

The P9336A AWG provides multiple independent or synchronized signal outputs with exceptional performance in USB form. The small, compact form-factor makes it ideal for creating complex waveforms, without taking up much bench space.  It can supply digitally modulated waveforms for wideband communication systems and high-resolution waveforms for radar and satellite test. Industry standard waveforms for the AWG can be easily generated using Keysight software applications tools such as Signal Studio or Waveform Creator. In addition to these tools, you can generate your own waveforms using MATLAB or custom tools. The AWG provides standard IVI compliant drivers for integration with multiple application development environments.

 

The new Keysight Streamlines Series offers VNAs, oscilloscopes, and an AWG in compact form with zero compromise in performance. Check out the details of each of these new instruments at www.keysight.com/find/streamline-series.

Have you ever been fighting noise on your measurements and can’t tell where it’s coming from? There are four possible contributors:

  1. Your device under test
  2. Your probe
  3. Your oscilloscope
  4. Or a combination of all three.

 

Learn about the different ways you can minimize noise impacts and improve the quality of your measurements.

 

System Noise Consequences
You should look at your probe and oscilloscope together as one measurement system that can add noise to your measurement. Figure 1 below shows two possible noise sources from your system: one source from your probe and the other from your oscilloscope. Some amount of noise will come from the probe’s amplifier system and ride on your DUT’s signals, which is sent to the attenuator of the oscilloscope. All scopes use an attenuator to vary the vertical scale on your oscilloscope screen. Most oscilloscopes can detect your probe's attenuation ratio and will automatically adjust its vertical scale accordingly. For example, for a 10:1 probe, the oscilloscope will simply amplify both the signal and noise by a factor of ten. Be sure to keep this in mind as you minimize the noise in your signal.

 

 

Signal to noise diagram

Figure 1: Signal-to-noise diagram of the oscilloscope and probe

 

Probe Noise Impacts
Due to added inductance, the probe ground/signal loop formed by the probe and tip contributes to the noise on your signal. This noise can be reduced by:
1) Selecting just enough scope/probe bandwidth to measure your DUTs signals. Excessive bandwidth will contribute to the system’s overall noise.
2) Setting your oscilloscope’s vertical range to the most sensitive voltage range possible while still seeing your DUT’s complete signal on the oscilloscope. This will reduce the amount of gain the oscilloscope needs.

 

Oscilloscope noise floor with no probe connected

Figure 2: Oscilloscope noise floor with no probe connected

 

Oscilloscope noise floor probe connected

Figure 3: Oscilloscope noise floor probe connected

 

In Figure 2, you can see the noise floor of an oscilloscope with no probe attached is 295 µV root mean square (rms).
In Figure 3, with the probe attached to the oscilloscope, the noise increases from 295 to 485 µV rms. So, the probe itself is adding around 200 µV rms (or 67% more noise)! This noise level will reduce when your probe is well grounded, but it is worth noting the increased noise level just by adding a probe. Keep your ground and tip lengths as short as possible to reduce this effect.

 

Probe Attenuation Impacts
The probe attenuation you need is going to depend on the Voltage of the signal you are measuring. The attenuation ratio changes how the signals are fed into your oscilloscope. For example, a 10:1 probe connected to a 1V signal will pass 100 mV to the scope’s input.The oscilloscope will either read (or you can manually enter) the probe’s attenuation ratio. Then the oscilloscope will display the correct signal, factoring in the probe’s attenuation ratio. Having a higher attenuation ratio (100:1, 1000:1) will allow you to view higher voltages, but it will also make the scope’s internal amplifier noise more pronounced. The higher the attenuation ratio, the more scope noise you’ll see. For example, a 10:1 probe will show 10x the noise.

 

One easy way to estimate the amount of your probe noise is to check the attenuation ratio and the probe noise level from the probe’s data sheet or manual. Many probe manufacturers characterize the probe’s noise as equivalent input noise (EIN) and will be listed in volts rms.

 

The probe pictures in Figure 4 (from bottom left going clockwise) are examples of a 10:1 passive; 10:1 single-ended active; 50:1 or 500:1 high voltage; and a 1000:1 high voltage probe. These attenuation ratios are needed to reduce the probed signal down to levels that the oscilloscope attenuators can handle and display on the screen without clipping.

 Examples of probe attenuation and voltage levels
Figure 4: Examples of probe attenuation and voltage levels

 

Common attenuation guidelines and limitations are shown in Figure 5 below. Keep this in mind when determining the attenuation ratio you need.

 

1:110:1100:1
Suitable formeasuring low voltage, low frequency signals (<~25MHz)general purpose measurementhigh voltage measurement
Limitationslimited bandwidth, dynamic rangetypically up to 300 Vhigh probe noise

Figure 5: Probe attenuation guidelines

 

Figure 6 below compares the same signal measured by both a 1:1 and a 10:1 passive probe. The screen shot clearly shows how attenuation from a 10:1 probe can cause the oscilloscope to amplify both the signal and noise. The result is an exaggerated level of noise in the signal (green trace).

 

A 50 mVp-p sinewave measured with bot 1:1 and 10:1 probes - overstated 36%

Figure 6: A 50 mVp-p sine wave measured with both a 1:1 and 10:1 probe.

 

Higher attenuation ratios lead to higher levels of noise shown on the oscilloscope. This might lead you to believe that you should always use a 1:1 probe. But that’s not true! Lower attenuation probes typically have much higher loading on your system and may have lower dynamic range. There are tradeoffs, and you will need to pick the probe that fits your measurement best. You want a probe that can effectively measure the level of voltage on your DUT with the least amount of attenuation and lowest loading effects.

 

Conclusion
Your system noise can be exaggerated by probe and oscilloscope noise levels. Selecting the correct probe for your application with the correct attenuation ratio will lower the probe/oscilloscope added noise. As a result, your measured signal is a cleaner representation of what is on your DUT.

 

Are you falling into oscilloscope probing pitfalls? Avoid making the same mistakes as others with the Oscilloscope Probing Pitfalls eBook

Get the most benefit from your DDR4 and next generation memory designs

 

DDR4 DRAM technology with fast data rates of 1600 MT/s and higher forced the industry to adopt high speed serial interface testing methodologies. Traditional setup and hold time tests are now replaced with eye diagrams and mask tests to account for bit error rate. Many DDR4 designers are still spending a lot of time trying to understand the specification and translate them into measurement methodologies. DDR5 is the next generation memory after DDR4, offering speeds up to 6.4 GT/s, and DDDR5 will present more design and testing challenges than DDR4. Learn how to better understand the DDR4 specification and its testing methodologies to help prepare yourself for the migration to DDR5.

 

When to Take the Leap - The Technology

There are many factors that drive technology migration. It helps to know the technology availability, the application, the speed, and the power requirement. There are three classes of volatile memory that are optimized for the different classes of applications – computer/server, graphics, and mobile applications. DDR4 is designed for the computing and server industry and has been around for quite some time with the fastest defined speed at 3.2 GT/s. JEDEC is currently working on the next-generation DDR memory, DDR5, to fulfil the request for faster data rates. It is anticipated that DDR5 will be able to operate up to 6.0 GT/s or higher. Graphics memory has the highest operating speed now at above 5.0 GT/s. GDDR6 is the latest graphics memory technology. The last class of volatile memory is the mobile DRAM, which is dominated by low power DRAM using DDR technology. LPDDR4 speed now surpasses DDR4 speed at 4.2 GT/s.

 

DDR technology trend.

Figure 1. DDR technology trend.

 

DDR4 Specification and Measurement Methods

JEDEC DRAM specifications are defined at the balls of the DRAM package. This is very different than other high-speed serial interfaces. For the longest time, DDR technology has adopted conservative design specifications. DDR technology uses a bi-directional, singled-ended, parallel bus. The specification is written to focus on interconnect and signal integrity characteristics with negative margin for timing budget. The industry over designs the parts to account for the negative timing margin.

 

One of the new measurements in DDR4 specification is receiver input masks. This one test would replace the traditional setup and hold time test. Memory timing specifications for previous DDR technologies like DDR2 and DDR3 were based on assumptions that the data capture will be error free if the data setup and hold time meet the specification. Lower data transfer rates in DDR2 and DDR3 have worked with these assumptions. This is not true for DDR4. DDR4 reflects the fact that random jitter and bit error rate are important parts of the specification, and changes have been made to the specification to help address these issues. Additionally, the specification now includes noise considerations such as the effect of noise-reducing eye openings. With all these specifications added to the standard, the lost margin can now be regained, and designers now can simplify their design and reduce design cycles, ultimately saving cost.

 

DDR4 receiver input mask.

Figure 2. DDR4 receiver input mask.

 

The Next Generation Memory

As previously mentioned, the next-generation DDR memory is anticipated to have data rates up to 6.5 GT/s. A potential consequence of running at these speeds is that the data eye could be closed at the receiver. This behavior is observed with other high-speed serial interfaces, such as PCI Express Gen 3. PCI Express Gen 3 implemented equalization on the receiver to open the eye for measurement and de-emphasis on the receiver side. The same methodology can be applied to DDR5 technology with several modifications. Memory is crosstalk dominant unlike PCIE, which is loss dominant. Memory is a single-ended, parallel bus and not a differential serial bus. The data bits in the DDR memory are not aligned. There is also no loopback mode in DDR4, so there is no way to perform receiver test. In DDR5, the receiver would need to tune itself via the training mode to minimize bit error rate. The specification describes what happens at the ball of the device. If you have an equalizer, the specification is written at the output of the equalizer, which is inside the DRAM die. You don’t know if you have an open eye in the die because you can’t probe inside the die directly. Hence, there is no eye mask specification at the ball. The eye would need to be opened using a decision feedback equalizer. The specification would model the impulse response of the channel. A decision feedback equalizer would back away impulse response effects of the channel, which would generate inter-symbol interference. These DDR5 measurements can be made using an oscilloscope with BER contour extrapolation. A loopback signal inside the memory would be required to figure out the receiver mask the memory would require.

 

Putting it All Together

Key components are required to ensure success of implementing DDR5 memory designs. Test equipment like oscilloscopes and bit error rate testers could help with some of the measurement challenges. Being able to make the measurements accurately would require advanced BGA probing capabilities, as the DRAM package is a BGA component. Parasitic loading is a big challenge to overcome when probing these high-speed buses. Early design stages could benefit from using modern simulation tools to model and optimize the system design. Power measurements are also a challenge, and the specification itself is not clear on how much noise is allowed in the power rail.

 

In summary, understanding the technology and specification helps you explore the best options for your design implementation. DDR4 memory is the first technology to adopt the receiver input mask concept. Understanding this concept allows you to quickly make meaningful measurements to ensure that the design works. DDR5 memory is the most advanced memory technology in its class. Measurement challenges associated with this new technology can be addressed using high-speed serial standard measurement techniques like equalization. A well-thought-out approach to the measurement requirement will enable high confidence in your memory system design.

Power Quality Determines the Performance of Your Device

Your product’s functional reliability is directly proportional to the quality of the DC power inside your product. Intuitively this makes sense: Stable DC supplies should not cause issues. Unstable DC supplies can cause unreliable performance.

 

In today’s products, IC density is increasing to provide more features faster. This means there are a larger number of smaller components packed onto each board, which makes your product more susceptible to the effects of poor power. To minimize the trouble power can cause, your design must convert and deliver DC power from the converters to the gates on the IC as effectively as possible. In other words, you want your design to have high power integrity, testing and verifying the integrity is crucial.

 

Tests Required to Validate Power Integrity

Evaluation usually consists of these four steps:

1. Analyze the output of your DC/DC converters without the rest of the circuit turned on.

  • This is to test the supply’s stability, looking for drift and PARD (Periodic and Random Disturbances).

2. Turn on your system and stress the supply under various operating conditions.

  • For example, test static and dynamic load to check the response and high frequency switching while keeping an eye out for transients and noise.

3. If your system has different power saving modes, you’ll evaluate your programmable power rails.

  • You want to ensure your supplies are reaching their intended level with the appropriate latency.

4. Lastly, run some (or all) of these tests again in a temperature chamber or accelerated life tester.

  • It is important to check operation in extreme environmental conditions and how your device will perform over time.

 

The Challenges of Making Power Integrity Measurements

For all the tests described above, you have a specific tolerance band. If the AC signals riding on your DC signal deviates too much, you have poor power integrity and your design is flawed. 

 

There are two major challenges to measuring your power integrity: noise and offset.

 

Noise

Noise from your oscilloscope, probe, and the connection to the DUT, are mixed in to your signal when you measure it. The result is that you don’t see an exact version of your signal on the oscilloscope screen. In light of this, make sure you are using a high-quality measurement system.

 

That means:

  • Choose an oscilloscope with low noise.
  • Choose a probe with low noise and 1:1 attenuation.
  • Connect to your DUT using as short of a lead as possible, with minimal to no probe-tip accessories.

 

Following these guidelines ensures you won’t mistake measurement system noise for power rail noise.

 

Offset

Viewing your AC swing can be difficult when your DC signal is large. To see the full signal on screen, you have to zoom out really far, but then you aren’t looking closely at the AC details. So, what do you do? Use a probe with support for power rail voltages. This is a probe with enough offset to be able to center the signal on screen without blocking DC so you can zoom in on the details of your waveform. What about a DC block, you ask?

 

Probe offset is better than using a DC block because:

1. Blocking capacitors not only block DC, they also block or filter low frequency AC.

 

  • This inhibits the ability to see drift, droop, sag, and other changes to the DC value of the power rail. These attributes are often critical to observe when your FPGAs and microprocessors turn on and off.

5V on a USB device measured with a DC block

Figure 1. 5V on a USB device measured with a DC block.

 

  • Probe offset passes all the AC content to the oscilloscope unfiltered.

5V on a USB device measured with probe offset

Figure 2. 5V on a USB device measured with probe offset.

 

  • In Figure 1, you can see the DC block shows what looks like a stable DC supply. In reality, the supply has some issues that become visible using the power rail probe in Figure 2. The issues can’t be seen with the DC block because it filters out the low frequency drift in the supply.

 

2. When using a DC block, the capacitor can discharge into your oscilloscope and blow out its front end. This is because the power rail you are measuring may exceed the input voltage of the oscilloscope, and the capacitor is being charged with that voltage. You may think you are protecting the oscilloscope from the voltage of your device, but if the capacitor discharges, all that energy will be sent into the front-end of your oscilloscope. This could be a costly repair.

 

3. DC blocks can make documentation of results tedious.

 

  • A DC block blocks all DC information from arriving to your oscilloscope. As a result, the oscilloscope will show the waveform centered at zero volts. Therefore, you need to use a DMM (digital multimeter) to see what the nominal value of the supply is and then manually type this information into any saved data or screen shots. Using a probe with offset means the oscilloscope knows the DC offset and can display things correctly, which makes record keeping easier. The DC offset is considered in any automated measurements or applications.

 

Additional Challenges – Loading and Bandwidth

 

Probe loading can cause your power supply to behave differently than it does without the probe connected or cause measurement errors like sag. So, you’ll also want to use a probe with very low loading.

 

You also want to choose a probe with high bandwidth. As I mentioned in the introduction, devices are now trying to do more at faster speeds. These increased speeds can introduce crosstalk on boards with small dimensions and lanes close together. And with the risk of crosstalk occurring, you’ll need to see transients, which requires high bandwidth. Having more bandwidth is also helpful for viewing high frequency supply noise, which can cause electromagnetic interference.

 

The Right Probe for Power Rail Measurements

Here is a summary of the tips provided above for overcoming power integrity measurement challenges:

 

Use a probe with:

1. Low noise

2. Support for popular rail voltages

3. Low loading

4. High bandwidth

 

If you need a specific product suggestion, use the Keysight N7020A or N7024A (New!) power rail probes.  They both meet the criteria suggested above and summarized below.

 

1. Low noise

  • The N7020A adds only 10% of the oscilloscope noise.
  • The N7024A adds only 30% of the oscilloscope noise.

2. Support for popular rail voltages

  • The N7020A has an offset range of ±24V.
  • The N7024A has an offset range of ±15.25V.

3. Low loading

  • The N7020A has an offset range of ±24V.
  • The N7024A has an offset range of ±15.25V.

4. High bandwidth

  • The N7020A has 2 GHz of bandwidth.
  • The N7024A has 6 GHz of bandwidth.

 

Power rail probe

 

Both probes work with Keysight Infiniium oscilloscopes, which have amazing signal integrity, low noise, and plenty of bandwidth. Additionally, they are compatible with special probing tips that help probe common surface mount capacitors packages.

 

Attribute

N7020A

N7024A

Probe bandwidth (-3dB)

2 GHz

6 GHz

Attenuation ratio

1.1:1

1.3:1

Offset range

± 24V

±15.25V

Input impedance at DC

50kΩ +/-2%

50kΩ +/-2%

Probe noise

0.1 * scope noise

0.3 * scope noise

Active signal range

± 850mV about offset voltage

± 600mV about offset voltage

Probe type

Single-ended

Single-ended

Included accessories

(orderable separately)

N7021A - Coaxial pigtail probe head (qty 3): 8”

N7022A - Main cable: 48”

N7023A – 350 MHz browser: 45”

Compatible, not included

N7032A 4 GHz browser for 0603 and 0805 packages (inch code)

N7033A 5 GHz browser for 0201 and 0402 packages (inch code)

1250-4403 Rotating SMA adapter

Output impedance

50Ω

50Ω

Extended temperature range

N7021A main cable, N7022A pigtail probe head: -40° to + 85° C

The diode is a crucial component to master if you want to grow your electronics prowess. So, it’s crucial to have a solid understanding of how diodes behave under varying loads. Today we’re going to look at some diode fundamentals, then take a look at a video covering 4 ½ practical uses for a diode.

 

Download the "6 Essentials for Getting the Most Out of Your Oscilloscope" eBook.

 

What are Diodes?

Diodes are relatively simple but more complicated than many basic passive components you probably already know. Diodes are non-linear components. Resistors, capacitors, and inductors are linear devices, meaning they can be characterized using first order differential equations. As much as I’d like to geek out on the math, I’ll spare you. That’s what Wikipedia is for. So, what are diodes?

 

Diodes are nonlinear devices. They don’t follow Ohm’s law, and for circuit analysis, you can’t replace them with a Thevenin equivalent.

 

Diodes are passive devices, which means they don’t need power to function.

 

Diodes are two-port devices. There’s a positive input, which is known as the anode, and there’s a negative output, which is known the cathode (Figure 1).

 

A diode’s circuit symbol. The anode is on the left, and the cathode is on the right.

Figure 1. A diode’s circuit symbol. The anode is on the left, and the cathode is on the right.

 

Diodes may be simple, but they are extremely useful because of their V-I curve, shown in Figure 2. The X-axis is voltage, and the Y-axis tells you how much current can flow through the diode at that voltage level.

 

A diode’s IV curve.

Figure 2. A diode’s IV curve.

 

Let’s take a closer look at the V-I curve. What does it mean? More importantly, how can you use it to your advantage?

 

Positively Biased Diodes

In Figure 2, there are a few things worth noticing. Let’s start with positively biased diodes. At moderate positive voltages, a diode basically acts as a short. However, there is a small voltage drop, usually called the “forward voltage drop.” It’s also known as the “cut-in voltage” or simply “on voltage.”

 

You can see the forward voltage drop on the V-I curve in Figure 2 around .6/.7 V, where the current spikes. A .6/.7V drop is standard for silicon diodes, but for other diode materials, the forward voltage drop will vary.

 

You can measure the forward voltage for a specific diode using a multimeter with a diode testing capability. You can see that this silicon diode has a forward voltage of roughly .62V (Figure 3).

 

Using a Keysight U1282A multimeter to measure the forward voltage drop of a diode.

Figure 3. Using a Keysight U1282A multimeter to measure the forward voltage drop of a diode.

 

But what you need to remember is that, when exposed to a moderate voltage – say 5V, a diode will pass through 5V minus the forward voltage. So, 4.3V for a standard silicon diode. There are some methods for compensating for this drop, but that’s beyond the scope of this article.

 

Negatively Biased Diodes

Let’s now move to the left side of the Y-axis in Figure 2. When exposed to a negative voltage, there will be a nano-amp reverse current. You can generally approximate it as 0A in most situations. That is, until you get to the other big swing on the VI curve, known as the breakdown voltage.

 

If your diode is exposed to a high level of reverse bias, you blew it. Often literally. Diodes essentially can’t hold up to that level of negative voltage, and the device physically breaks down, allowing negative current flow.

 

Long story short, you can essentially think of a diode as a one-way conductor with a voltage drop. Enough preface, let’s look at a few different ways you can use diodes for your circuits.

 

How to Use a Diode in Your Designs

Now that you know how a diode works, how can you use one in your designs? Check out this video for 4 ½ practical uses for a diode:

 

 

Diodes are Great!

Diodes are extremely useful components, and if you’re working with electronics, you need to build a solid knowledge of how diodes work. The video covered a few ways to use diodes, but that’s just the beginning! How are you using diodes in your designs? Let me know in the comments here or on the Keysight Labs YouTube channel.

 

Did you know that when you probe your DUT, the probe becomes part of the circuit? All probes have a loading effect on your circuit to some extent. These effects can manifest into overshoot, ringing, slow rise/fall times, propagation delays and DC offset problems. In addition, the loading impacts vary as you probe from DC voltages through high frequency ranges. Over this large frequency span, your probe impedance can vary greatly.

 

RCRC vs RC Impedance Characteristics

The capacitive and inductive components in a probe are what causes loading on your device. The traditional model off a probe’s looks more like the read trace in figure 1. However, newly developed high-end and high frequency active probes two knee or crossover points (RCRC) and provide different loading responses. Understanding the probe’s input impedance characteristics over frequency enables you to make the best probe selection for the circuits you are testing.

 

Let’s look at some probe input impedance vs frequency curves in Figure 1 below to understand the impact to your measurements. A probe’s input impedance is shown on the vertical axis and frequency is shown on the horizontal axis. Both RC and RCRC probe curves are shown.


The red trace is a typical RC probe response over frequency. Note that from DC to around 10 MHz, the RC probe holds steady at a 50 K? of differential impedance. Higher than 10 MHz, the RC probe’s capacitive reactance comes into play at 210 fF, and the probe impedance continues to decrease as the frequency increases. This is what is called an RC input impedance profile of most conventional probes on the market.

Lower impedance will have accumulative loading impacts on the circuit you are probing.

 

The blue trace is an RCRC probe’s response. Notice from DC to 10 Hz the inductance is at 100 K? and then falls to 1K? from 10 Hz to 10 KHz. The 1 K? inductance will load your circuit more than the RC probe’s 50 K? in this frequency band, but past 10 MHz, the RC loading will be much worse because the RC probes impedance decreases rapidly driven by the capacitance of the probe. The RCRC holds this 1 K? impedance from 10 KHz to around 1 GHz. Past that, the capacitive reactance at 32 fF starts to come into play, reducing the 1 K? impedance further. So, you can see at higher frequencies, above several hundred MHz, the RCRC probe proves to be the better choice because it will decrease loading effects at higher frequencies.

 

input impedance vs frequency of probes
Figure 1: Input Impedance vs Frequency of modern high-performance probes

Red = RC probe example
Blue = RCRC probe example
Pink = RCRC probe example

 

The pink trace is another RCRC probe’s response for additional comparisons. Note that from DC to around 100 MHz the impedance is 100 K?. But from 100 KHz to 10 MHz the probe’s 110 pF capacitive reduces the inductance to 450 ?s. This change in impedance results in a significant amount of additional loading relative to the initial 100 K?s at lower frequencies. And then at 100 MHz and above, the probe’s 65 fF capacitance reduces the impedance further.

 

To summarize the curves in Figure 1, your probe selection for the lowest circuit loading should be:

  • RC probes Higher input impedance for lower loading at mid band (kHz to GHz)
  • RCRC probes – Higher input impedance for lower loading at higher bands (>GHz)

 

Applications tips for each probe type

Use an RCRC probe for:

  • Accurate high frequency content above GHz due to low loading
  • High speed signals with low source impedance, such as a 50 ohm transmission line
  • Reproducing wave shapes with fast edge speeds

 

Use an RC probe for:

  • Mid-band frequencies due to low loading
  • Buses that transition to a “high Z” state such as DDR and MIPI signals
  • Signal sources with high impedance
  • Signals with long time constants

 

Conclusion

A common misconception is that a higher priced, higher bandwidth probe can more effectively measure signals across all bandwidths. However, this is not the case. The best probe for your application will be dependent on what frequencies you are working with. Always factor in the probe loading effects on your measurement.

 

Are you falling into oscilloscope probing pitfalls? Avoid making the same mistakes as others with the Oscilloscope Probing Pitfalls eBook.

The terms “bit error rate” and “bit error ratio” are used interchangeably on many websites and publications. However, the definitions are very different. Understanding the difference will help you effectively analyze your system’s performance.

 

What is a BERT?

To find the bit error rate or bit error ratio of your system, you need a Bit Error Rate Tester or Bit Error Ratio Tester (BERT). BERT refers to a class of test equipment; depending upon the manufacturer or distributor, BERT stands for Bit Error Rate Tester or Bit Error Ratio Tester. A BERT tests the complete transmitter/receiver system for any data loss. It transmits data into a system, and then measures how well a system transmits and receives the data. To do this, a BERT requires a pattern generator and error detector.

 

What is the difference?

There is a very clear difference between the error ratio and rate. Understanding that difference is important to assess your system performance.

 

Bit error ratio (BER) is the number of bit errors divided by the total number of bits transferred during a specific time interval.

Bit error rate (also BER) is the number of bit errors per unit time.

 

Essentially, the bit error rate refers to errors with respect to time, and the bit error ratio refers to errors with respect to the quantity of transferred bits.

 

The bit error ratio is a unitless performance calculation and is expressed as a percentage. It is an estimate of the bit error probability which is the expected value of the bit error ratio. This estimate is more accurate over a longer time interval and when capturing a high number of bit errors.

 

bit error ratio

Why it’s important differentiate?

It’s important to differentiate between bit error rates and bit error ratios. If your BERT pattern generator sends 100 bits to your device under test and your BERT error detector sees 10 errors, the bit error ratio is 10 percent.

 

bit error ratio

The bit error rate is the bit error ratio multiplied by the bit rate. For example, if your BERT pattern generator sends bits to your unit under test at a rate of 100 bits/second and your BERT error detector sees 10 errors every 100 bits, the bit error rate would be the bit error rate equals 10 bits/second.

 

bit error rate

 

The bit error rate is used more often because it tells you how long it will take to encounter an error. For example, using the calculated 0.1 bit error ratio above tells you the ratio between errors received and number of data bits sent.

 

But what does knowing the bit error ratio really tell you about your system performance? Not much - you need to know your data rate. If your system data rate was 1 bit per week, then your system calculated bit error rate would only be one error in 10 weeks. Another example would be if your system data rate was 100 G bits/second, then your bit error rate would be 10,000,000,000 errors every second!

 

Summary

The bit error ratio is the number of bit errors divided by the total number of bits transferred during a specific time interval. Bit error rate is the number of bit errors per unit time. The bit error rate gives you an indication of your system’s performance relative to bits transferred vs bits received. Visit Keysight.com to learn more about Keysight’s bit error ratio test options.

You just bought a high frequency single-ended and differential probe and are feeling good about your measurements. But when using the two probes side-by-side, you notice differences in vertical voltage measurements and rising edge times. This can cause you to question the accuracy of your measurements relative to what your device under test (DUT) signals should look like. Don’t worry, you’re not alone here. Let’s walk through the causes of these differences and the steps needed to correct them.

 

The Problem

The signal path from the DUT to the oscilloscope can create distortions in your signal and result in amplitude and timing differences, which can cause errors in your design. A typical signal path is shown in Figure 1 below. The original signal is what is on your DUT before probing. When you probe your DUT, the probe may add distortions to your signal due to probe capacitance, inductance, or slight impedance mismatches. See my other probing blogs for these effects and how to avoid them. The signal on the probe tip is amplified and sent to the oscilloscope input. The oscilloscope then converts this analog signal to a digitized version using digital signal processing (DSP).

 

Oscilloscope Probing: Simplified signal path.

Figure 1. Simplified signal path.

 

Incorrect skew and amplitude measurements can cause problems with your measurements. Skew is defined as the difference in bit timing relative to the same point on two waveforms. Along with these pesky timing differences, the amplitude differences can cause your DUT’s eye patterns to look more closed than they really are. This may make it seem like there is a lot jitter or noise in your DUT, when in reality it is coming from the signal path. Not seeing a true representation of your DUT’s signals on the oscilloscope can make it difficult (if not impossible) for you to debug or complete your design work.

 

Probe Calibration

Probe calibration is sometimes forgotten, but this is an important step to ensure the most accurate measurement possible. Both single-ended and differential probes come with a calibration certificate. These certificates will give you confidence that the probes will meet their stated specifications. However, the probe and oscilloscope are a measurement system, which means each probe’s output and each oscilloscope’s input characteristics will vary. The coupling needed in the signal path above creates three variabilities: (1) coupling from your DUT to the probe, (2) coupling from the probe output to the oscilloscope input, and (3) coupling from the oscilloscope input through its DSP circuitry. Let’s take some of this variability out of the system by calibrating your probe to improve amplitude and skew.

 

Keysight oscilloscopes can recognize Keysight active probes and give you a message on the screen if your probe is not calibrated to your oscilloscope. Figure 2 below shows a 50 MHz signal from a signal generator before probing, using our E2655C deskew fixture (yellow trace). The signal is also sent through a differential probe and shown on Channel 2 of the oscilloscope (green trace). The generator output on Channel 1 is 1.04 Vp-p (volts peak-to-peak), and the probed signal on Channel 2 is 965 mV (millivolts). In addition, the skew from Channel 1 to Channel 2 is massive (around 3 ms), which means the rise times do not line up at all.

 
Oscilloscope Probing: Generator output and probed signal.

 

Figure 2. Generator output and probed signal.

 

Click on the Channel 2 green button; you will see the probe calibration button in the lower right corner (see Figure 3 below showing a Keysight S-Series screenshot). Click on this button and walk through the amplitude and skew calibrations in less than five seconds each. The oscilloscope will prompt you when each calibration is complete.

 

Oscilloscope Probing: Channel 2 probe calibration selection.

Figure 3. Channel 2 probe calibration selection.

 

Note the calibration results in Figure 4 below. This screenshot is after amplitude and skew calibration. The amplitude is now improved to 972 mVp-p, and the skew has been corrected with both rise times lining up.

 
Oscilloscope Probing: After amplitude and skew calibration.

 

Figure 4. After amplitude and skew calibration.

 

Conclusion

The system links between your DUT, probe, and oscilloscope can cause errors in your measurements relative to what signals are really on your DUT. Always calibrate your probe with the oscilloscope you are using for the best fidelity in your measured signal.

 

Are you falling into oscilloscope probing pitfalls? Avoid making the same mistakes as others with the Oscilloscope Probing Pitfalls eBook.

Many people think that differential probes are made to only probe differential signals. Did you know you can also probe single-ended signals with your differential probe? Today we’ll learn how to maximize the usage of your differential probe and get the best signal fidelity possible. You will need to examine the performance and usability trade-offs between your differential and active probes to make this decision. The simplified models of each probe are shown in Figure 1 below and will be referenced throughout this blog.

 

Differential Probes - Simplified models of single-ended and differential probes.

Figure 1. Simplified models of single-ended and differential probes.

 

Signal Fidelity

Because the physical geometries of the two probes are similar, the values of the capacitors and inductors will also be similar. The ground connection of the single-ended active probe (lg) is sensitive to the length the ground accessory used in the measurement. Since you are not measuring relative to ground with a differential probe (you are measuring relative to another voltage), you are not making an additional LC circuit with a ground lead, so you see much less inductance with a differential probe.

 

Note that the differential probe tips connecting to the DUT (device under test) have a tip resistor on both tips (Zp and Zm). Whereas the single-ended probe has a tip resistor (Zs) on the signal connection and none the ground connection. These tip resistors damp the resonance caused by the probe input inductors and capacitors (ls, lg, and cs in the single-ended probe and lp, lm, cp, and cm in the differential probe).

 

There is a lack of damping on single-ended probe ground connections. Because of this, a voltage can develop between your probe’s ground and your DUT’s ground. This happens at higher frequencies and reduces the input to your single-ended probes amplifier input, thereby reducing your probe’s output. This disadvantage of your single-ended probe can be minimized by keeping your ground lead length as short as possible. 

 

Common Mode Rejection

Unwanted common mode signals can also affect the signal you see on screen. They can be caused by electromagnetic interference, cross talk, and other noise characteristics on or around your DUT. Your probe needs to be able to reject common mode noise in order to give you the most accurate signal. The amount a probe can reject is represented by a ratio: common mode rejection ratio CMRR. But this rejection is only relative to the signals that appear simultaneously and in-phase on both probes’ inputs. You can see in Figure 2 below that the single-ended probe (green line) has less rejection over the frequency band measured than the differential probe (dotted red line). Around 1.5 GHz, the difference is close to 20 dB. This is significant since the common mode (noise) will be amplified by the single-ended probe, causing considerably more noise on its output relative to the differential probe.

 

Differential Probes - CMRR of the single-ended vs. the differential probe.

Figure 2. CMRR of the single-ended vs. the differential probe.

 

Measurement Comparisons

In Figure 3 and Figure 4 below you can see:

  • The input signal to the oscilloscope is show in yellow on all the following figures (un-probed and fed directly into Channel 1 of the oscilloscope). We will call this signal Vin.
  • Figure 1 below shows the single-ended probe measurement of Vin on Channel 2 of the oscilloscope (blue trace).
  • Figure 2 below shows the differential probe measurement of Vin on Channel 2 of the oscilloscope (red trace). Note: one side of the differential probe is connected to Vin and the other is connected to the same ground as the single-ended probe.

 

Single-ended measurement. Differential measurement.

    Figure 3. Single-ended measurement.                             Figure 4. Differential measurement.

 

Notice that the two probe measurements look like they carry the same amount of noise relative to the un-probed signal in yellow. But let’s look at little closer and change the volts per division on the oscilloscope from 200 mV/div (millivolts per division) to 20 mV/div. You can now see in Figure 5 below the single-ended measured signal in blue, and in Figure 6 you can see the differential measured signal in red. Notice that the single-ended measurement in blue has much more noise than the differential measurement in red due to less common mode correction by the single-ended probe.

 

 

 Single-ended measurement. Differential measurement.

     Figure 5. Single-ended measurement.                             Figure 6. Differential measurement.

 

Conclusion

Differential probes can make the same types of measurements that single-ended probes can perform. However, due to the common mode rejection on both inputs of the differential probe, the differential measurements can have significantly less noise. The common mode rejection in the differential probe reduces the amount of noise that goes into its amplifier, resulting in much less noise displayed on the oscilloscope. This allows you to see a better representation of your DUT’s signals and not be misled by random probe noise.

 

Are you falling into oscilloscope probing pitfalls? Avoid making the same mistakes as others with the Oscilloscope Probing Pitfalls eBook.

Did you know that when you probe a circuit, you change the electrical characteristics of that circuit? Oscilloscope probes add resistive, capacitive, and inductive loads to your circuit. These loading affects can change the operation of your circuit under test. Understanding these loading impacts helps you avoid selecting the wrong probe for your specific circuit or system.

 

Figure 1 below shows a circuit under test and the electrical model of a probe connected to it. In a perfect world, Vin (voltage at the input of the probe) should be the same as the Vsource (voltage of your circuit before it is probed). But because of probe loading effects, the impedance of your circuit and probe determines the voltage at the input of the probe. It is a simple resistor divider circuit. Increases in frequency can also become a major source of loading because the probe’s capacitive reactance gets smaller. This loading alters not only the amplitude but also the shape of your original circuit waveform.

 

Oscilloscope Probe Electrical Circuit

Figure 1: The probe’s electrical circuit

 

When the probe is connected to the circuit, the impedance matching of the circuit and probe determines the voltage at the input of the probe.

 


Capacitive Loading

Capacitive loading can be the main culprit of your measurement errors. For general-purpose measurements less than 500 MHz, passive 1:1 and 10:1 high-impedance resistor divider probes are good choices. These passive probes begin to impose heavier capacitive loading as the frequency of the signal being measured increases. As the frequency of the signal goes up, the probe impedance drops and can load your circuit. High-impedance passive probes are a great choice for general-purpose debugging and troubleshooting on most analog or digital circuits below 500 MHz.

 

High-impedance active probes are the best selection below 500 MHz.


Inductive Loading

It is critical to remember that your probe’s impedance is not constant over frequency. Most of the inductance is created from the ground lead you chose for your probe. At DC and low frequency ranges, the probe’s impedance starts out at the rated impedance, but as the frequency goes up, inductance comes into play. The result is higher frequency ringing on your rising edge and across the top of your waveform. Figure 2 below shows the four different types of ground lead solutions’ stepped responses of a Keysight N2796A 2 GHz active probe. The three grounding solutions below decrease in inductance starting with the highest inductance in Case 1 to the lowest inductance solution shown in Case 4. Notice that the Case 4 black line solution has the least amount of overshoot and ringing.

 

Active oscilloscope probe step response

Figure 2. An active probe’s stepped response with different accessories.

 

Resistive Loading

Resistive loading is the least likely to induce nonlinear or low amplitude behavior in your circuit. Your circuit’s output resistance and the probe’s own resistance form a voltage divider circuit. This divider circuit distorts the signal being measured because the probe is seen as a load to the circuit under test.

 

1:1 passive probes can cause resistive loading of you circuit under test above 500 MHz.

 

Passive and Active Probes

The higher the passive probe’s attenuation ratio, the lower the capacitive loading will be. 1:1 passive probes have capacitive loading around 100 pf, while a 10:1 probe is around 10 pf. But there is a tradeoff here. 1:1 probes transfer lower noise levels to the oscilloscope. 10:1 passive probes get both their signal and noise amplified by 10x because the oscilloscope accounts for the fact that the probe output is one tenth of the actual measured signal. 

 

10:1 passive probes increase the noise level on the oscilloscope because both the signal and noise floor are amplified by the oscilloscope.

 

Active probes are another way to reduce probe loading. They have around one tenth the input capacitance of passive probes. Active probes can achieve this lower tip capacitance due to the active circuit at the tip of the probe. See these active vs. passive probe relationships below in Table 1 to aid you in your probe selection.

 

Active probesPassive probes
FeaturesFeatures
Low loadingHigher resistance
High bandwidthHigh dynamic range
High bandwidthRugged
Least intrusiveLow cost
TradeoffsTradeoffs
Higher costBandwidth limited to 500 MHz
Limited input dynamic rangeHeavy capacitive loading

Table 1. Passive vs. active probe selection.

 

For faster frequency of rise time signals, use active probes with lower capacitive loading.

 

 

Higher-End Probes

Higher-end oscilloscopes use digital signal processing to help compensate for probe loading but do not eliminate probe loading altogether. To minimize loading, you need to factor your design parameters with the impedance values of the probe you are using.

 

Conclusion

All probes have some type of impact on your circuit under test. It is up to you to determine what is most important for your tests. Understanding some of the common pitfalls helps you select the right probe. A probe draws a portion of the circuit energy and supplies this energy to the oscilloscope. All probes present a capacitive, resistive, and inductive loading element to your circuit. In order to avoid using a probe that adversely impacts your circuit and changes the signal from its original state, you need to factor in the probe’s resistive, capacitive, and inductive characteristics with the properties of your design.

 

Are you falling into oscilloscope probing pitfalls? Avoid making the same mistakes as others with the Oscilloscope Probing Pitfalls eBook.

 

Probe impedance changes with frequency –
The bigger the probe resistance and smaller the probe capacitance, the less the loading your probe will have.

Bandwidth is one of the key oscilloscope probe banner specifications, and if you pick the wrong bandwidth, you’ll get inaccurate measurements. However, picking the right bandwidth probe isn’t as simple as you might think. Avoid making these common mistakes when selecting the right probe bandwidth, and have confidence that the signal you measure is the best representation of your device under test (DUT).

 

Probe Bandwidth

Probe bandwidth is a point on the frequency response curve where the amplitude has decreased by 3 dB. This 3 dB roll-off is shown below in Figure 1.

 

Diagram showing 3dB roll-off point on probe bandwidth

 

Figure 1. Probe 3 dB roll-off point.

 

For example, measuring a 500 MHz, 1 V peak-to-peak sine wave using a 500 MHz probe results in a measurement error of –3 dB or decrease by –0.3 V peak-to-peak. This 3 dB roll-off diminishes your actual 1 Volt peak-to-peak signal to only 0.7 V peak-to-peak. You can fix this pitfall by calculating the correct bandwidth for your desired frequency. A common rule of thumb is that your probe bandwidth should be three times the sine wave frequency you wish to measure. So, to measure a 500 MHz sine wave, you need to choose a probe with 1.5 GHz or more bandwidth.

 

Rise Time and Bandwidth are Related

Now let’s dive a little deeper into this theory. You need to know more than just your signal frequency. To calculate a more accurate probe bandwidth, you need to know your signal’s rise time. Rise time is the time it takes your signal to get from the 10% level to the 90% level of a rising edge.

 

Diagram showing 10 percent and 90 percent rise time points

Figure 2. 10% and 90% rise time points.

 

Let’s use the universally accepted formula that states: bandwidth times the rise time equals 0.35 when evaluating a rising edge from 10% to 90%.

Or you can configure the same formula another way:

If you are working with a communication standard, often the rise time specification is listed along with other banner specs. For example, if your rise time for the 500 MHz clock signal is 350 ps (pico seconds), use the formula below to calculate the probe bandwidth you need.

The calculated bandwidth now equals 1 GHz. This means the real bandwidth of your signal is 1 GHz, not 500 MHz. 500 MHz is your clock frequency for an entire cycle, but your clock’s rising edge is much faster at 1 GHz. You need a probe with a bandwidth specification of 3 times your calculated 1 GHz bandwidth, or a 3 GHz probe. By doing this, you avoid the pitfall of attenuating any of your signal’s frequency components.

 

The Harmonic Nature of Square Waves

You have been calculating your bandwidth needs based on measuring a sine wave. Let’s transition into the pitfalls associated with measuring square waves. If you remember your schooling on Fourier series, you recall that it is a way to represent a function as the sum of sine waves. In Figure 3 below, an original signal is shown in yellow and is made from its fundamental harmonics. Note that the first harmonic in green has the same period and duty cycle, but its rising edges are slower, and the corners are more rounded. The first and third harmonics combined in purple have faster rising edges, and the corners are crisper. The first, third, and fifth harmonics combined in pink have faster edges, crisper corners, and detail on the top and bottom.

 

Square wave signals with harmonics below it

Figure 3. Square wave signal with its harmonics below it.

 

Now let’s take this harmonic knowledge a little farther and use a 100 MHz square wave clock example. Figure 4 below shows the result of measuring the 100 MHz square wave with a 100 MHz probe. The harmonics of the 100 MHz signal are well below the 3 dB point of the probe, resulting in what looks like a sine wave. Any measurements made with this signal will be inaccurate.

 

Distorted square wave signals due to wrong bandwidth probe selection

Figure 4. Distorted square wave due to the wrong bandwidth probe selection.

 

Now let’s measure the same 100 MHz square wave with a 500 MHz probe. The resulting crisp square wave is shown in Figure 5 below because the critical harmonics of the 100 MHz signal are captured in the measurement. You can see how much difference it makes when you use a probe with the appropriate bandwidth.

 

Square wave with the correct bandwidth probe selection

Figure 5. Square wave with the correct bandwidth probe selection.

 

System Bandwidth, The Weakest Link

It is also worth noting that you should consider your entire system bandwidth. You need to factor in both the bandwidth of your probe and your oscilloscope to determine the bandwidth of your probing system (probe + scope). See the formula for your probing system bandwidth below.

 

Let’s say both your oscilloscope and probe bandwidths are 500 MHz. Using the formula above, the system bandwidth would be 353 MHz. You can see that the system bandwidth degrades greatly from the two individual bandwidth specifications of the probe and oscilloscope. Now, let’s say that the probe bandwidth is 300 MHz and the oscilloscope bandwidth is still 500 MHz. Using the above formula, the system bandwidth reduces further to 257 MHz. You can see that the total system bandwidth is always lower than your weakest link or lowest system component bandwidth.

 

Accessory Choice

There is always a trade-off between measurement flexibility, usability, and resulting bandwidth. Many probe tip accessory solutions are available, but all have an impact on your bandwidth. The added inductance of longer ground leads brings with it stray capacitance, creating RC circuits that resonate within your measured bandwidth. These circuits reduce your bandwidth and the ripple on the DUT’s signals. For example, users can use longer lead solutions for quick checks to ensure the circuits are functioning. However, for critical measurements, lead length (both on the signal and ground side) should be kept to a minimum.

 

Measuring Your Probe’s Bandwidth

People often use a VNA (vector network analyzer) to measure an oscilloscope probe’s bandwidth, but VNA’s are expensive and require some training to operate them effectively. In addition, because the VNA is a 50 Ω system and passive probes should be terminated into 1 MΩ, the VNA is not a good test solution for passives.

 

Another way to test bandwidth is to use a sine wave source, splitter, and power meter to sweep the response. This method requires a remote interface such as GPIB or USB and programing skills in MATLAB.

 

An easier way to measure the bandwidth of probes with bandwidths below 1 GHz is the time domain approach, utilizing only an oscilloscope with a built-in step signal source, and ‘differentiate’ and ‘FFT’ functions. Apply a step function to your system, then apply the differentiate (or derivative) to this step response. You now have an impulse response and can take the FFT of the impulse response to obtain the system’s frequency response.

 

Conclusion

Choosing a probe with adequate bandwidth is crucial to making accurate measurements and good engineering decisions. Understanding the impact of rise times, harmonics, and system constraints can help you determine your bandwidth limitations.

 

Are you falling into oscilloscope probing pitfalls? Avoid making the same mistakes as others with the Oscilloscope Probing Pitfalls eBook.

Knowing key Arbitrary Waveform Generator (AWG) specifications lets you make the best purchase decision, saving you time and money. Learn how to compare these characteristics across arbitrary waveform generator types and vendors: memory, sample rate, dynamic range, and bandwidth. Let’s discuss these specifications in detail.

 

Memory Size

Memory size is the amount of memory available for storing long strings of user-defined waveforms. This specification is listed in giga samples (GSa). Data is fed into the digital-to-analog converter (DAC), which creates a voltage stair-stepped representation of you desired signal. High sample rates and large memories are needed to accurately create the defined signal.

 

Sample Rate

Sample rate is the number of samples the DAC can take in a given time interval. The specification is listed in giga samples per second (GSa/s). The sample rate determines the maximum frequency component of the arbitrary waveform generator output signal. Other industry terms for sample rate are “clock rate” and “sample access rate.” A key formula to understand the relationship between memory size and sample rate is shown below.

 

Memory / sample rate = play time

 

You can see from the formula above that as the sample rate increases, more memory is used and play time decreases. Play time determines the total length of the unique waveform an arbitrary waveform generator can generate. This play time length is also called time before repeating. For example, a memory size of 256 kSa and a sample rate of 64 GSa/s results in a play time of 4 micro seconds. This is not a very long pattern and is why large amounts of memory are needed for more play time.

 

Dynamic Range - Vertical Resolution (ADC Bits)

This is the output of the DAC, and it is expressed in voltage as vertical bits of resolution. The value is listed in decibels relative to carrier amplitude (dBc). For example, an 8-bit DAC can output two to the eighth bits of vertical resolution or 256 different voltage levels creating the desired waveform. When comparing arbitrary waveform generator ADC bits specification across different brands, it is important to know that for every ADC bit added, the vertical resolution is doubled.

 

Bandwidth

Arbitrary waveform generator outputs are limited to a specific upper-end output frequency. The bandwidth of the arbitrary waveform generator is the range of frequency outputs that it can reliably provide. The value is also called “data rate” and is listed in giga bits per second (Gb/s). Note that the bandwidth is determined by the sample rate, but it will not be a one-to-one correlation. Let’s walk through the reasons why this is the case:

  1. The DAC must accurately create the signal in memory and needs to create at least two data points per period. This is called the Nyquist Theory. So, a sample rate of 1 GHz results in a DAC output of 500 MHz or one half the sample rate.
  2. The DAC output signal is not a smooth sinewave but is a voltage stepped representation of the pattern in memory. Because of this, the DAC output needs to be filtered. Within the arbitrary waveform generator, this filtering is done by what is called a reconstruction filter, which creates a smooth sinewave. However, there is a price for this filtering in the form of an additional 10% loss to the sample-rate-to-AWG-output frequency ratio. You can reference the formula below. For example, the 1 GHz sample rate mentioned above results in an arbitrary waveform generator maximum output frequency of 400 MHz.

 

Maximum arbitrary waveform generator output frequency = sample rate x 40%

 

Spurious Free Dynamic Range (SFDR)

This is measured in the frequency domain and is the distance in dB from the selected frequency to the highest visible spur or harmonic within the stated bandwidth. The value is listed in decibels relative to the selected frequency amplitude. The screen shot in Figure 1 below shows an example of an AWG output frequency in the center of the screen with a spur to the left that is 94.54 dB lower than its amplitude.

 

 Arbitrary Waveform Generator (AWG): Center frequency relative to a spur.

 

Figure 1. Center frequency relative to a spur.

 

 

Effective Number of Bits (ENOB)

The effective number of bits is derived from the DAC bits. It is a lesser value than the DAC bits due to impacts of harmonics, spurious signals, and the AWG noise floor. Note that this specification changes over the bandwidth of the arbitrary waveform generator, and you should look at ENOB vs. frequency plots like the one below to see the value associated with your signal out frequency of choice. Note the plot in Figure 2 below references a 14-bit system. But after the impacts of receiver signal to noise and distortion (SINAD), you can see that at 1.5 GHz, the actual bit of resolution is deceased to around 9 bits. 

 

Arbitrary Waveform Generator (AWG): An ENOB curve over frequency.

 

Figure 2. An ENOB curve over frequency.

 

ENOB is a great specification to see the actual performance of the arbitrary waveform generator after factoring in these effects on signal quality. ENOB can be measured or calculated. (The formula is below.) Note that SINAD is the ratio of total signal power to unwanted signal noise.

Effective Number of Bits (ENOB) formula

Jitter

A waveform’s jitter can cause misalignment of edges and voltage levels. This can cause the AWG to inject data errors into your system. The jitter value is usually listed in ps peak-peak between the sync clock and the direct data output.

 

Conclusion

Know your key arbitrary waveform generator specifications to properly select the AWG that fits your application:

  • Memory, sample rate, and play time are interrelated.
  • Your bandwidth will not match your sample rate but will be 60% of that number.
  • ENOB is a better representation of resolution than ADC bits.
  • Factor in the jitter specification to account for true signal fidelity.

 

To learn more about arbitrary waveform generators, check out: A High-Performance AWG Primer

1. For better oscilloscope probing, an active probe is generally better than a passive probe in terms of wider bandwidth and _________.
A. Ruggedness
B. Cost
C. Voltage range
D. Probe loading

Answer: D Probe loading


2. When using a DC power supply, remote sensing is one of the great features. Remote sensing will help:
A. Increase power supply capacity
B. Increase resolution of power supply output setting
C. Compensate DC drop by testing lead load
D. Safety operation

Answer: C. Compensate DC drop by testing lead load


3. A true RMS multimeter is a better choice than an average-responding multimeter to measure (select all that apply):
A. Sine waves
B. Square waves
C. PWM signal
D. Rectifiers

Answer: B. Square waves, C. PWM signal, D. Rectifiers

Prove yourself as an engineer! The Schematic Challenge is the perfect opportunity to test your skills. On March 5, 6, and 7, we will be posting a new schematic or problem-solving challenge. If you, as a community, are able to answer questions 1, 2, and 3 correctly by Thursday, March 8 at 11:59 PM MST, we will add three 1000 X-Series oscilloscopes to the overall Wave 2018 giveaway! Answers should be posted in a comment on the #SchematicChallenge posts on the Keysight Bench or RF Facebook pages. Work with your family, friends, coworkers, or fellow engineers in the Wave community to solve these problems. If you haven’t already, be sure to register for Wave 2018 at wave.keysight.com.

 

Question 1:

By Ryan Carlino

 

Status: SOLVED! (minimum of 8 bits)

 

Week1 Question 1 SchematicYou need to design a circuit to determine the resistance of an unknown ID resistor.
A voltage divider provides a bias that creates a voltage at the input of an ADC.
You’d like to be able to distinguish between a 15K and 20K ID resistor.
The ADC has a 0.5% internal 3.3V reference. The resistors are all 1%.
What is the minimum number of bits of resolution that the ADC needs in order to have at least 10 codes (LSBs) between a 15K and 20K resistor?

 

Question 2:

By Ryan Gillespie

 

Status: SOLVED! ( V(d)=(0.72 - 0.13i) V )

 

Given the doubly terminated transmission line, calculate the voltage at d = 100 µm.

Hint: First you may want to solve for Zo, Wave Speed, Wavelength, V(x) and I(x)

Your answer should be in the form of V(d) = ( # + #i )  where # are the numerical answers.

 

 

 

Useful formulas:

 

Question 3:

By Patrick Mann

 

Status: SOLVED!

 

Given the block diagram in figure 1, is the additional explicit trigger input shown in blue in figure 2 required? Select the correct answer and post the respective letter on the Keysight Bench or RF Facebook pages:

 

  1. The explicit trigger is not required since the sampling oscilloscope can trigger off the data.
  2. The explicit trigger is required since the sampling oscilloscope cannot trigger off the data.
  3. The explicit trigger is not required because the precision waveform analyzer module can recover a clock and feed it to the sampling oscilloscope’s trigger circuitry.
  4. The explicit trigger is required because the precision waveform analyzer module cannot recover a clock and feed it to the sampling oscilloscope’s trigger circuitry.
  5. The explicit trigger is not required because the external time reference feeds into the sampling oscilloscope’s trigger circuitry.
  6. The explicit trigger is required because the external time reference feeds into the sampling oscilloscope’s trigger circuitry.

 

Figure 1: Precision Waveform Analyzer module (blue) and sampling oscilloscope mainframe block diagram (green)

 

Figure 2: Connection diagram of a sampling oscilloscope and module (left) to a pseudorandom binary sequence (PRBS) generator (right)

If your sample rate is not fast enough, you won’t be able to see your signal accurately on the oscilloscope screen. Sample rate is the number of samples an oscilloscope can acquire per second. This determines the resolution of your waveform. Read on to learn why.

 

The Basics

A sample is a single value at a point in time. You could think of a sample like one piece in a puzzle. The more pieces you assemble over time, the more apparent and complete the picture becomes. 

 

Oscilloscope sample rate: Puzzle

 

But unlike a puzzle, reconstructing a waveform on an oscilloscope is not solely dependent on the number of samples that are strung together. The speed at which you sample matters too. A puzzle is a static picture. Therefore, it doesn’t matter how long it takes you to assemble a puzzle – the result will still be a complete picture in the end. However, electric waveforms change with time. So, to get a complete picture of the waveform, we need to sample fast enough to capture it. That is why we talk about the specification in terms of a rate. We need a fast sample rate to properly display our device’s signals on our test equipment.

 

We know from Harry Nyquist that we need to take equally spaced samples of a signal at at least twice the rate of the signal’s highest frequency component to represent that signal without errors. 

 

Fsampling 2Fsignal

 

This definition is given as a minimum requirement for proper sampling. You want your oscilloscope to provide more than just a minimum requirement.

 

Oscilloscope Sampling

There are two key oscilloscope specs that determine if your signal will be displayed properly on screen: bandwidth and sample rate. In my previous blog “What is Bandwidth and How Much Do You Need,” we discussed the importance of bandwidth. From that blog you’ll know that without enough bandwidth, you’ll have an attenuated and distorted signal. But, it’s also important to know that without enough sample rate, you will be without all the waveform information that is necessary to display the frequency of your signal, exact rise and fall times, the height and shape of your signal, and any glitch or anomaly that may be occurring.

 

When you probe your device and connect it to an oscilloscope, you are sending an analog signal into the oscilloscope. Then, the scope samples and digitizes the signal, saves it in memory, and displays it on screen. 

 

Oscilloscope sample rate: Simplified block diagram of signal flow from a DUT through an oscilloscope.

Figure 1. Simplified block diagram of signal flow from a DUT through an oscilloscope.

 

The default sampling setting on your Keysight oscilloscope is automatic in real-time sampling mode. Automatic sampling will select the sample rate for you. The scope will choose the highest sampling rate possible, using as much memory as necessary to fill the display with your waveform information. In real-time sampling mode, all the samples of the waveform are taken from one trigger event and are evenly spaced in time. (If you aren’t familiar with the term trigger, that is basically the event that time-correlates your device’s waveform within the oscilloscope, allowing the waveform to be steadily displayed on screen.) The scope may also apply interpolation to fill in gaps between samples. 

 

If you don’t want the oscilloscope to select the sample rate for you, most oscilloscopes allow you to set the sample rate yourself. If you set the sample rate yourself, remember: two times the frequency is the absolute minimum rate you should use. When it comes to oscilloscopes, I recommend choosing a sample rate faster than this. Usually choosing a sample rate that is 3 to 5 times the bandwidth of the oscilloscope will give you a high-enough sampling rate to capture the details of your signal, including its frequency of oscillation and the rise times of your waveforms. You need a sample rate that will provide enough detail to see any unexpected glitches or anomalies.

 

The more samples you have in each period, the more signal detail you'll capture.

 

One last thing to double check is the sample rate of the oscilloscope when all channels are turned on. Typically, when multiple channels are in use, the sample rate is split up among the channels. If you are using more than one channel, you’ll want to make sure the sampling rate is still sufficient.

 

The Specs You Need to Know

While bandwidth is the number one oscilloscope specification, sample rate is a close second. The oscilloscope sample rate determines the amount of waveform information captured and displayed on screen. You need a sample rate that will accurately show all aspects of your signal including its standard shape, accurate rise times, and glitches. You could be missing vital design flaws without being able to view a glitch, or you could waste hours trying to determine why your signal looks differently than you expected just because your scope was under sampling. 

 

To learn about the other need-to-know oscilloscope specs to set you up for successful measurements, check out the Basic Oscilloscope Fundamentals application note.