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Oscilloscopes Blog

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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.

 

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.

There are many cases where certain signals can cause your device to malfunction. This may be a problem your customer ends up finding if you don’t properly test during product development. Designers and test engineers frequently use an Arbitrary Waveform Generator (AWG) to simulate worst-case conditions during design verification. An AWG is the ideal tool for creating degraded or stressed signals to verify product performance limits. System or product noise susceptibility, timing problems, signal-level abnormalities, bandwidth loss, harmonic distortion, or a host of related maladies can be determined.

 

The AWG is a very powerful tool and can create waveforms or waveform bursts needed for your specific application. An AWG combines the capabilities of a function generator with that of a pulse generator, modulation source, noise generator, sweep generator, and trigger generator. It is a good tool for everyday use in the design lab or test environment. You can create custom solutions for a wide range of applications spanning many industries. AWG applications range from high dynamic range to high bandwidth output requirements.

 

 Arbitrary Waveform Generator (AWG) applications

Figure 1. Arbitrary Waveform Generator (AWG) applications.

 

Below is a list of common applications covered in this blog:

  • Radio Frequency (RF) signals
  • Radar signals
  • Environment signals
  • Coherent optical
  • Generic Orthogonal Frequency-Division Multiplexing (OFDM)
  • High-speed serial
  • Simulating real-world aberrations in 100Base-T physical layer
  • Dual Tone Multi-Frequency (DTMF)
  • Pacemaker
  • Automobile suspension testing
  • Power line testing

 

Radio Frequency (RF) Signals

Creating the signals required for RF conformance and margin testing is increasingly difficult. Digital RF technologies require wide-bandwidths and fast-changing signals that other generators cannot produce. These types of signals are seen in RF communications and ultra-wide band radio applications.

 

Radar Signals

Radar signals demand AWG-level performance in terms of sample rate, dynamic range, and memory. AWGs can oversample the signal in instances where phase and amplitude quadrature signal generation is desired. This improves signal quality, creating a spurious free dynamic output. AWG’s also provide Linear Frequency Modulation (LFM), Barker and Polyphase codes, step FM, and nonlinear FM modulation signals. They also generate pulse trains to resolve:

  • Range and doppler shift ambiguity
  • Frequency hopping for electronic counter-counter measures
  • Pulse-to-pulse amplitude variation

 

Environment Signals

Radar signals must coexist with commercial signals and not affect each other. Use your AWG to thoroughly test all the corner case issues at the design or debug stage. An AWG can be programmed to output many industry-standard signals:

  • WiMAX
  • WIFI
  • GSM
  • GSM-EDGE
  • EGPRS-2A
  • EGPRS2B
  • CDMA
  • WCDMA
  • DVB-T
  • Noise
  • CW radar

 

You can also define the carrier frequency, power, start time, and duration of these signals. This allows control of the level of signal interaction or interference.

 

Coherent Optical

Today's web driven world is pushing the demand for high-speed short and long haul coherent optical solutions. Phase modulation, high baud rate, high sample rate, high bandwidth, and high resolution are all critical to optical applications. Multiple synchronized AWGs can be used to generate many desired coherent optical signals.

 

Generic Orthogonal Frequency-Division Multiplexing (OFDM)

OFDM has become the modulation method of choice for transmitting large amounts of digital data over short and medium distances. Wide bandwidths and multiple carriers are needed to test RF receivers in today’s wireless world. AWG OFDM packets can specifying the spacing between the symbols or frames or stressed by adding gated noise.

 

High-speed Serial

Serial signals are made of binary data (simple ones and zeros). These signals have begun to look more like analog waveforms with analog events embedded in the digital data. The textbook zero-rise time and flat top of the theoretical square wave no longer represent reality. Today’s serial communication environments are negatively impacted by noise, jitter, crosstalk, distributed reactances, and power supply variations. Your arbitrary waveform generator can create all these signals!

 

Using direct synthesis techniques, AWGs can simulate the effects of propagation through a transmission line.

 

 

Rise times, pulse shapes, delays, and aberrations can all be controlled by your AWG. You can also create a variety of digital data impairments such as jitter (random, periodic, sinusoidal), noise, pre/de-emphasis, duty cycle distortion, inter-symbol interference, duty cycle distortion, and spread spectrum clocking.

 

Simulating Real-World Aberrations in 100Base-T Physical Layer

To simulate physical layer test signals for 100Base-T transceivers, your AWG will create several analog parameters:

  • Undershoot and overshoot
  • Rise and fall time
  • Ringing
  • Amplitude variations
  • Specific timing variations such as jitter

 

AWGs provide an efficient method for generating signal impairments like these for testing product margins.

 

Dual Tone Multi-Frequency (DTMF)

Touch-tone signals on push button telephones are created by combining a low frequency and a high-frequency signal. Simulating the superimposed frequencies creates a special challenge if the frequencies are not harmonically related. An arbitrary waveform generator can generate these signals along with controlled levels of noise and harmonic content.

 

Pacemaker

A simple square wave or sine-wave pulse was used to test pacemakers in the past. Today’s AWGs can create a simulated heartbeat waveform that pacemakers are designed to detect.

The arbitrary waveform generator can customize pacemaker testing for particular heart rate types.

 

Automobile Suspension Testing

An AWG can simulate automobile sensor outputs just as a car would when it hits a bump. The suspension’s response and reliability can be tested under virtually any simulated road condition because the size of the “bumps” can be precisely controlled.

 

Power Line Testing

Multichannel AWGs can simulate three-phase power. Transients or glitches can be created to simulated problematic waveforms. For example, you could simulate a transient on one phase and signal dropout on another.

 

In addition to all the applications above, there are many more across several different industries, and the arbitrary waveform generator will support them all:

  • Sequencing and deep memory
  • Creating long scenario simulations
  • Leading edge physics, chemistry, and electronics research
  • Validation and compliance testing of high-speed silicon and communications devices
  • Stressing testing receivers with a wide array of signal impairments
  • Generating high Baud rate baseband signals with higher order, complex modulation
  • Radar, satellite, electronic warfare, and multilevel signals
  • Jitter margin testing for analog-to-digital converters

 

Conclusion

We have now covered the importance of an arbitrary waveform generator to ensure your device is working properly for your specific application. As you can see, AWGs excel in creating mixed-signal waveforms that can mimic real world conditions. To learn more about arbitrary waveform generators, check out: A High-Performance AWG Primer.

If you are using an oscilloscope, make sure you are using the right bandwidth! Choosing the wrong amount could adversely affect your measurement results. Let’s look at what oscilloscope bandwidth is and why you need just the right amount. 

 

What is Bandwidth?

 

Bandwidth is often regarded as the single most important characteristic of an oscilloscope. Measured in Hertz, the bandwidth of your oscilloscope is the range of frequencies that your oscilloscope can accurately measure. Without enough bandwidth, the amplitude of your signal will be incorrect and details of your waveform might be lost. With too much bandwidth, you will capture excessive noise, providing you with an inaccurate measurement. Here’s why: 

 

You can think of an oscilloscope like a low pass filter, meaning it will only pass frequencies from 0 Hz up to a specified frequency. An oscilloscope’s bandwidth is specified as the 3 dB down point of the filter. What the heck is a 3 dB down point? Read on. 

 

Low pass filters allow signals to pass through them at full amplitude until the signal frequency approaches the high end of the frequencies that the filter can pass. Then a filter attenuates signals passing through them until the signal’s amplitude is dampened to nothing. When the signal is attenuated by three decibels (3 dB), that is the cutoff point for an oscilloscope’s bandwidth specification. If you aren’t familiar with decibels, the 3 dB down point is when the amplitude of a sine wave is 70.7% of its actual height. Look at the diagram below to visualize the frequency response of a low pass filter, depicted in blue.

 

Oscilloscope bandwidth: Frequency response of a low pass filter, depicting the 3 db down point and cutoff frequency.

Figure 1. Frequency response of a low pass filter, depicting the 3 dB down point and cutoff frequency.

 

So, if you have an oscilloscope that has a bandwidth of 200 MHz, you know that the cutoff frequency of that oscilloscope’s filter is 200 MHz. Why does this matter for your measurements? 

 

Too Little Bandwidth

 

You can see from Figure 1 that if you are measuring a signal that has a higher frequency than the cutoff frequency, you’ll either see an attenuated and distorted version of your signal or not much of a signal at all. Even measuring a signal as fast as the bandwidth of the scope is not a good idea. Measuring a 200 MHz signal on a 200 MHz oscilloscope will not provide you with the best representation of your signal, as the filter has already begun to roll off and distort your input.

 

Measuring with too little bandwidth will provide distorted results

 

Here is the rule of thumb for choosing the right bandwidth:

  • Digital signal measurements: five times higher bandwidth than the fastest digital clock rate in your system
  • Analog signal measurements: three times higher bandwidth than the maximum signal frequency on an oscilloscope with a flat frequency response

 

For more detail on these rules, read Evaluating Oscilloscope Bandwidths for Your Application.

 

So why not just use an oscilloscope with the highest bandwidth possible?

 

Too Much Bandwidth

 

Oscilloscopes can capture environmental noise. Oscilloscopes also add noise to your signal from filtering, processing, and digitizing (though a high-quality oscilloscope will do all of this properly and add less noise than a poorly-designed scope). And noise occurs at all frequencies. So if you have a 200 MHz oscilloscope, that scope is only going to show noise up to 200 MHz. But, if you have a 33 GHz oscilloscope, it will add noise to your measurement through its entire measurement range up to 33 GHz, regardless of the frequency of your signal. 

 

Increasing bandwidth increases noise

 

If you want to measure a 50 MHz signal, a 200 MHz oscilloscope will give you plenty of bandwidth to clearly display your signal without attenuation and filter distortion but not so much that it adds high frequency noise content to your measurement.

 

Insider tip: If all you have access to is a high bandwidth oscilloscope, but you are measuring low frequencies, turn on hardware filters in the oscilloscope to eliminate that high frequency noise and get a cleaner measurement.

 

The other reason why you probably don’t want to buy the highest bandwidth oscilloscope out there is price. The higher the bandwidth, the higher the price. If you are worried the bandwidth you need today will not be enough for future measurements, look for an oscilloscope that lets you upgrade the bandwidth with a software license. That way you can buy the bandwidth you need now and upgrade later without having to purchase a new oscilloscope or send it in to the factory for a hardware update. (Most Keysight oscilloscopes can be bandwidth upgraded with a software license for this very reason.)

 

Demonstration

 

Don’t be afraid to be the Goldilocks of bandwidth. Did she settle for the porridge that was too hot or too cold? No. She went for the one that was just right. And lucky for us, we won’t be eaten by bears if we set our bandwidth to just the right amount. Here is an example of how even a simple sine wave can be falsely represented on an oscilloscope without the right bandwidth.

 

In this demonstration, I am measuring a sine wave oscillating with a frequency of 80 MHz and a peak-peak voltage of about 2 volts.

 

Oscilloscope bandwidth: Measuring a sine wave oscillating with a frequency of 80 MHz and a peak-peak voltage of about 2 volts

 

I am using an 8 GHz oscilloscope. This is an excessive amount of bandwidth for an 80 MHz signal. The rule of thumb for analog signals is to use about 3 times the frequency of the signal. While this measurement doesn’t look horrible, let’s see how much better it can get when I apply the rule of thumb.

 

With only 240 MHz of bandwidth, look at how much cleaner my measurement is.

 

Oscilloscope bandwidth: Clean measurement with only 240 MHz of bandwidth

 

If I just want a quick check on the basics like voltage and frequency, the difference might not be crucial. But if I’m proving the quality of my design or attempting to pass strict performance or compliance specs, I would want the best (and cleanest) representation of my signal.

 

Now, I’ll decrease the bandwidth even further. As I mentioned earlier, you shouldn’t measure a signal at the bandwidth of the oscilloscope. The signal will be passing right through the 3 dB down point of the filter.

 

Oscilloscope bandwidth: Bandwidth decreased further

 

Here I’m measuring my 80 MHz signal with 80 MHz of bandwidth. You can see that the voltage is decreased from 1.92 V to 1.36 V. This is 70.8% of the voltage we should be seeing. The signal is attenuated by the filter. 

 

To demonstrate the effects of the filter above the cutoff frequency, here is my measurement of the same signal with only 75 MHz of bandwidth. The signal is attenuated even further to 161 mV. The period of my measured signal is displayed as 12.74 ns. This would imply that the frequency of my signal is only 78 MHz, which we know to be false.

 

Oscilloscope bandwidth: Measure the same signal with only 75 MHz of bandwidth

 

And here I’ve measured the same signal again with only 70 MHz of bandwidth. It barely looks like there is a signal at all.

 

Oscilloscope bandwidth: Measured same signal with only 70 MHz of bandwidth

 

You can see how dramatically the signal is attenuated when you try to measure a signal with frequency beyond the bandwidth of the oscilloscope.

 

Summary

 

Bandwidth is the most important characteristic of an oscilloscope

 

While there are many important features of an oscilloscope that you’ll need to evaluate before choosing one for your measurements, clearly bandwidth is the number one spec that you must check before any other. If you don’t have enough bandwidth you’ll see distorted or attenuated signals, giving you inaccurate measurements. If you have too much bandwidth, your measurements will be noisier than necessary. You have to choose a bandwidth that can support a clean and accurate representation of your test signals.

 

Now that you understand why bandwidth is the most important characteristic of an oscilloscope, check out Basic Oscilloscope Fundamentals to learn the other important oscilloscope characteristics and how to use an oscilloscope.

Picture the heart rate monitor that you always see next to hospital beds on “House” or “Grey’s Anatomy.” You hold your breath as you wait for the next beep and jump of the line on the screen, and you dread the flat line as the TV show reaches its apex.


Well, when my family asks me what I do for a living, this is how I describe an oscilloscope. But instead of displaying the signal of a human heart, oscilloscopes show the heartbeat of electronic devices. They give us all kinds of insights into whether or not an electronic device is operating correctly, allowing us to check its vitals.


The vitals of our devices could be voltage or current. And just like we don’t want our hearts to beat too fast or too slow, we want those voltages to oscillate at the right pace or frequency. We all know heart murmurs are bad. Well, we don’t want any glitches in our electrical signals either, and an oscilloscope can help us find them. Having insights like this into your electronic devices allows you to validate it is operating as expected. And if it’s not, oscilloscopes help you diagnose the problem and correct it. If you are an electrical engineer, chances are you could use an oscilloscope ─ whether you’re a test engineer or student or work in manufacturing, repair, research, or development.

1000 X Series Oscilloscopes1000 X-Series oscilloscopes making a variety of measurements.

 

Oscilloscope Basics

The basic operation of an oscilloscope displays voltage versus time, with voltage on the vertical axis and time on the horizontal axis. This allows you to double check that your device’s signal is as you expect, both in magnitude and frequency. And because oscilloscopes provide a visual representation of the signal, you can view any anomalies or distortion that might be occurring. But before you start testing, there are some things for you to consider.

 

Oscilloscope displayOscilloscopes display voltage on the vertical axis and time on the horizontal axis.

 

Oscilloscopes come in many flavors. You want to select an oscilloscope with the right bandwidth, signal integrity, sample rate, and channel inputs. You also want to make sure it is compatible with any applications and probes you may need. Here is a list of some of the features you should check when deciding what oscilloscope to use:

 

  • Bandwidth – The range of frequencies the oscilloscope can measure accurately. Oscilloscope bandwidths typically range from 50 MHz to 100 GHz.
  • Sample Rate – The number of samples the oscilloscope can acquire per second. The greater the samples per second, the more clearly and accurately the waveform is displayed.
  • Signal Integrity – The oscilloscope’s ability to represent the waveform accurately. This is a topic I’m particularly passionate about and you’ll find me writing about this a lot. You wouldn’t want a heart rate monitor that displays incorrect information. It would do no good to declare a patient dead whose heart is still beating. The same is true for your device under test. You wouldn’t want to declare your device is malfunctioning and spend weeks trying to find the root cause when there isn’t actually a problem.
  • Channels – The input to the oscilloscope. They can be analog or digital. There are typically 2 to 4 analog channels per oscilloscope.
  • Probe Compatibility – A probe is the tool used connect the oscilloscope to your device under test. There are a large variety of passive and active probes, each made for specific use cases. You want an oscilloscope that is compatible with the type of probe you need for your specific tests.
  • Applications – Signal analysis, protocol decode, and compliance test software can greatly reduce the time it takes to identify and capture errors in your designs. Analysis software can help you find and evaluate jitter, perform Fourier transforms, create eye diagrams, and even identify and quantify crosstalk. Protocol decoding software can identify digital packets of information, trigger on different packet conditions, and identify protocol errors. Not all oscilloscopes are compatible with every application.

 

What are Oscilloscopes Used for?

Now that you’re armed with the lingo, you’re ready to get going. The most basic testing only requires an oscilloscope with 50 to 200 MHz of bandwidth, a passive probe, and sufficient sample rate, signal integrity, and channel inputs.


Armed with these basics, you can spot-check your printed circuit boards (PCBs) to find faulty parts, noisy power lines, shorts, and I/Os (inputs and outputs) that are not working; dive into different trigger modes to search for runts, glitches, and timing errors; and capture signals and data to prove the quality of your designs. Some basic oscilloscopes even provide Bode or frequency and phase response analysis. And this is just the start.

 

Frequency response analysis on InfiniiVision oscilloscopeFrequency response analysis performed on an InfiniiVision oscilloscope.

 

Oscilloscopes are versatile and widely used instruments. Automotive technicians use oscilloscopes to diagnose electrical problems in cars. University labs use oscilloscopes to teach students about electronics. Research groups all over the world have oscilloscopes at their disposal. Cell phone manufacturers use oscilloscopes to test the integrity of their signals. The military and aviation industries use oscilloscopes to test radar communication systems. R&D engineers use oscilloscopes to test and design new technologies. Oscilloscopes are also used for compliance testing such as USB and CAN protocols where the output must meet certain standards.

 

Get Started

Now that you know what an oscilloscope is and some of the crucial oscilloscope specs, it’s time to get testing. So throw on your scrubs (or maybe an ESD strap instead) and get started!


To learn more about how to operate an oscilloscope and understand measurement fundamentals, you can read the Basic Oscilloscope Fundamentals application note.

Quick note: We usually post oscilloscope tips and tricks to this blog, but today we want to share with you about another test & measurement tool often used with or alongside scopes.

 

In my previous post, I outlined the different types of signal generators in the market today, and what you need to consider when selecting the right fit for your application. I also highlighted why the arbitrary waveform generator (AWG) is my recommendation for you to simulate real world stimulus.

 

Arbitrary waveform generators (AWGs) are the most versatile signal generators available. An AWG can generate any mathematically-characterized signal, including sine wave, pulse, modulated, multitone, polarized, and rotated signals. The AWG is commonly seen as the workhorse piece of test equipment and can perform the functions of any other generator type. A typical block diagram of an AWG is shown below. The signal flow through the functional blocks starts with a numeric description of a waveform stored in memory. Then the selected waveform samples are sent to a digital-to-analog converter (DAC), filtered, conditioned, amplified and output as an analog waveform.

 

Diagram of a common Arbitrary Waveform GeneratorA common AWG block diagram

 

 

A Closer Look at Each Arbitrary Waveform Generator (AWG) Functional Block

 

1. Memory

A digital representation of a waveform is loaded into AWG memory through a variety of software applications, such as MATLAB, LabView, Visual Studio Plus, IVI, and SCPI. The memory is clocked at the highest sampling rate supported by the AWG. The size of the memory will dictate the amount of signal playback time available. A rule of thumb to determine the playback time is: memory depth divided by sample rate equals playback time. The faster your sample rate, the quicker you will use up the available memory.

 

2. Sequencer

The sequencer circuitry can solve memory depth limitations by arranging (sequencing) the waveform into segments to create your desired waveform. Memory sequencing (or memory ping-pong) does this by only enabling memory during critical waveform portions and then shutting off. You can think about it like this: when recording a round of golf, imagine how much recording time you would save if you only recorded the players striking the ball and not all the walking and setup time. The sequencer does the same thing by only recording waveform transitions and not idle time. Synchronization is maintained by the trigger generator, which enables the waveform. Trigger events can be internal, external, or linked to another AWG.


3. Markers and Triggers

Marker outputs are useful for triggering external equipment. Trigger inputs are used to alter sequencer operation, resulting in the desired waveform entering the DAC. Hardware or software triggers can be used for applications requiring exact timing, like wideband chirp signals. They can also be used where multiple AWGs are synchronized together and need to be triggered simultaneously.

 

4. Clock Generator

The timing of the waveform is controlled by an internal or external clock source. The memory controller keeps track of waveform events in memory and then outputs them in the correct order to the DAC. The memory controller saves space by looping on repetitive elements so that the elements are listed only once in the waveform memory. Clocking circuitry controls both the DAC and the sequencer.

 

5. Digital-to-Analog Converter (DAC)

Waveform memory contents are sent to the DAC. Here the digital voltage values are converted into analog voltages. The number of bits within the DAC will impact the AWG’s vertical resolution. The higher the number of bits, the higher the vertical resolution and the more detailed the output waveform will be. DACs can use interpolation to reach an even higher update rate than what was supplied by the waveform memory.

 

6. Low Pass Filter

Because the DAC output is a series of voltage stair steps, it is harmonic-rich and requires filtering for a smooth sinusoidal analog waveform.

 

7. Output Amplifier

After the signal passes through the filter, it will enter an amplifier. The amplifier controls both gain and offset. This gives you the flexibility to adjust output gain and offset depending on your application. For example, you may need high dynamic ranges for radar and satellite solutions or high bandwidth for high-speed and coherent optical solutions.

 

Use this blog’s functional building blocks to help you understand just what is happening within your AWG and fully utilize the AWG’s capabilities. For a deeper understanding of arbitrary waveform generator fundamentals, I recommend that you download the comprehensive "Fundamentals of Arbitrary Waveform Generation" guide. 

 

Takeaway and the Demands of Our Connected World

Image of a connected worldInternet technologies have driven advanced AWG solutions

 

Our connected world demands increased speed and data complexity. To support this demand, today’s AWGs must:

  • Reach higher frequencies while providing wider bandwidth
  • Handle complex modulation techniques that cram more data into available bandwidths
  • Work with ideal and real-world signals
  • Generate signals that stress devices to their limits
  • Provide reliable and repeatable results

 

More on this topic in my posts to come.