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2018

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.

 

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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.

 

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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.

 

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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.