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Oscilloscope Probing: Skew and Amplitude Pitfall

Blog Post created by BoonCampbell Employee on Apr 24, 2018

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.

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