Technical Tidbit - March 2003

High Frequency Measurements Web Page
Douglas C. Smith

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Technical Tidbit -March 2003
Minimizing Errors in Oscilloscope Measurements

noise on waveform
Figure 1. High Frequency Noise on a Waveform
Vertical scale = 10 Amps/div, Horizontal scale = 20 ns/div

Abstract: Digitizing oscilloscopes have become one of the most important overall lab instruments over the last ten years. As useful as they are, some interesting problems can occur that affect measurement results. Two such issues are described. Avoiding the problems associated with these issues is straightforward.

Discussion: Figure 1 shows a waveform taken of an ESD current on a system cable. The interesting feature in the middle of the waveform is likely an artifact of the measurement and not really present. The cause is easy to understand after a little investigative work.

Many modern digitizing scopes have a minimum sensitivity of only a volt or two per division referenced to their input connectors, especially high performance scopes. In the example shown in Figure 1, the current being measured had a peak amplitude of over 30 Amperes. The current probe transfer impedance (Fischer F-33-1) was about 5 Ohms (voltage out divided by current through the probe) so the probe output voltage was actually something like 150 Volts! In order to keep the waveform on the screen, two 20 dB attenuators were used for a total attenuating factor of 100:1.

The problem lies that while the scope sees a signal of a few volts, the event being measured is quite large, resulting in the 30 Ampere current (150 Volt signal) displayed. The ESD event in this case had many times the energy of that used for normal ESD testing, such as used for CE mark European testing. Some fraction of that energy was entering the scope via a path different from the signal input, possibly as a common mode current on the probe cable or as direct radiation. The result shown in Figure 1 is suspicious because when the trace is expanded, the noise riding on the waveform has a di/dt of about 2000 Amp/ns or greater (current probe frequency response is factored in). This value for di/di is much too high and reason for doubting the accuracy of the plot.

A problem that comes as a result of interpreting the high frequency noise as real is wasting time trying to find it. To check for interference, use a null experiment to gauge the measurement error. A example of a null experiment is given in the July 1999 Technical Tidbit, The Shorted Scope Probe. One possible null experiment in this case is to remove the current probe from the cable and replace it with a 50 Ohm termination. At this point the scope just has an input cable with a termination. The resulting display should be very small compared to the actual measurement. The next step would be (for this case) to connect the current probe, fold the current carrying wire to be measured and insert it into the current probe. This exposes the probe to the electric field on the wire, but with no net current, so the result should be zero. This null experiment checks the current probe as well as the scope and the connection to the scope. This technique is detailed in "Current Probes, More Useful Than You Think," a paper on this website (click on the paper title to get the paper in pdf format).

Another case of measurement error is shown in Figure 2. Normally, the default for many digitizing scopes is to turn on sin(x)/x interpolation. The problem is that this interpolation makes waveforms look very smooth and hides the fact that the sampling rate may be not fast enough to properly display the waveform. In Figure 2, a high frequency waveform is displayed with sin(x)/x interpolation turned off. One can see the straight lines connecting individual sample points. Clearly, the scope is not sampling fast enough to display the waveform accurately. If the sin(x)/x interpolation was on, everything would look fine with something close to a modulated sine wave shape.

Figure 2. High Frequency Signal Displayed with Sin(x)/x Interpolation Off
Horizontal scale = 500 ps/div

Recommendations:

If you use an attenuator, or a probe with a high attenuation factor, on an oscilloscope to keep a waveform within the amplitude limits, check for interference to the scope. Placement of the scope 3 or 4 meters from the measurement with several ferrite cores on the scope input cable may be needed. In extreme cases, the scope may need to be placed in a Faraday (shielded) cage.
Turn off sin(x)/x interpolation in digital scopes for most measurements

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