Oscilloscope probes are one of the determining factors that determine the accuracy of fast sampled signals. To ensure accuracy during a live measurement of a high-speed signal or high-frequency waveform, a technique known as compensation is often needed. The role of a compensation circuit in an oscilloscope probe is to tune the probe’s bandwidth so that it can properly collect the signal with maximum accuracy.
How Oscilloscope Probes Are Compensated
All oscilloscope probes and the front-end on an oscilloscope have some defined input capacitance that affects the bandwidth and impedance matching for a desired attenuation level. In order to compensate for a particular input capacitance on the scope and the cable’s capacitance, the probe’s capacitance can be adjusted with a precision variable capacitor. This is better known as oscilloscope probe compensation.
Probe compensation is performed manually and it involves an adjustment through a screw or dial on the oscilloscope probe. This adjusts the capacitance seen by the signal as it reaches the oscilloscope input. A common implementation of oscilloscope compensation in a passive probe is shown below.
These methods are implemented for passive probes. Compensation can also be applied in active probes, which could involve adjusting the capacitance before the amplification stage in the probe. Some compensation circuits will have a variable snubber circuit with a resistor instead of a lone variable capacitor. Other compensation circuits may be located at the probe end near the tip, rather than being placed near the BNC plug at the connecto end of the cable.
Verifying Probe Compensation
One reason that probe compensation is performed is to ensure maximally flat response up to the oscilloscope’s available/rated bandwidth. This can be performed by tuning the variable capacitor while monitoring measurement of a reference waveform on the oscilloscope readout. Oscilloscopes will make this reference waveform available through a port on the front panel.
The reference waveform that is typically used to verify compensation is a square wave, although a single sine wave could be used. When a square wave is used, the edges of the wave will exhibit overshoot or undershoot when excessive/insufficient compensation is applied, respectively. The various waveforms involved are shown below.
Verification is done visually rather than being calculated; when the probe is properly compensated the response will appear most closely to a true square wave.
The function of compensation that produces this response is two-fold:
- When undercompensated, the variable capacitor is reducing the amplitude of high-frequency components without limiting the bandwidth.
- When overcompensated, the variable capacitor is band-limiting the response and the scope is producing an overshoot artifact through its sampling action.
In the first case, the probe is attenuating higher frequency components, which rolls over the edge of the measured square wave. In the second case, the scope is interpolating an overshoot on the rising edge of the pulse. In both cases, if the probe remains imperfectly compensated, a measured signal would appear distorted, but the probe user might not realize the probe is producing an inaccurate result.
Can Compensation Be Automated?
Oscilloscopes that can interface with high-precision probes through a BNC connection cannot automate the compensation mechanism on their own because the probe is a separate piece of equipment. As was shown above, compensation is applied by hand and verified visually. That being said, it may be possible to automate the probe compensation while also automating the square wave comparison process numerically. This would require a separate piece of equipment and would complicate the compensation process.
Mathematically, probe compensation is essentially a process of comparing a measured wave with a reference square/sine wave. When the deviation between these is minimal, then the probe has been maximally compensated. In principle, this is something that could be automated, but it remains to be seen if oscilloscope manufacturers will implement such a feature.
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