USING THE RIGHT OSCILLOSCOPE PROBE

A piece of test equipment’s performance is always limited by its peripheral equipment, and the same holds true for digital storage oscilloscopes. Most people tend to focus on an oscilloscope’s specifications — especially considering that specifications generally determine price — but an oscilloscope probe’s performance is every bit as important. A substandard oscilloscope probe will impair the performance of even the very best oscilloscope.

You want your oscilloscope probe to provide a simple means of presenting the signal on a circuit board or whatever is being tested to the oscilloscope’s input. A standard oscilloscope probe will consists of a probe tip, length of shielded wire, and a compensation network.

Passive probes are the most common kind of oscilloscope probe and there are two noteworthy types of passive probes: X1 and X10. X1 probes present a signal as it exists to your oscilloscope. Ordinarily an oscilloscope’s input impedance is one megohm, but this impedance can load the device you’re testing and distort the waveform. Moreover, an X1’s tip capacitance can be as high as one hundred picofarads. To avoid these limitations and lessen the load on the circuit you’re testing an X10 probe can be used instead. Because an X10 probe has an input impedance of ten megohms and a tip capacitance of around ten picofarads, this type of probe will distort the waveform far less than an X1.

Active oscilloscope probes are another option when you need even greater levels of performance. These probes have very low levels of capacitance and significantly higher input impedances as a result of having an active element quite close to the oscilloscope probe’s tip.

Calibration

It’s very easy to simply plug your oscilloscope probe in and start taking measurements with your oscilloscope; however, your oscilloscope probe needs to be calibrated before you use it to make sure that its response is flat. Almost every oscilloscope has a built-in calibrator for this reason. The calibrator provides a square wave output, and the oscilloscope probe has a small preset adjustor. You connect the probe to the calibrator’s output and manipulate the present adjustor until the shape of the displayed waveform is perfectly square. When the oscilloscope probe’s high frequency response is down the edges of the square wave on the display will be rounded and, if the high frequency response is up, the probe’s wave will overshoot the edges.

This simple adjustment is imperative for ensuring that the oscilloscope probe performs perfectly.