10x Passive Probes and Cable Reflections

Figure 1: With unequal impedances at either end of the coax,
are cable reflections a concern in 10x passive probes?

We've been discussing the ubiquitous 10x passive probe here on Test Happens, beginning with an overview of the probe-oscilloscope system. We turned to the 10x passive probe itself and the issues posed by its constitutive circuitry. Then we covered what about that circuitry makes it usable at all, namely, its built-in equalization circuit.

How Tip Inductance Impacts a Probing System's Bandwidth

Figure 1: Shown are FFT plots of a 10-MHz, fast-edge square
wave reaching the oscilloscope via direct coax connection
(orange-yellow plot) and 10x passive probe fitted with a coax
tip adapter (straw-colored plot)

If you're using 10x passive probes with your oscilloscope, it's important to understand the bandwidth of your probing system and how it's affected by various methods of probing the signal of interest. There's a relatively easy way to determine this parameter by probing a fast-edge, 10-MHz signal from a square-wave generator. Doing so can also instruct us in the effects of tip inductance on the probe's bandwidth.

How Equalization Works in 10x Passive Probes

Figure 1: The adjustable equalization circuit on the oscilloscope
end of the coaxial cable compensates for the 10x passive
probe's inherent low-pass filter characteristics

We've been discussing 10x passive probes and their inner workings; our last post covered all the ways in which a 10x passive probe is apt to be a liability. They'd be basically unusable for any measurements at all but for one attribute: their equalization circuit (Figure 1). Without it, the 10x passive probe makes a pretty good low-pass filter, but the equalization circuit counters that with a high-pass filter to balance things out.