Probing Techniques and Tradeoffs (Part II)

Figure 1: A snapshot of available probes from
Teledyne LeCroy

Our first post in this series concentrated on connectivity and various means by which one might apply an oscilloscope probe to a circuit or device under test. Now, we'll look at an "ideal" probe vs. a real-world probe, and then begin a discussion of probe specifications.

An "ideal" probe is just that: an ideal. The ideal probe would have a perfectly flat magnitude response over its entire infinite frequency range. It would offer a perfectly linear phase response, and would present zero loading to the circuit under test, implying infinite impedance.

Unfortunately, neither we at Teledyne LeCroy, nor anyone else for that matter, has been able to create an ideal probe, as the realities of physics render it impossible. So we're stuck with the probes we are able to make. The good news is that these probes are pretty good. We can get fairly close to making a real-world probe behave in an ideal fashion. But achieving a perfectly flat magnitude response and a perfectly linear phase response from DC to tens of gigahertz is a rather difficult exercise in hardware design. The same applies to loading, as impedance is finite. Thus, real-world probes diverge somewhat from the ideal, but there are ways to compensate for these non-ideal behaviors to greater or lesser extents.

Oscilloscope makers all bring to market a wide variety of probes for users to choose from, depending on the application (Figure 1). Along with the probes comes a plethora of accessories as well as the oscilloscopes themselves.

There are many things to consider when choosing a probe for an application. What key specifications of oscilloscope probes should you bear in mind when making that choice? Here's a short list:

• Bandwidth (frequency response)
• Rise time (step response)
• Dynamic range (gain/attenuation)
• Noise
• Loading impedance (AC vs. DC)

Next time, we'll begin an in-depth discussion of probe bandwidth.

Meanwhile, for further reading, please revisit our earlier series on oscilloscope probes:

• Back to Basics: Probes (Part I)
• Back to Basics: Probes (Part II)
• Back to Basics: Probes (Part III)
• Back to Basics: Probes (Part IV)