Putting Probes in Perspective

Figure 1: Probe, cable, and oscilloscope form a system
that makes or breaks the accuracy of signal acquisitions

Few aspects of using an oscilloscope are as important as the probe: after all, the probe forms both the mechanical and electrical interfaces between the device under test (DUT) and the oscilloscope itself. To feed a signal into an oscilloscope, we're limited to a coaxial connection. Thus, we need a geometry transformer that picks up the signal of interest from the DUT and transfers it to the oscilloscope's coaxial connection.

Probing Techniques and Tradeoffs (Part X): More Best Practices

Figure 1: Chip clips;
they're not just for
snacks anymore

In probing circuits, as with most endeavors, there are some best practices you can use to enhance your chances of obtaining optimal measurements. We began exploring this concept in our last post, and we'll continue here with more best practices.

Probing Techniques and Tradeoffs (Part IV)

Figure 1: Applying bandwidth filters to a 2.5-GHz clock
signal clearly shows the effect of bandwidth on rise time

The topic of probe bandwidth is a broad and deep one. We began our discussion of bandwidth in an earlier post with some basic information about what bandwidth means and the importance of the -3 dB point. Next, we looked at a Fourier deconstruction of a square wave into its fundamental and the lower-order harmonics, and covered the importance of bandwidth in capturing enough harmonic content to understand the signal's overall shape.

Probing Techniques and Tradeoffs (Part III)

Figure 1: Bandwidth is defined as the frequency at which
the ratio of the displayed amplitude to the input amplitude
is -3 dB (or 0.707)

Any discussion of oscilloscopes and/or probes must include the topic of analog bandwidth. Bandwidth is one of a short list of key specifications for a testbench setup. All oscilloscopes and probes come to market with a bandwidth specification, which is defined as:

The frequency at which the ratio of the displayed amplitude to the input amplitude is -3 dB (or 0.707).

This is known as the "-3 dB point," or the half-power point (Figure 1). At this frequency, a sine-wave input signal is attenuated to 70.7% of its true amplitude. Any higher frequencies will likely be distorted on the display, making accurate measurements and calibration impossible.

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.