Testing Multi-level Bullets & Latex

All too often signal integrity aberrations are visible in digital signals displayed on an oscilloscope. Fortunately signal fidelity can be significantly improved by a simple homebrew 3D-printed accessory that can be retrofitted to any standard scope probe. The “workhorse” scope probe usually supplied with scopes is the high-impedance 10:1 passive scope probe. While very useful in many circumstances, like all scope probes it has its limitations and can introduce distortions into the displayed waveform. This post complements the scope probe reference material and simply:

  • concentrates on one problem that is frequently encountered in medium speed digital circuits
    • illustrates commercial accessories used by high-end passive probes to avoid the problem
      • describes a simple homebrew 3D-printed accessory
  • demonstrates improved signal fidelity

The Problem and Fidelity Improvement

The examples below use an HP10074C scope probe (150MHz, risetime <2.33ns, 15pF, 6inch/15cm ground lead) and Tektronix TDS340 100MHz digital scope. Without the modification, a fast (<1ns) digital edge incorrectly appears to “ring” with a half-period of 5.5ns (91MHz), 20% overshoot, 33% peak-peak amplitude, and 10s of nanoseconds duration:

Standard 15cm Ground Lead

HP10074 with Standard 15cm Ground Lead

With the homebrew accessory the ringing is removed, revealing the undistorted waveform.

Homebrew Bayonet Ground

HP10074 with Homebrew Spear Ground

There is a little ringing at 270MHz, 0.5% overshoot, 4% peak-peak amplitude and pre-transition ringing is visible (even using a 1.5GHz HP10020A probe), but it is not visible on an equivalent analogue scope. Hence that ringing is probably an artefact of the scope itself.

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