EEVblog Electronics Community Forum

Electronics => Projects, Designs, and Technical Stuff => Topic started by: tggzzz on March 11, 2015, 04:16:00 pm

Title: Improving Signal Fidelity For 10:1 High Impedance Scope Probes
Post by: tggzzz on March 11, 2015, 04:16:00 pm
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 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 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:
(https://entertaininghacks.files.wordpress.com/2015/03/hp10074-falling-6inch.gif)

With the homebrew accessory the ringing is removed, revealing the undistorted waveform.
(https://entertaininghacks.files.wordpress.com/2015/03/hp10074-falling-bayonet.gif)
There is a little ringing at 3.6ns (270MHz), 0.5% overshoot, 4% peak-peak amplitude and <5ns duration. Some 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.

For more information, see Scope Probe Accessory Improves Signal Fidelity (https://entertaininghacks.wordpress.com/2015/04/23/scope-probe-accessory-improves-signal-fidelity/)