ROBUST HIGH-BANDWIDTH PASSIVE DIY PROBES




Introduction

Before presenting this great tip I must admit I did not come up with this idea. This type of measurement probe is presented in the book High Speed Digital Design – A Handbook of Black Magic by Howard W. Johnson and Martin Graham. I first saw the idea written by fellow DIYer Janne Ahonen but he also gives credit to Howard Johnson. Please refer to Janne’s article for more technical explanation of the probe, followed by measurements.

ROBUST HIGH-BANDWIDTH PASSIVE DIY PROBES

Here are instructions how to build and use these probes along with plenty of photos. There are no measurements done to present for now.

Problem of conventional probe

Conventional oscilloscope probes are ok for basic measurements but have several limitations. A practical one is that they may be clumsy to use with big plastic case and a clip that does not stay anywhere. The probes of my relatively expensive Keysight scope are good example; in fact they are among the worst ones I have used. Traditional scope probes are a bit of a remnant from the past. If you want to buy an active high-bandwidth probe from your scope manufacturer, prepare to pay 1000+ dollars even for the ‘low-bandwidth’ models (that is maybe 1 GHz).

Another limitation is bandwidth. Many hobbyists may not be concerned about that as it becomes significant when going beyond 300 MHz or so, usually higher than hobbyists’ scope bandwidths. Bandwidth is mainly limited by the probe capacitance.

However, in order to get even close to the bandwidth of even a conventional passive probe, one must not use the alligator ground clip. This creates a huge ground loop with significant inductance. If you want to measure higher bandwidth signals, the provided barrel spring connector must be used. However, that thing is incredible cumbersome to use!

READ  Lux Meter Module

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