Digital Signal Integrity and Bandwidth: Signal’s Risetime is Important, Period is Irrelevant

What bandwidth should your scope and scope probe have? When do you have to worry about “high frequency” effects? This is basic theory relating the time and frequency domains, but too often people quote rules of thumb that, while strictly accurate, are very misleading. Typical misleading statements are:

  • the scope’s bandwidth should be at least 5 times the frequency, so you can see a reasonable waveform
  • I only have a 1kHz (i.e. 1ms period) signal, which isn’t high frequency (certainly not RF), and an audio frequency scope is sufficient

This post is a basic illustration of the “maximum frequency” in a 1kHz signal, to help dispel some misconceptions.

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Scope Probe Accessory: Higher Frequency Results

This takes the scope probe accessory described previously, and repeats the measurements with a higher frequency oscilloscope (>350MHz) and higher frequency probes (250MHz). The results are even more impressive at higher frequencies.

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Renovating a Tektronix P6013A 12kV 100kHz Scope Probe

I’ve recently had to debug several scope’s 2kV-3kV HV supply and the CRT’s Z-axis waveforms at 2.5kV. So far I’ve got away with using a homemade 1000:1 voltage divider and a multimeter. Since that is crude and not particularly safe, I’m not going to mention the details in order to avoid someone apeing me and hurting themselves.

Then, at a recent auction I managed to pick up:

  • a 40kV meter for measuring 17kV anode voltages, but which barely registers 2kV
  • a Tek P6013A 12kV 1000:1 100kHz scope probe

The probe was functional but missing part of the handle. While not strictly necessary, I wanted to have a little fun fabricating the missing part…

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Happy days: a new-old 1.5GHz Scope Probe

For a while my best probes have been HP10020A 1.5GHz passive probes, but only the 10:1 variant. They are really pleasant to use because they are light, robust and cheap (unlike active probes), have very convenient spear tips, and most importantly, don’t distort the signal (unlike the common 10:1 high impedance probes). For the latter reason, these poorly named “low impedance Z0” probes have been a preferred way of looking at high-speed signals. For more information on their characteristics, see my scope probe reference material.

The only problem has been that I only have two, and so have been reluctant to use them in case they were damaged.

Well, that’s changed thanks to Ebay and an Australian vendor: I was able to buy a new-old HP10020A with the 1:1, 5:1, 10:1, 20:1, 100:1 tips, and all accessories in a case remarkably cheaply. That’s the definition of a good transaction: both parties are pleased.

Happy days.

Scope Probe Accessory Improves Signal Fidelity

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.

With the unmodified ground lead, 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. With the homebrew spear ground the ringing is removed, revealing the undistorted waveform.

There is a little ringing at 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.

The updated results with a higher frequency oscilloscope and higher frequency probe are even more impressive.

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PLA Scope Probe Accessory Improves Signal Fidelity

This page describes an earlier version, and is retained for historical interest.

See this new page for details of the theory, the performance and manufacturing.

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.

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Oscilloscope Probes

Questions that are frequently seen on blog threads include:

  • why are there so many different types of scope probe?
  • which probe should I use?
  • why is my signal doing this?

Those are very sensible, common and important questions – so, if you know where to look, there are good answers on the web. I’ve gathered together a small collection of high quality references that have helped me in the past. I hope they enable people to swiftly answer their common questions.