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.
I recently picked up an early Tek 475 scope plus service manual plus probes at a hamfest. I asked what was wrong, and the seller indicated the timebase was unreliable. Fair enough and, given the low price, at worst it would be a parts mule.
It was a bit mucky on the outside, as would be expected of something that had been in storage. A quick look the scope showed it was cleaner inside than I expected, and playing with it made me feel it was worth resuscitating. If nothing else, the puzzles involved would be more interesting and worthwhile than crosswords and soduku.
Overall the renovation involved some traditional problems and solutions, plus a couple that are less well-known – and a few diversions down blind alleys. Maybe the hints and tricks outlined below will save other people some time.
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.
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…
A non-functional Tektronix 465 oscilloscope with a dented case and several serious faults was recently donated to my local HackSpace. Since they didn’t want another broken scope, I took it to see what was wrong. Eventually, after learning about CRT theory, with the help of very knowledgable people on the TekScopes forum, and gingerly measuring waveforms at -2450V, I came to the conclusion the CRT’s grid was faulty. Reluctantly removing the CRT, I found that it could not be repaired. Since parts of the CRT are rather beautiful, I decided to salvage those parts for display. This note outlines the original tube, the fault, and the process of turning it into something fit for a display cabinet.
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.
HP10074 with Standard Ground Lead
HP10074 with Homebrew Spear Ground
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.
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.
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.