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.
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.
My previous post used a Tektronix 1502 to examine discontinuities in cables. This post examines the discontinuity introduced by a “nominally invisible” protection diode on a PCB; it is clearly visible with the TDR, but probably won’t affect the final application.
What can you see, test and measure with a traditional time domain reflectometer (TDR)? The answer is “more than you might expect”:
- measure impedance variations in connectors/filters/antennas/PCBs
- locate short/open circuits and damage in cables
- locate intermittent faults in cables and connectors
- locate connectors in cables
and can resolve discontinuities around 2cm apart. That resolution is at least 10 times better than can be obtained with the typical homebrew logic pulse + oscilloscope combination.
I recently bought a couple of cheap 1970s Tektronix 1502s in the hope that I could make a single working frankenmachine. My initial assessment was depressing: one had a cracked and broken case (so I assumed the CRT was also broken), the other’s electrolytic caps had spewed acid across the PSU and had a faulty 2kV PSU, and both had defective NiCd batteries – and it won’t even start without a working battery. But eventually I managed to get both working: I recapped the PSUs, rewelded the case with methylene chloride, used my “new” 12kV scope probe and 40kV meter to repair the HV PSU, created a “NiCd emulator”, and the CRT wasn’t damaged after all. Later reading of a TekScope magazine indicates it isn’t surprising the CRT survived: it is mechanically completely isolated from the chassis to protect against up to 26 12″ drops.
So I am now the proud possessor of two nice little portable waterproof instruments, literally designed for field use – one of the service manuals indicates they were used with Patriot missile defence systems.
Tektronix 1502 TDR Cable Tester
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.