EEVblog 1445 - How to Simulate an Oscilloscope Probe in LTSPICE - Page 1
Forum section: electronics / general

author: EEVblog
author_rank: Unknown
timestamp: December 21, 2021, 09:42:30 pm
content: How to simulate a x1 oscilloscope probe in LTSPICE.And how to use the lossy transmission line model in practice with the spice directive.Mysteries of x1 oscilloscope probes revealed:

author: radioing
author_rank: Newbie
timestamp: December 23, 2021, 08:13:34 pm
content: Hello Dave, thank you for nice video - good point with the Spice DC setting by big R to enable synthesis of 'floating' circuits.Several years ago I found the Joe Weber's >100 pages document aboutTektronix osc. probes design from 1969 - 'Oscilloscope Probe Circuits'.Not sure you saw/read it but anyway, even it's a little bit ancient, the theory holds and it's nice reading for anybody who's interested in.

author: EEVblog
author_rank: Unknown
timestamp: December 23, 2021, 11:27:12 pm
content: Quote from: radioing on December 23, 2021, 08:13:34 pmI haven't seen that, is there a PDF available?

author: Tom45
author_rank: Frequent Contributor
timestamp: December 24, 2021, 12:17:45 am
content: Quote from: EEVblog on December 23, 2021, 11:27:12 pmSearch for TekProbeCircuits.pdfIt can be found at a number of places.

author: David Hess
author_rank: Super Contributor
timestamp: December 24, 2021, 01:39:41 am
content: Oscilloscope probes are actually tested and specified with a 25 ohm source impedance from a parallel terminated 50 ohm source. The probe tip adapter used for testing is driven from a 50 ohm generator and has a 50 ohm parallel termination built into it directly at the probe tip, like a 50 ohm feedthrough terminator. So your bandwidth results are only half of what they should be.Quote from: EEVblog on December 23, 2021, 11:27:12 pmIs there a PDF available. Sheesh, only on days ending in "Y": (1)https://w140.com/tekwiki/images/6/62/062-1146-00.pdf(1) Yankee humor.

author: DL4JY
author_rank: Regular Contributor
timestamp: January 24, 2022, 11:07:47 pm
content: Hi Dave,when watching your video, I was a bit struggling with the assumption of having a 50 Ohm cable. I recall when I tried to repair a broken cable of a probe many years ago, that the inner conductor was thin as a hair. This would rather give an impedance of 120 Ohms or so. (~0.1mm inner conductor, ~2mm outer conductor). I believe they use this higher impedance cable to decrease overall capacitance, especially in the 10:1 mode.This would change the values a bit. The capacitance of the 1.2m cable would be more like 50pF and the inductance 730nH. Together with the compensation capacitor & a bit of stray capacitance we come into the direction of your measured value.How much this will change your simulation, I have no working spice currently. But I tried unsuccessfully to use a VNA/Distance-to-Fault measurement with one of my probes with a potentiometer at the end - to adjust to the line impedance, but no luck - it was almost no difference what I connected to the other end. And yes , I was in the 1:1 position.thanks for the great video & best regards from the other side were its now cold & wet.Juergen

author: shapirus
author_rank: Super Contributor
timestamp: April 22, 2024, 10:09:39 am
content: An idea for a future video: simulation of a 10x probe.