EEVblog 1445 - How to Simulate an Oscilloscope Probe in LTSPICE

[EEVblog]

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:

[radioing]

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 about
Tektronix 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.

[EEVblog]

Several years ago I found the Joe Weber's >100 pages document about
Tektronix 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.

I haven't seen that, is there a PDF available?

[Tom45]

I haven't seen that, is there a PDF available?

Search for TekProbeCircuits.pdf

It can be found at a number of places.

[David Hess]

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.

Several years ago I found the Joe Weber's >100 pages document about
Tektronix 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.

I haven't seen that, is there a PDF available?

Is 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.

[DL4JY]

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

[shapirus]

An idea for a future video: simulation of a 10x probe.