EEVblog 1445 - How to Simulate an Oscilloscope Probe in LTSPICE

How to simulate a x1 oscilloscope probe in LTSPICE.
And how to use the lossy transmission line model in practice with the spice directive.

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#ElectronicsCreators #Simulation #LTSPICE

[FPiorski]

I think you forgot to divide the measured resistance by 1.2, as you did with the capacitance. Still, we *do* get the idea so it's all good.

[AlphaPhoenixChannel]

Very interesting stuff! I clearly have a lot to learn on the practical side here - I admittedly approached Derek's question as a physicist not an EE... On the validity of the signal I measured, my current (ha) confidence stems from the fact that when I spread the wires out into two big loops, the flat "pre-lightspeed" transient dropped to zero, so I doubt it was actually CAUSED by the probes, but the probes may have significantly messed up my measured amplitude on an absolute scale - I anxiously await the results of your simulation! Frankly if I got it right, I was lucky - I SHOULD have cared about the probes' impact more than I did. I actually had all four wired up and tuned in 10x mode in the morning, but there was so much noise on the line at one point in frustration I flipped everything back to 1x and never remembered to set it back... I'm glad there are good engineers out there with practical knowledge to keep the scientists honest =D

[Factory]

Using LTSpice makes me feel like it's 1994 again. :-)

It is still an amazingly useful and relevant tool. Fun to see a probe considered and simulated.

[lawrencemiller]

Last week I was shown a drive signal which ideally should be a rectangular pulse, but was oscillating. The engineer added a series resistor and got rid of the oscillation, but the rising edge was rounded. Thinking of a 'scope probe circuit, I suggested a variable capacitor across the resistor and simulated a simpler model. It consisted of a RC parallel circuit (R1 || C1) to model the probe, in series with a RC parallel circuit to model the 'scope input (R2 || C2). The source was a 0-to-1 V 1 kHz square wave. Simulations were done with compensation C too large, too small, and tuned for best pulse visually. Simulation was as-expected. ... Cheers!

[p_mouse]

Calling LTSpice GUI nice is kind of overstatement 😄

[PeervanTienen]

I just love the content waterfall that Derek (veritasium) has caused 😂

[nicholasroos]

Will never stop appreciating the analog knowledge dumps.

[zazio]

Some OpAmp models that come with LTSPice has also similar problems with grounding. Those encrypted models sometimes include GND referenced connection inside. If put to floating operation it will result in error..

[marcoperches]

At first I saw the video title as how to stimulate an oscilliscope probe and I was thinking...yeah I can tell you how to stimulate my probe.

[markburton]

Should R=318/1.2 as the lead is 1.2m long?

[ledzappelin]

Simulation Software Micro Cap: versions 10, 11, and 12 by Spectrum Software are now free and require no key, just to mention...

[WacKEDmaN]

cool..thanks for the tips Dave.. always gotta remember capacitance, inductance and resistance is everywhere.. nothing is "ideal"

[WildEngineering]

Looking forward to the next video on the subject (;

[Taylor_GE]

Hi Dave, how come you included a 50Ohm series resistor on the "scope input". I was under the impression we tend to have just a 1M to ground with an optional Series capacitor for AC Coupling. But when a 50 Ohm "Terminator" is fitted or switched in, we would get 50 Ohm to deck, still not in series.

[PafiTheOne]

Dave, your older video was very much better! Almost everything was correct. Haven't you watched it before making this one? Back then you actually disassembled a probe and you've seen and said there is no series resistor. You have actually measured bandwidth. You said many other important things that you've got correctly then, but forgot by this time. :-(

[kmkessler]

Should you have adjusted the value of the series resistor to get 6MHz without the oscilloscope input capacitance in circuit (like you did in the last simulation), since the datasheet should just be measuring the bandwidth of the probe and not the bandwidth of the probe/scope combo. That could be different depending on what scope you are using.

[whowhatisthat]

I seen the thumbnail video just yesterday!

[andyhaas]

You don't need all this to figure out the bandwidth of the 1x probe.. the RC low-pass cutoff frequency of your 250 Ohm and 100 pF gives you about 6 MHz.
Now where it would get interesting is in modeling a 10x probe, trying to get the bandwidth up to 500 MHz... That's where the lossy transmission line, plus your 68 Ohm termination (not compensation!) resistor and 25 pF cap are really needed!

[firedeveloper]

when you changed the R value you should have recalculated the L with sqr root as you did the first time. (Also divide R by 1.2 as others have noticed)

[PlasmaHH]

This reminds me of that I wanted to figure out a good way to build 100x probes myself with good performance and minimal equipment for calibration...