Products > Test Equipment
making an 20:1 coax probe
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tggzzz:

--- Quote from: bson on July 11, 2022, 07:26:27 pm ---
--- Quote from: sicco on July 10, 2022, 02:58:44 pm ---If your scope does not have a 50 ohms input option (selected) then you do need to add something like this on the scope BNC connector:

https://www.distrelec.nl/en/feed-through-termination-50-ohm-2w-rohde-schwarz-hz22/p/11085041?ext_cid=shgooaqnlnl-p-shopping-fallback&gclsrc=aw.ds&?pi=11085041&gclid=EAIaIQobChMIiK7X08Tu-AIVy513Ch2y8wCQEAQYASABEgJO2_D_BwE

A 50 ohms coax cable connected to just a 1 Mohm scope input is a pure reflection point for sharp fast pulses. The coax cable is a transmission line, a waveguide. Waveguides shall be terminated, on both ends, with a resistor that is as many ohms as the waveguide’s characteristic impedance. 50 ohms that is for the coax you’re considering to use. For a Z0 probe, one can omit the 50 ohms resistor in the probe because there is no need to cancel echoes from what might travel as pulse/edge in the cable back from scope to probe. But then we must first guarantee that nothing could have bounced at the scope BNC input. The 50 ohms scope input resistor does give that guarantee. If it is there…

The spice models posted just above confirm it all…

--- End quote ---
But the thing is to increase its impedance as a pure 50Ω Z0 probe is practically useless; there's almost nothing you can observe with it.  Put it on an XO µC pin and the oscillator stops.  On a 100Ω LVDS signal, and the transceiver faults.  On a device D+/D- USB pin, and the host wants to reset and reenumerate, and likely completely quarantines unless you're very quick to remove the probe.  On an ethernet RMII interface and it goes belly-up.  There are such extremely limited uses for it that its existence is mostly an academical novelty.

--- End quote ---

All tools have their limitations.

The (up to) 5kohm input impedance of an HP10020 Z0 probe is two orders of magnitude higher than a so-called "high" impedance *10 probe, and much more robust to misapplied voltages than an active probe.
David Hess:

--- Quote from: G0HZU on July 11, 2022, 11:18:31 am ---
--- Quote from: David Hess on July 11, 2022, 03:33:11 am ---I thought Tektronix might have made a separate set of low-z probes for their early or late oscilloscopes that used feedthrough termination that include the t-coil termination to prevent the problem, but I have not been able to find them.  The only low-z probes in that category are the ones I mentioned which work with a 1 megohm input and have the termination built in.  I know those show no such glitch.

From what I remember, the t-coil termination sort of transforms the capacitive load into a resistive load.
--- End quote ---

That's interesting stuff, thanks! I'll have a play this evening with that t-coil circuit.
--- End quote ---

There is quite a bit of documentation now on the t-coil derivation.  Back when Tektronix first started using it, it was a big secret.


--- Quote from: tggzzz on July 11, 2022, 11:59:30 am ---With most scopes, including that majority with an internal 50ohm resistor banged in parallel with the 1Mohm input, the best that can be hoped for is to use an attenuator to reduce the effect of the 15pF+1m cable.

Lose gain, gain fidelity; that's always the story at RF. But hey, gain is cheap nowadays :)
--- End quote ---

Double terminating the cable would halve the end impedance to 25 ohms, so the series resistor would be half value to keep the same attenuation.  Is that such a big sacrifice?  That would make it ... 225 instead of 450 or 2475 instead of 4950 ohms?


--- Quote from: bson on July 11, 2022, 07:26:27 pm ---But the thing is to increase its impedance as a pure 50Ω Z0 probe is practically useless; there's almost nothing you can observe with it.  Put it on an XO µC pin and the oscillator stops.  On a 100Ω LVDS signal, and the transceiver faults.  On a device D+/D- USB pin, and the host wants to reset and reenumerate, and likely completely quarantines unless you're very quick to remove the probe.  On an ethernet RMII interface and it goes belly-up.  There are such extremely limited uses for it that its existence is mostly an academical novelty.
--- End quote ---

Mostly they were used for measuring ECL signals where the impedance would be 25 to 50 ohms from the 50 to 100 ohm terminated microstrip.  But the DC offset caused by a low-z probe was still a problem, so they had "offset" probes where the DC bias could be adjusted preventing interference with the level of the signal:

https://w140.com/tekwiki/wiki/P6230
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