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making an 20:1 coax probe
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David Hess:

--- Quote from: G0HZU on July 10, 2022, 08:59:40 pm ---
--- Quote ---My fastest oscilloscope with an internal switched termination is only 300 MHz and I have never seen any extra aberration from using a 50 ohm source or probe interacting with the input capacitance.  There is something there, but a fast high impedance probe shows the same thing.
--- End quote ---

A typical shunt 15-20pF capacitance from a scope input will cause quite a discontinuity up at VHF. At the tip end of the cable is a 950 ohm resistor so the cable is therefore mismatched at both ends up at VHF. This means the initial positive edge of the pulse hits a shunt capacitance at the scope. This is a lower impedance than 50R so there will be an inverted reflection that lasts only a brief amount of time as the capacitance charges up. This inverted blip will travel back up the cable where it hits the 950R resistor. This huge mismatch causes nearly all of the blip to reflect back but this time it will stay inverted as 950R resistance is much higher than 50R so there is no waveform inversion with this reflection. If the cable has a delay of 5ns then after 10ns from the rising edge of the pulse the blip will arrive back at the scope input where it will put a little dent in the top edge of the waveform about 10ns after the rising edge seen on the scope just as in the spice simulations.

This all assumes the pulse has a fairly fast rise-time. I'd expect a rise time of 2-3ns to cause visible artefacts on a typical 200MHz scope. The Pico scope only has 13pF input capacitance so maybe it won't be as affected as some other scopes that can have 15-20pF input capacitance.
--- End quote ---

Has anybody actually observed that with a real low-z probe when a feedthrough termination is used?

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.
Performa01:

--- Quote from: G0HZU on July 10, 2022, 11:33:36 pm ---This should be a lot more insightful than looking at a few plots from a simulator. You can see I tinkered with the rise-time of the pulse and the length of the probe cable and the capacitance of the scope input. I also added a variable attenuator at the input at the end of the video.

--- End quote ---

Thanks a lot. This is very enlightening indeed!
Marco:
Does the Douglas Smith version with termination at the probe side perform better? (at the cost of more noise.)
tggzzz:

--- Quote from: David Hess on July 11, 2022, 03:33:11 am ---
--- Quote from: G0HZU on July 10, 2022, 08:59:40 pm ---
--- Quote ---My fastest oscilloscope with an internal switched termination is only 300 MHz and I have never seen any extra aberration from using a 50 ohm source or probe interacting with the input capacitance.  There is something there, but a fast high impedance probe shows the same thing.
--- End quote ---

A typical shunt 15-20pF capacitance from a scope input will cause quite a discontinuity up at VHF. At the tip end of the cable is a 950 ohm resistor so the cable is therefore mismatched at both ends up at VHF. This means the initial positive edge of the pulse hits a shunt capacitance at the scope. This is a lower impedance than 50R so there will be an inverted reflection that lasts only a brief amount of time as the capacitance charges up. This inverted blip will travel back up the cable where it hits the 950R resistor. This huge mismatch causes nearly all of the blip to reflect back but this time it will stay inverted as 950R resistance is much higher than 50R so there is no waveform inversion with this reflection. If the cable has a delay of 5ns then after 10ns from the rising edge of the pulse the blip will arrive back at the scope input where it will put a little dent in the top edge of the waveform about 10ns after the rising edge seen on the scope just as in the spice simulations.

This all assumes the pulse has a fairly fast rise-time. I'd expect a rise time of 2-3ns to cause visible artefacts on a typical 200MHz scope. The Pico scope only has 13pF input capacitance so maybe it won't be as affected as some other scopes that can have 15-20pF input capacitance.
--- End quote ---

Has anybody actually observed that with a real low-z probe when a feedthrough termination is used?

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 ---

Tek 485 5ns/div with an HP10020A probe using a spear at the source (<500ps risetime).

First picture is with the 485's internal 50ohm attenuator. The inductive blip at the 3rd division indicates the probe cable length.

Second picture is with the 485's 1M input and an external 50ohm through termination.

Interpretation is left as an exercise for the student.


iMo:
The internal wire in the oscope probe's coax isn't made of copper, but it is a higher resistance wire usually, something like 200ohm, afaik..
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