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making an 20:1 coax probe
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G0HZU:

--- 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.
Performa01:
Quite obviously there are at least three different fundamental approaches when it comes to 50 ohms inputs:

* Providing no internal 50 ohms termination at all. Users are forced to use external through terminators. Depending on the actual input impedance of the scope, this might work if the requirements are low, but usually yields poor results. Btw, the popular solution with the BNC-T and external end-terminator works just as well in my experience. This is not too surprising, since the quality of the terminator becomes almost irrelevant in view of the input capacitance of the 1 meg scope input.
* Providing internal 50 ohms termination by a simple 50 ohms termination resistor switched in parallel to the input (by means of a relay). This can be as bad as the external termination. In any case it is far from ideal.
* Providing an internal 50 ohms termination with reduced input capacitance. This means more or less a dedicated 50 ohms signal path, but it’s up to the frontend designer how far that goes. Such a solution which avoids excessive input capacitance can provide a much better internal 50 ohms termination with reasonable VSWR.I happen to have some old measurements. See first screenshot.

RL_M-Pico_Ext_C-SDS_Int_Y-SDS_Ext

It shows the internal 50 ohms termination of an old low end 300 MHz DSO (cyan) and compares it to an external through terminator (yellow). Internal is a little bit better than external, but both are of very limited use. Here the internal termination clearly was just a resistor switched in parallel to the input without minimizing the capacitance.

There appears to be some industry standard, where a VSWR of 1.5 is considered acceptable for a 50 ohms input of an oscilloscope. This is equivalent to a return loss of 14 dB. Using this criterion, the scope in question barely reached 100 MHz with its internal termination and the external one barely exceededs 50 MHz.

As a comparison, the magenta trace shows the external terminator on a 200 MHz PicoScope (3000 series). This proves that this approach can actually work, as long as the requirements are low. Better than 12 dB return loss (VSWR < 1.67) up to 500 MHz is quite a respectable result for just an external termination. Since the input capacitance is 14 pF as well, there must be a simple means like a 50 ohms series resistor in the input to mitigate the reflections.

By contrast, a modern upper entry level DSO like the SDS2000X Plus or HD provides a much better input match:

SDS2354X+_SWR_200mV

The maximum VSWR is 1.31, which is equivalent to 17.45 dB return loss.

tggzzz:
I don't know why I've never noticed the series resistor before. Embarassing.


--- Quote from: G0HZU on July 10, 2022, 08:43:54 pm ---Here's a old plot comparing a basic model against the input of a Tek 465 scope when measured with a VNA. The plot shows parallel resistance and parallel capacitance. You can see the input is only 1Meg || 20pF below about 1MHz.

By 100MHz the parallel input resistance Rp has fallen to about 160 ohms. The model shows very good agreement with the real scope.

--- End quote ---

Good result; that't a good justification for my nanoVNA :)
G0HZU:
Yes, I think this stuff should be predictable as long as the scope input impedance vs frequency is known.

To give a demo, I've measured my old Tek TDS2012 DSO input with a VNA and produced a model for the input to match the VNA measurement. See the first plot below. I then put this model into Genesys (I could have just used the raw s1p data rather than the model) and then ran a time domain analysis using a pulse with a 2ns risetime and a 950R probe tip with a cable 1.2m long feeding into the TDS2012 but with a 50 ohm termination modelled at the input.


The second plot below shows the simulation vs the actual measurement on a fast logic gate that has about a 2ns rise-time. I used a probe with a series 1k ohm resistor and 1.2m of RG174 cable followed by a crimped BNC and an unbranded 50R through terminator and this was fed to the scope input on channel 1. I think the agreement is quite good. The other trace on channel 2 of the scope is a measurement using my probe that is well terminated with an SMA connector, SMA attenuator, SMA through termination and a SMA to BNC adaptor at the scope input. This looks a lot better.
G0HZU:
I loaded a different simulation engine into Genesys and this allows a real time analysis of the probe system. This simulator isn't as accurate as the previous transient simulator but it is really fast. Therefore, some people may find the simulation to be useful in understanding how the length of the probe cable and the capacitance of the scope affect the response seen on a scope.

Sorry, there's no sound but it's fairly obvious what is being done in the video. 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.

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