Products > Test Equipment
Inexpenive 50 Ohm Feed Through Terminations
langwadt:
--- Quote from: vk6zgo on July 16, 2019, 04:39:49 pm ---
--- Quote from: TimFox on July 16, 2019, 01:08:46 pm ---An unterminated 20 dB attenuator will show a return loss of 40 dB, with a complete reflection at the other end but 20 dB attenuation each way.
--- End quote ---
I do believe that is exactly what I said happened under test.
For most practical purposes in RF work, a return loss of 40dB is "near as dammit" to a correct termination.
One would assume that the return loss of a 20dB attenuator in cascade with a 50 Ohm termination connected to an oscilloscope with an input impedance made up of 1MOhm resistance in parallel with 16.5pf would be 40dB better than with the termination alone.
The OP seems to get a best figure of around 40dB
--- End quote ---
an unterminated PI attenuator (61.11R,247.5R,61.11R) is a ~51R load, which is ~40dB return loss
tautech:
The Tek pads Chris used are listed in the attached pdf
bob91343:
My main criticism is that I can't understand all of the discussion. I guess my ears are getting old.
Bottom line, since most of my work is below 100 MHz I guess I can stop worrying about termination quality.
I have a home brew 50 Ohm termination that I use sometimes with a T connector and it works reasonably well also.
ejeffrey:
--- Quote from: graybeard on July 15, 2019, 10:19:02 am ---Then I do review a low-cost (2 for $10) feed-through termination and compare it to ones from Tektronix and HP including measurements with a network analyzer.
The results are interesting and not what you might expect. The low-cost ones and expensive name brand ones all perform the same when connected to an oscilloscope input.
--- End quote ---
This isn't that surprising to me. I don't doubt that there are plenty of ways to screw this up but it isn't like they are magic. The performance of a feedthrough terminator and its advantage over a tee is mostly due to geometry.
graybeard:
--- Quote from: David Hess on July 16, 2019, 01:19:11 am ---Is it possible that at 100mV/div and lower, the signal level from the VNA is triggering the oscilloscope's shunt input protection? If there was really that big a difference, it should show up as a change in probe compensation between 100mV/div and 200mV/div and if that happens, the oscilloscope is broken.
--- End quote ---
No the input shunt protection was not activated. The VNA 10 MHz waveform only exceeded the screen limits on 10mV/div and and more sensitive settings. There was no change in the measured input impedance from 500µV/div to 100mV/div. There was one tiny change above 200mV/div I noted in the video, there was an associated relay click when it happened.
--- Quote from: David Hess on July 16, 2019, 01:19:11 am ---Older oscilloscopes with 3 or even more high impedance input attenuators perform better.
--- End quote ---
This is not always true. I tested the the inputs to my 200 MHz 7A26 plug-in on my Tektronix 7904A 500MHz CRO and it was worse than the Siglent SDS1104X-E since the 7A26 input capacitance is 22pF vs. 15pF for the SDS1104X-E. The 7A26 measured input impedance with the feed-through termination varied a bit on almost every input attenuator setting.
I omitted the measurements on 7A26 (200 MHz 1MOhm||22pf) and my 7A29 (1GHz 50Ohm) from the video to keep the run-time reasonable.
Chris
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