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How to Impedance Match (Compensate) 50ohm Coax from Hi-Z source to DSO

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say we have high impedance source signal. we want to probe with oscilloscope (15pF || 1Mohm input impedance) through 50ohm coax. and i have control to put parallel capacitor for compensation.

i tried to do compensation from a high impedance (1Kohm) source (from c1) for coax cable about say 3 foot (modeled by 3xRLC, is it right?) but changing compensation capacitor (50pF paralled to 1Kohm impedance) will come out with funny bode plot. i tried lumped RLC which give nicer bode attenuation, but...

1) how to properly compensate this?
2) is my coax cable model right? 0.1ohm, 0.5nH, 30pF per foot? (3foot = 3xRLC)
3) can i model coax with just one lump RLC network? (iirc i heard from one high speed transmission line guy here we cant)
4) any better idea or correction here?


Why not make a voltage divider across the high Z source,with 50 Ohms as the lower section of the divider?
Or it you want to be more high tech, make a multisection impedance matching attenuator (pad)?
Then just use the coax with a 50 Ohm inline through termination at the Oscilloscope end.
The cable will be operating at its characteristic impedance,so it should not need any corrections,other than for the fact that its loss will increase with frequency.
This will give better results than trying to correct for impedance mismatches with strange RC networks.
The downside is that your 'scope may not be sensitive enough to give a useable display.

Do not try to simulate a coax with L's and C's, you'll need too many of them to model work properly. Transmission line is not a low-pass filter. Use a proper transmission line component instead. I don't know about TINA but at least LTSpice has one. Matching requires that coax sees 50 ohms at either driving or load end (or both, the best case), otherwise there is no match possible.

If you care about DC loading only, then you could do an AC termination at scope end, i.e. 50 ohm resistor in series with a suitable capacitor for the frequency of interest.


Build yourself a simple FET probe. What sort of bandwidth do you need?


--- Quote from: Zad on March 31, 2012, 02:45:51 pm ---Build yourself a simple FET probe. What sort of bandwidth do you need?

--- End quote ---
this is somewhat relevant. fet opamp output, will feed signal to coax to dso. this is my OP question ;) except... 1/10x probe (9M divider) is my last option. i try find solution for 1X probing first. or maybe it is just impossible (i guess thats why 1/10X probe were invented) :( ps: actually my signal is already attenuated to some degree, so i dont think further attenuating it for the sake of coax is a better idea. i'll study further, thanks guys for the link, i will need time for that.

edit: forgot to answer, bandwidth.... 100MHz.


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