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Cable tester/TDR strange result

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jmw:
I made a signal generator based on W2AEW's 74-series oscillator design. When I tested it with a length of 50 ohm coax, the results were as expected. The voltage just about doubles on the second step up, so the generator is a good match to the cable.



When I terminated the cable with a short, the voltage levels change. If the signal initially sees an load impedance of 50 ohm from the coax, the voltage across a 50 ohm load should be about 1.65 V, and it can't "know" about the short until the wave reflects back, why is the initial voltage now around 2.2 V?

dmendesf:
Show us the circuit... But if the initial condition is different it's probably because a DC current is passing thru the 50 ohms.

jmw:
Sure. This is what I used for simulation. The inverter is 74LVC14A. R6 is the cable load, and R7 + C4 are the oscilloscope input impedance.

tggzzz:

--- Quote from: jmw on January 19, 2020, 02:20:55 am ---Sure. This is what I used for simulation. The inverter is 74LVC14A. R6 is the cable load, and R7 + C4 are the oscilloscope input impedance.

(Attachment Link)

--- End quote ---

Oh come on, give us all the information: what's

* Rs?
* the output impedance or invert14a?
* decoupling capacitors?
* physical layoutThe latter two are critical.

Please say you didn't use a solderless breadboard; if you did, you'll need to include parasitics in your simulation.

magic:
For the record, here's the setup you should be using:
generator - short cable - BNC tee on the scope - long cable - optional termination

Also, the period between pulses should be at least two propagation delays long, for the whole length of cable to settle at 0V before another pulse.

If you can get these two waveform from the above setup by simply switching on/off a short across the free end of the cable, I would guess that you must have been visited by aliens :-//

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