#76: Measuring RF Impedances WIth an Oscilloscope

[@aduedc]

Nice presentation. Thanks, I learned a new method for measuring impedance. Here how to measure phase difference automatically with out he need for oscilloscope. Use a fast clock and count V2 from raising edge to raising edge with the help of zero cross detector and a counter. Use a fast clock and count from raising edge of V1 to raising edge of V2 with the help of zero cross detector and a counter. Divide the second # by the first # and multiply by 2PI, and this is your phase. Measure V1 & V2 using a peak detector and A2D converter and do the last two lines.

[@w2aew]

Here's a link to a video that I did that shows this technique using an analog scope: kzbin.info/www/bejne/m4qxaJeemdlketk

[@jacobytheo1766]

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[@lianellis2943]

@Jacoby Theo instablaster :)

[@jacobytheo1766]

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[@jacobytheo1766]

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[@lianellis2943]

@Jacoby Theo happy to help :)

[@galileo_rs]

From what I have seen, most single port VNAs work like this so this could be easily be titled VNA basics or something like that. Very nice presentation, as usual.

[@w0qe]

Dragan, You are correct that many (maybe most) of the little VNAs work like this. However as I mentioned near the end of the video other circuits such as a dual directional coupler could also be used but the detection circuits would need to be more sensitive. I struggle with titles for videos all the time. The title can't be too many words and I want it to be understood by most folks, I don't want it to be "click-bait", etc. Glad you enjoyed the video. Larry, W0QE

[@johneslaughter]

I enjoyed the video, thank you.

[@w0qe]

Glad you enjoyed the video John. Larry, W0QE

[@tomthompson6911]

Larry, this is a good video, and I think it will be very useful for most people. I guess the same thing could be done in Excel, but you don't have the nice Smith chart visualization about how close the calculation is. Anyway, this was one of your best.

[@w0qe]

Tom, Yes, the complex math help could come from SimSmith or several other programs like Excel, Matlab, Mathematica etc. but you're right that having the Smith Chart visualization is really nice. I have done the complex math manually many times (which is good practice) but it is very easy to make mistakes. Thanks for the comment. Larry

[@fishyrider3528]

Can this method be used to mesure the unknown impedance of a receiver at a specific freq? In order to build an antenna for it?

[@w0qe]

This technique can be used to measure the RF impedance of any circuit but with an oscilloscope being the detector the voltage levels will need to be fairly large and you need to be careful not to damage the front end of the receiver. S9 is typically a signal of about 50uV so a voltage of 100mV would be S9+66dB and 100mV will barely be enough unless you have a pretty good scope. A receiver that is mismatched to the antenna + feed line will suffer what I have mentioned as mis-match loss in previous videos. The mis-match loss for a 3:1 SWR is only 1.25dB so little is to be gained in the HF region where band noise dominates receiver noise. At higher frequencies this is not the case but you should probably be most interested in the input impedance of a receiver with the pre-amp enabled and again you need to be careful not to damage the receiver. Larry, W0QE

[@dl9ar245]

I setup the circuit, pasted the the first line of the calculation Wave("V1pk", V1, V); but got the error"Wire, variable, or component already declared or built in:V1". What am i doing wrong here?

[@dl9ar245]

Problem fixed after upgrading to Version 16.6, thank you.

[@w0qe]

I see that you got the problem fixed. This can be a little confusing since many of the parameters are optional. Look at video #69 where I went through many of the options. Larry, W0QE