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Author Topic: RTL-SDR V3 as a RF probe? for signal tracing through a defective IC-78.  (Read 110 times)

ZMG3251

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I was wondering about using my RTL-SDR as a RF probe/spectrum analyzer - as my Oscilloscope only has a very limited BW of 20Mhz - and I want to check the PLL and up to 90Mhz.

I could connect a 100x oscilloscope lead to the SMA connector of the RTL-SDR v3 and use GNUradio or Gqrx to demodulate etc.

It would of course need AC coupling ?10pF? and more attenuation as I guess the input signal to the SDR needs to be around  -70 dbm

The input impedance of the RTL-SDR seems to be 3.3K Ohms when direct sampling, so I guess I would need an 64/1 impedance transformer (something like a efhw transformer)?

Did I miss anything? I googled and haven't found anything online.

Thanks for any advice/input - I'm a newbie to this a bit.


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G8FXC

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Re: RTL-SDR V3 as a RF probe? for signal tracing through a defective IC-78.
« Reply #1 on: September 23, 2020, 12:37:24 AM »

I don't see why it should not do the job provided that you adequately protect the input. I have done similar using an SRDPlay RSP1a as a spectrum analyser.

Martin (G8FXC)
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W9WQA

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Re: RTL-SDR V3 as a RF probe? for signal tracing through a defective IC-78.
« Reply #2 on: September 23, 2020, 06:37:52 AM »

no need to z match...you dont with a scope or dmm...
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W9IQ

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Re: RTL-SDR V3 as a RF probe? for signal tracing through a defective IC-78.
« Reply #3 on: September 23, 2020, 08:53:33 AM »

Scope probes rate their field level attenuation based on the assumption that the input impedance of the scope is 1 meg ohm. Since your SDR has a much lower input impedance, a scope probe will provide much greater attenuation than its usual 10 or 100 times rating

If the input impedance of the SDR is indeed 3.3 k ohms then a 10x probe will attenuate voltage by ~2728 while a 100x probe will attenuate voltage by ~3090.

You want to avoid transformers on the SDR input if at all possible. The scope probe or equivalent resistance should provide sufficient attenuation and isolation. You can add a DC isolation capacitor if you wish - select a value that provide less than 100 ohms of impedance at the frequencies you are measuring.

If you do not need to measure the voltages in the circuit under test and there are not too many competing signals, often simply getting a probe near the point of measurement, without making contact, will provide sufficient amplitude to confirm the presence of the signal.

- Glenn W9IQ
« Last Edit: September 23, 2020, 08:55:56 AM by W9IQ »
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- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

ZMG3251

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Re: RTL-SDR V3 as a RF probe? for signal tracing through a defective IC-78.
« Reply #4 on: September 23, 2020, 01:27:15 PM »

Thanks all - great info and advice! :)
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