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Old 5th Feb 2019, 10:23 am   #8
MrBungle
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Join Date: Jun 2016
Location: London, UK.
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Default Re: Looking to build RF detector probe for low frequency

Just a couple of warnings here as I've gone down this route too and burned myself a couple of times. Any measurement device will load the circuit in some way. In the case of the probe there, any stray capacitance will drag your IF measurement off and/or reduce the Q of the circuit being measured. This may or may not be an issue but it's worth remembering it.

Since discovering this I tend to adjust IF shape by injecting noise into the front end of the receiver from a noise source and using an audio FFT on my smartphone to shape it. Surprisingly effective!

I am using a clone of the noise source here, without the bridge (everything after C4 removed) to do the job: https://qrpguys.com/wp-content/uploa...ide_120616.pdf ... this is loosely coupled to the antenna with a bit of wire wrapped around it usually.

On the probe front, your standard scope probe would have 15-20pF of capacitance across it. I would build the probe as is if you want to go down that route.
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