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I've become very accustomed to the waterfall display on my SDR, especially for digital modes over HF. I'm thinking about tying into the IF on my IC-718 and connecting to an SDR to add a waterfall to the Icom, but I hesitate.

The electronics work isn't the issue. My hesitation is with regard to what hooking into the IF could do to introduce noise into the IF. Hence, my question: What sort of impact would hooking into the IF signal with a short piece of coax have on the overall behavior of the radio? Are there things that could be done (adding an RF transformer perhaps?) to induce the signal into another wire rather than tying in directly?

My research is telling me that I'd want some sort of a high impedance buffer. What exactly would that be? Something like an optoisolator?

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An optoisolator would not likely accomplish what you're wanting here (relatively low feed impedance, with not-so-great linearity).

Op Amps seem to be the generally preferred implimentation for buffer amps now; something like this should work for you:
Op Amp IF buffer amp

Alternately, if you don't want to directly connect anything to your IF line, you could use a close-coupled parallel wire (like in your SWR meter) to sense the IF signal, then put it through 1-2 amp (LNA) stages before feeding to your SDR.

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  • $\begingroup$ Strangely, I have another question asking about using this design where I'm being told this is absolutely the wrong way to do it. ham.stackexchange.com/questions/6328/… Can you shed any light? $\endgroup$
    – David Hoelzer ♦
    Commented Jun 21, 2016 at 23:20

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