On Sat, Oct 12, 2024 at 12:11 PM, Everett N4CY wrote:
There is no way to run a proper IMD without a band stop filter that rejects the 2 tones and a band pass filter for each of the tones.
A decade back, I made use of an old tube-type Stoddart NM-22A Field Strength EMI/RFI receiver for this purpose, as, a tuned RF front end serves the purpose of fundamental filtering or notching. Beware using broad-band SDR receivers for this application w/o taking similar precautions (of course) as one would for a broad band Spec An.
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Anything as broad in response as a spectrum analyzer is going to require assurances (such as outboard LP/HP/BP filtering) to assure the Spec An is not overloaded, or its dynamic range is not exceeded.
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I was running tests on an active antenna mounted on a car. The 'antenna' (wire) was about 30 inches and driving the gate of an JFET. I could vary the bias point and observe 3rd order distortion products as created by several local AM radio stations in the area? ... different circuit configurations were tried, including buffer stages following the JFET input amplifier/Source follower stage. Sorry, no detailed notes were taken of this quick proof-of-concept testing.
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de AA5CT Jim
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