Hi Martin & Fred,
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Thanks for the article links. I will dig into them later.
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At this moment, I don't have notch filters for the 5-30 MHz range that I want to measure. To quickly verify whether the IP is internal to the TinySA, I will try to increase its attenuation to see if the displayed IMD reduces.?
What I find hard to understand is why the measured OIP3 is 30 dBm at 20 MHz (
see graph), but progressively drops as the frequency pair is lowered, and finally reaching 16 dBm at 5 MHz. My expectation is internally generated IMD should be quite constant over frequency. On the other hand, if the external & internal IMD are combining constructively and destructively at different frequencies, as Martin has speculated, I would expect the OIP3 trace to be wavy. Or did I miss something?
Anyway, more investigation is required to understand the setup's eccentricities.
At the project onset, I thought it would take 1 month max to build a working active loop, but now it has already taken a year of my life ?. What a deep rabbit hole!
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73, Chin-Leong Lim, 9W2LC
