Good work Oliver. I¡¯ll try to duplicate this test tomorrow?
dave
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On Mar 23, 2025, at 7:36?AM, Oliver KI3P via groups.io <oliver@...> wrote:
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I found that the 25 MHz and 30 MHz bands oscillate, but the lower bands don't, matching what others have found.?
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To figure out why I measured the insertion loss from the output of the PA to the input of the PA. i.e., I connect port 0 of my NanoVNA to J8, and port 1 to J7. What this measures is the magnitude of the feedback loop.
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What I found is that this is a lot higher than it should be, particularly at the higher bands.
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| 7 MHz | 14 MHz | 21 MHz | 24 MHz | 28 MHz |
|-------|--------|--------|--------|--------|
|<-80dB | -63 dB | -53 dB | -48 dB | -47 dB |
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I would expect roughly 40dB of insertion loss each through the T/R switch and the BPF selection switch U8. Clearly, I'm not getting that. It seems that the feedback amplitude is getting high enough at the higher bands to cause oscillations. What I would like to do next is:
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1) Repeat this measurement for a known-good version of the code as identified by Jerry. Is this feedback path amplitude different?
2) Find the reason for the change in the feedback amplitude path by examining the LPF control signals between known-good and known-bad code.
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Unfortunately, I am tied up for the week and won't be able to do this until next weekend.
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