Thank you @Ronald , unchecking "Flatten" did help against the low-frequency peak in the spectrum, so that turned out to be a separate problem.
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The lack of image rejection remains. Some more research on that: I connected a signal generator (tinySA) to the QDX input, and tuned it near the reception frequency. With an oscilloscope I measured an audio-frequency signal at both sides of C58 and C60, with a 90 degree phase shift between the two capacitors.This indicates that T2 is installed correctly. I also confirmed that the audio signals reach the ADC chip.
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In IQ mode with a sound card oscilloscope program, I see a nice sine signal in the right channel and zero in the left.
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Two possibilities come to mind:
- the ADC chip is broken so the left channel is dead
- some pins of the ADC chip are not properly soldered so that the left channel doesn't work
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I'm considering trying to re-solder the ADC chip by dragging a ball of solder along the pin rows with a soldering iron, then use a solder wick to clear any bridges created.
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Did anyone else run into this problem?
For the future, would it be possible to add a test in the firmware showing the two audio channel amplitudes?
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Fredrik (OH1HSN)