On Feb 12, 2024, at 14:13, Dean Souleles <dsouleles@...> wrote:

?Hi all -

Seeking wisdom (to borrow a term from FFT lore) from the crowd about a frequency problem wiht my Homebrew SBITX.??if you happened to listen Soldersmoke #250 I talked about it there and I'll illulstrate it here.?

First I should explain the diffferences between my build and the HF Signals build.? My digital board consists of a Raspberry Pi 3b+ and a WM8731 codec prototype board from .? My SI-5351 an Adafruit style module which does not have a precision tuned crystal referency oscilaltor and therfore it requires a crystal calibration offset.

First issue - I zero beat 10MHz WWV and used the displayed value to calculate the crystal calibration offest I needed for the frequeny display to be exactly 10MHz.? After adjustging the offset the radio is perfectly alligned at 10MHz.? To linearity I set the frequency to 15MHZ and found the WWV center frequency was approximately 200Hz too high (15.000.200). I then tuned to 5MHz and the center frequeny was approximately 200 Hz too low (4.999.800).? ?Pictures below (signal in the picture is from a signal generator, not WWV, to make it easier to see.

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Does anyone have any thoughts on why the display frequency should vary so much from the tuned frequency and why the delta would vary with the HF frequency?

Second issue - in testing the codec prototype board stand-alone (that is disconnected from my analog receiver),? I injected a 24KHz sine wave into the line-in and expected to see a peak at exacty 0 on the waterfall display,? I did not. The waterfall shows the peak about 600Hz below the waterfall center point.? In discussing with Farhan the best guess is the audio prototype board crystal reference is off frequency.? ?I'm going to try a different card to see what happens,

Would appreciatte any thoughts on either of the above.

73,
Dean
KK4DAS