Re: #ubitx-help Strong Audio Tones in and outside audio receive passband #ubitx-help

Jerry Gaffke
 

 Adding a cap across the 25mhz crystal (or adjusting the loading capacitance in the si5352 initialization code)
and going into calibration and adjusting the bfo setting both do the same thing.
They move the BFO frequency a bit down, further away from the 12mhz crystal filter passband.
(I'm assuming this 16mhz harmonic is above the 12mhz harmonic, as most raduinos have it)

I'd recommend trying the latter, as it is simpler. 
But however you do it, transmit and receive audio will change, becoming higher pitched.

I think the ideal solution would be to move the 16mhz Nano oscillator by a few khz.
If we're lucky, we can add capacitance in parallel with it and move it well below 16mhz.
Unfortunately it will have to move pretty far, as that 3'rd harmonic is going from above
the 4'th harmonic of the bfo (at 48mhz) to below the bfo harmonic. 
Pushing the 16mhz crystal up in frequency is more difficult to do, perhaps a series inductor.

Replacing the 16mhz crystal entirely might be best.
The max spec for the ATMega328P is 16mhz, so if it's a significantly different freq it should be lower.
Hard to get in the dinky form factor used on the Nano, but other crystal package sizes would be fine
if you hot glue it to the board and use short wires, being careful to keep the crystal case insulated.

Mouser has their 449-LFXTAL002057BULK which is 15.625mhz in stock, quite cheap and relatively small.
Also 449-LFXTAL003555BULK which is 15.81mhz, somewhat bigger.
Mouser 449-LFXTAL012514BULK is at 16.02355, I doubt the ATMega328P would notice it being out of spec.
That last one looks best to me.

The ones I show above are all pretty big physically, you might shop for smaller

Another possibility is to shield the Nano as best you can, try to keep that 16mhz from getting to the radio.
Perhaps grounding the 16mhz crystal case is all that is needed.
I doubt there is much 16mhz floating around there except the at the crystal and the two ATMega328P pins
that go to the crystal.

Jerry


On Fri, Feb 2, 2018 at 06:10 pm, Ashhar Farhan wrote:
I suspect the 16 mhz crystal must have aged. There are two things you can try. First is go into the bfo settings and move it slightly so the spurs go away. Second is to add some capacitance to the 25 mhz crystal, about 10 pf. There is a third way too, which is to program the si5352 initialization code to enable a different load capacitance on oscillator. I havent tried the third method myself.
 

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