Re: a simple spur fix

Jerry Gaffke

We believe the audio tones on some rigs come from harmonics of the 16mhz processor clock
beating with harmonics of the 12mhz BFO.  Most rigs don't have this, since the 12mhz
ceramic resonator is not very accurate, can be off by 100khz or so. 
That, and us old guys have lost the ability to hear anything much over 3khz.

Could drive the processor clock from the BFO, near 12mhz.
Should not create tones when it beats against itself.
The si5351 can glitchlessly transition that BFO by 3khz if we wish to switch from
low side to high side BFO (to switch between LSB and USB, keep clk1 on the high side.)

The 16mhz crystal resonator on the Nano is extremely small, will be tough for some to remove.
Perhaps leave it in place, overpower one end of it with a wire from the 12mhz BFO.
So this fix might be a matter of just adding one wire, and some new firmware on the Nano.

The ATMega328P has an internal processor clock generator that can be selected,
though it is even less stable than the ceramic resonator, and thus the birdies unpredictable.
Might be worth trying, as no hack is required except to the firmware.
Will be required to use the internal clock at startup until the si5351 is programmed
if the BFO is to be used as the processor clock.

Moving the crystal filter frequency to 11.052mhz should work, maybe that will show up on v5.
But matching 8 crystals for frequency, replacing the existing 12mhz crystals, evaluating
the filter passband given whatever crystal parameters you happen to have, and adjusting
caps and impedances for the desired bandwidth and filter ripple may be more than
most here want to take on.

If you have a good inspection microscope and no coffee, might be possible to swap
out the 16mhz ceramic resonator.  Any funny number for a frequency is likely to work
better than what we have.  We think we have trouble with 3*16 = 48mhz = 4*12.
If the processor were moved to 13mhz,  the first beat note I see is at 13*16= 208mhz = 16*13.
Those high harmonics will be very weak, won't get much better than that.
Remember to also avoid beats with clk1, at 45-12=33mhz and 45+12=57mhz.

Jerry, KE7ER

On Sat, Dec 8, 2018 at 04:57 PM, <freefuel@...> wrote:
the Atmel AVR chip can be clocked up to 20MHz, with that said what frequency would be best if we were to replace the crystal on the Arduino board to shift that birdiey to another less offensive location. 

-Justin N2TOH

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