David GM8ARV-
Yes, the ADAFRUIT white page on their SI5351 board I linked to does elude to setting the frequency to 1ppm. I have found and knew it for years, "ZERO BEATING" to WWV doesn't work, because humans can't hear lower than 20-30 cycles depending on age and history with cannon fire.
For those who are not hams FLDIGI is a program for decoding various ham radio digital modes. It uses a computer sound card to display frequency versus time versus intensity (amplitude) in a waterfall display. It has audio frequency readout. Now before someone pipes up - "Well how do you know that's accurate?" We don't .. but what we do know is that WWV and it's carrier is accurate, by superimposing our NanoVNA CW mode signal at 10.0000 or injecting it into the same receiver using nearby antennas.. we can see the WWV carrier and the NanoVNA carrier and see the frequency difference on the waterfall display. In my case it's 110 cycles. WWV is also at 5.0000, 15.0000, 20,0000 and there is also CHU Canada available. So, these days we don't need a capacitor to change, or slug tuning.. Even the SI5351 data sheet specified a crystal in the 25-29 mhz region.. That's because somewhere we deposit a number in a register that aligns the accuracy of the output of the SI5351.. granted to enviromental tolerances, but for a $50 "gizmo" that is in use world wide, that's fantastic.. I started with a US Army Signal Corps BC-221-AF frequency meter, which was the "best you could get" when I got my 1st class Commercial Radio Telephone license in 1971.. Cause a Service Monitor was $1000's. Now go read how to use a BC-221 and we could meet and do better than commercial standards. Back then we'd use a heathkit oscilloscope and display the oscillator "standard" versus the unknown oscillator and produce a Lissajous pattern and using a twiddle stick, get the pattern to look like an "O" and try to stop it from rotating. In the Detroit area we were homebrewing 100.0 cycle PL''s and used a 5:3 Lissajous pattern against 60 cycles. Detroit Edison AC was our frequency standard..
Larry W8LM
Larry Macionski