Vince says:
> crystal loading and the si5351 settings for the crystal. This caused the si5351 to be slightly off frequency.
This applies to pretty much all si5351 builds, so may actually be pertinent to the forum.
The 25mhz crystals typically available are tuned on the assumption that there will be maybe 18pF
of capacitance in parallel with the crystal.? (It's in the datasheet for your particular crystal.)
The si5351 allows you to program a parallel capacitance across the crystal of 6, 8 or 10pF.
There might be another pF or so due to traces, but we're typically well short of that 18pF
So our reference oscillator might be a khz above the expected 25mhz.
This is corrected by, wait for it ....? the correction factor.
As described in a previous post.
I did a quick check.
The Etherkit correction factor is in parts per billion, not in hz variance of the vco.
So Mark's change was 1.170 ppm, not 1170/875000000 * 1e6 = 1.337ppm
So his error was 12.5% less than I had previously calculated on the assumption
he was using the si5351bx routines.
In the si5351bx routines as I wrote them, you simply the value of si5351bx_vcoa
Farhan has added a level of indirection to this,
his correction factor get's added to the 875000000 default for si5351bx_vcoa.
Jerry, KE7ER
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On Sat, Nov 3, 2018 at 11:13 AM, Vince Vielhaber wrote:
I looked at your project and see you're using the adafruit si5351 board. There was an issue some time ago and recalling from a hazy memory it had something to do with crystal loading and the si5351 settings for the crystal. This caused the si5351 to be slightly off frequency. Jerry may remember more about it as I believe he addressed it in the si5351bx routines. I know it affected the Raduino, I'm just guessing the Adafruit is using the same or very similar crystal due to cost and availability.
