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To calibrate the si5351, just repeatedly adjust the value of the calibration variable si5351bx_vcoa,
until the uBitx is operating at the correct frequency (or you measure one of the three output clocks to be correct).
You must call si5351bx_setfreq() to update the output so the new si5351bx_vcoa value can take effect.

The above method says how to calibrate iteratively, without any math.
Alternately you can calculate the correct value for si5351bx_vcoa.
That variable is nominally 875000000, which is the frequency of the internal oscillator, 875.0 mhz.
All output frequencies (and the operating frequency of the uBitx) are strict ratios of that internal oscillator.

Example:?
Assume you are just starting out, and si5351_vcoa is at the nominal value of 875000000
You tell clk2 to be at 30mhz with? ? ?si5351bx_setfreq(2, 30000000);
But your frequency counter says clk2 is at? ?30.001000mhz
That means that the internal oscillator is actually? ? 875mhz * 30.001/30.000 = 875.029167mhz
Adjust the si5351bx_vcoa value to be 875029167, and the next call to si5351bx_setfreq() should be dead accurate.

In my own Raduino code I adjust si5351bx_vcoa directly, and save it to EEPROM when happy with the calibration.
Farhan added a level of indirection to this, his routine si5351_setcalibration() adds an offset to the nominal 875000000 value,
and stores that offset to EEPROM.

There have been lots and lots of posts in this forum about trouble getting the si5351 calibrated.
Which suggests to me that all available firmware out there is doing this wrong.
See posts?35235, 54501 44278, 44515

Regarding the crystal loading capacitance:
The crystals are manufactured to have a parallel resonant frequency that is on target
when there is a specific capacitance (perhaps 20pf) across that crystal due to?
traces and other parts connected to the crystal (parts such as the si5351).
The Raduino apparently has significantly less to 20pf,
so the outputs of the si5351 are consistently too high in frequency by a few hundred ppm.
See posts? 35275, 61287.

Jerry, KE7ER
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