LimeSDRmini1.2 - Trim reference clock frequency

I’ve got a LimeSDR Mini v1.2 that I would like to use at a reference clock frequency of 30.72MHz rather than the 40MHz it defaults to. For various reasons, I’d like to do this by trimming the VCTCXO using the DAC at IC9 on the board. How would I do this?

In LimeSuiteGUI under board controls, I see there is a VCTCXO (runtime) value that defaults to 180 and a VCTCXO (permanent) value that defaults to 65535. How do these two values correspond to the reference clock frequency? I’ve noticed that plugging in different values and clicking “write all” doesn’t change the CGEN Ref Clock frequency in the calibration tab.

Any help with this would be greatly appreciated.

The datasheet for the RTX5032A VCTCXO says:

Line Parameter Test Condition Value Unit
4.1 Control voltage range The nominal control voltage value is midway between the minimum and maximum (Note 4) 0.5 to 4.5 V
4.2 Frequency tuning Frequency shift from minimum to maximum control voltages 6 to 30 ppm

Note 4: VCO of 4.5V only applicable when Vcc of 5.0V is applied.

30 ppm of a VCTCXO with a Product Frequency Code of 40MHz is 1200 Hz and 6 ppm at 40 MHz is 240 Hz. I do not think that the current 40MHz VCTCXO can tune down to 30.72MHz, that would require a frequency variation of 9730000 Hz which at 40MHz is a 243250 ppm correction.

And IC9 is a DAC5311IDCKR 8bit, Single Channel, 80uA, 1.8V-5.5V DAC the output of which is limited by a resistive divider to a maximum output voltage of 2.538V at a guess to decrease the range and thereby decrease the granularity of the tuning to be a much finer resolution with only 256 possible output voltages from the 8-bits.

Maybe I’m wrong, but I do not think that your idea will work.