VCTCXO model and the issue with crystal aging - 2LO off the DAC range

igor-m <mikki@...>

Hi, as it has been discussed earlier I have been messing with the rather known potential issue - the TCXO's crystal aging.
The service menu REF hex value of FF does not tune my 2LO to the required 124.032MHz. Even with 100% OTH menu #51 I am still low by 5-10Hz at 21.2MHz.
I've made a model of the TCXO voltage tuning (based on the schematics in the Service Manual) and I've plotted the voltage range the 8bit DAC is producing at the TCXO's voltage control pin.The DAC is placed inside the HD64F2377 MCU.
It seems to me a potential solution could be a replacement of a resistor in the voltage divider - ie. the R10 with the stock value 22k (green) - after replaced by a larger value (ie. 27k, 33k) will shift the tuning voltage range such the TCXO will move higher with its frequency, imo.
Below the picture with the simulation.
PS: I would also increase the capacitor value there as the MCU's on-chip DACs are pretty noisy usually..
73 Igor

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