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

igor-m <mikki@...>

On the DDS pcb - the position H10-V5 - you may find the pads for a smd pot trimmer (not populated), wired to the VC pin of the TCXO. They apparently planned to have a pot trim there in order to find a sweet spot for the voltage tuning range..

igor-m <mikki@...>

Example: With R11 removed and the trimmpot soldered in (will be accessible from the top of the DDS board), for example 22k large, you may get following voltage ranges (the x-axis is the DAC output voltage from 00h to FFh, and the y-axis is the voltage at the CV pin of the TCXO):

igor-m <mikki@...>

I applied my TCXO mod above and it seems it works!
I removed the R11 short and soldered in a 10k smd resistor at the pads of the not populated trimp pot.
The 10k resistor is easily accessible now as it sits on "top side" of the DDS pcb when mounted in.
I also covered the TCXO and the pcb from the opposite side with 3mm piece of foam (against the "banana effect").

The measurement is as follows now:

Service menu:
HEX        FREQ 2LO
00            124.031.190
7F            124.031.640
FF            124.032.120

I get the exact 2LO frequency of 124.032.000 Hz with E6 hex.

While measuring at the dummy load (with TX RTTY method) I get my 21.200.000 with 89% set in OTH menu #51.

73 Igor

Steve W3AHL


Good job!  Glad you were able to get the REF in spec with a simple change.

Steve, W3AHL