we worked a lot on this issue during the last two weeks:
A megaset LNB was modified as decribed in cqDL 01/19. The crystal was removed and pin 14 of the mixer chip connected to one of the external connectors.
First I ordered a TCXO from farnell, 2.5ppm.
The TCXO was built on a PCB together with a voltage regulation, a 25 MHz filter and some decoupling caps.
The mid frequency was quite stable, but it jumped up and down for a few 100 Hz every second. I read about digital-TCXOs, so this one obviously is digital.
For the next try I purchased an analog-TCXO. This was also stable enough but now I got some random jitter for a few 100 Hz. The frequency moved up and down, like before with the other tcxo, but now it did'nt jump but was a continuous moving up and down.
So I gave up using TCXOs and purchased a 1-port GPSDO from sdr-kits. It was delivered yesterday. Again I added a 25 MHz filter (which also adjusts the voltage level to about 6 dBm, about 1.2Vpp). And also a voltage regulation so I can supply it with 12 volts (it needs 200mA at 5V).
The GPSDO was programmed to 25 MHz and connected to the LNB.
The frequency was stable, very stable (!), but the signal had a low frequency humm of about 10 Hz and significant amplitude. Still not usable for listening to QO100.
After some reading here in the forum I found an advise to alter the frequency by only a very little amount.
So I tried 25.000.001 002 003 ... and also 24.999.999 and so on.
With a setting of 24.999.999 MHz the signal was totally clean and stable like a rock.
This is my setup now and I am very happy with it.
best 73, Kurt