I have heard several reports about OMs struggling to reach the advertised speeds for the higher VARA speed levels. As people were suggesting the stability of the transponder LO frequency could be a problem I decided to investigate the required stability parameters a bit more.
According to the specification by the developer (Jose, EA5HVK) VARA is based on an OFDM with modulations between BPSK and up to 32QAM for the higher speed levels. See also his blog post for more details. He also states in his VARA specification document that short term frequency stability for both TX and RX should be <0.5 Hz/sec.
Many OMs have their ground setup fully locked to a 10 MHz GPSDO. While this seems perfect at first sight, I would like to remind everybody about the difference between stability and accuracy. Ulrich Bangert (DF6JB, SK) has written an excellent article about it (german only). A very popular GPSDO used in the QO100 community is the Leo Bodnar GPSDO. Several measurements suggest, that the stability at 1 second intervals is of the order of 1E-10:
1E-10 (@ 1s) is quite a respectable number for such a low cost device, but is it enough for the VARA requirements on QO100?
At 1E-10 any carrier generated at 2400 MHz will have an instability of 0.24 Hz in 1 sec. The downlink is on 10.5 GHz and assuming a perfectly stable transponder LO, this results in a downlink drift of 0.24 Hz here as well. However, the LO for the RX chain is also locked to the reference and should therefore drift by 1.05 Hz. The difference of the two (1.05-0.24=0.81 Hz) would be visible on the downlink as drift in a 1 sec time interval. As this is larger than the required 0.5 Hz a degradation in performance is to be expected.
Disclaimer: I cannot and will not give any technical details of the QO100 transponder due to NDA. But I think it is obvious that any space-qualified transponder oscillator will have very good stability values so for the sake of simplicity we assume it is non-significant compared to the numbers above.
The solution would be now to use a more stable (not necessarily accurate!) reference oscillator. James Miller G3RUH has an example with (now obsolete) hardware. In a few words good stability on short timescales requires (among other things) good thermal stability of the 10 MHz master oscillator. On short terms the GPS control servo is (should be) non-effective and a good free-running OCXO might be even better (again: stability vs. accuracy). This probably means something like a HP10811 (which many still regard as one of the best references available for hamradio) or even one of the double oven HP10811 versions used in the Z3801. They reach 1E-11 and below. But they also need to be kept running 24/7 as letting them reach thermal equilibrium may take a few hours..
73s Achim, DH2VA