Frequency Stability of the QO-100 NB Transponder

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:

http://leobodnar.com/files/Mic…eobodnar%20GPSDO_rev2.pdf

http://leobodnar.com/files/Inf…Frequency%20Reference.pdf

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

pe1hzg fully true. The example given above is a best-case scenario as TX and RX drift are closely correlated. If I listen to somebody else's signal the two drifts are uncorrelated and the worst case can happen (according to Murphy's law, it will..) with both drift having opposite signs and the result would be the sum of the two. So something of the order of 1.2 Hz/sec.

Again, this is for a 1E-10 oscillator at constant ambient temperature. If you open the window in the shack to let in some fresh air, hell will break loose. I have about half a dozen GPSDOs of different grades sitting here in my lab and you can clearly notice it.. Ulrich Bangert has described in his article linked above that he has placed his OCXO in a aluminum vault with 20mm (!) wall thickness. This is quite some kilograms of mass and therefore acts as a thermal lowpass. Do not try to pack the OCXO in an insulating box (styrofoam) as the oven will need a certain heat dissipation to actually work. The aluminium vault is actually great as the thermal conductivity is low but the thermal mass is high.. my best double oven HP10811 is placed in such a vault and then on top equipped with its own UPS to keep it running even in case of a power failure. Uptime 8 years..

G8UGD it's hard to give a clear answer but if you think about how the oven internally works, it might give a better understanding what can be done from outside to help the temperature controller.

Theoretically to keep the crystal oscillator at any given temperature, the controller would heat if the target is too cold and cool if the target is too hot. Heating is easy but for cooling the control loop requires that the energy has to go somewhere (no active cooling) so we need some way out.

The prime reason for anybody to isolate his OCXO is to separate the temperature variation from the outside from the device. That unfortunately means that heat cannot go out anymore as well thus changing the control loop time constants for both heating and cooling. It probably will still work (operating temp according to datasheet up to 70 degC) but the sweet spot is 25degC.

The reason for DF6JB's proposal to use thick aluminum walls is to keep the thermal conductivity to the ambient (aluminium being a good heat conductor) but averaging out any short term effects like opening the window or even the door. The total energy change will be the same with or without thick walls but the slope will be softened by the thermal capacity of the aluminum. Ideally, the slope will be slow enough so the OCXO temperature controller can fully compensate for it without the user noticing. Without this thermal capacity the temp controller might be just too slow and not be able to catch up resulting in a larger delta-T at the crystal and hence a delta-F.

Does this answer your question?

Quote from G8UGD

I do wonder how to check stability of any osc for both accuracy/drift over a period of time, is there an ultra-stable signal on HF that one can lock and compare to or any software designed for the measurement one can get gold of easily?

No. We receive here DCF77 from Germany and due to variations in the propagation, its stability is only of the order of 1E-9 on short timescales.

DD0CW any earth orbit is far from ideal: perturbation from the moon and sun, radiation pressure from the sun and non-spherical gravitational field of the earth itself (not a round sphere but more a pear). That is why any satellite operator has to correct the orbit every 2 weeks or so with tiny corrections to stay in it's designed orbital position. I think a 50 km box is the allowed space as staying too precise on a given spot is just too expensive in terms of fuel. And that is why a tiny amount of orbital wiggle around the perfect position is allowed. The level of doppler is usually not noticable (DVB-S2 couldn't care less) and therefore not an issue for commercial operators.

DD0CW ideal: correct, no doppler.. YES

radiation pressure is NOT random but has a 24h period, as the sun position relative to the satellite varies during the day. Over the course of one day, sun and moon position relative to earth stay more or less constant and therefore all is dominated by the 24h orbit period of the satellite. One the other hand, sunlight is VERY constant as the tv satellites are almost in constant daylight due to their distance of the orbit to the earth (6 earth radii) and the tilt of 23.5 deg of the equatorial plane to the ecliptic plane. Only around start of spring and fall when the intersection of both planes is aligned with the sun-earth direction the satellites have a some eclipse time. So rest assured, the satellite internal temperature is rather constant and on top of that the transponder LO is actually very stable. Cannot say more unfortunately

DD0CW about the observed 24h period: I would guess most of it is doppler. Neither is inclination exactly 0 deg (as of now, it is 0.01 deg), so this results in an apparent North-South movement over 24h. Next is the non-circular orbit, it is slightly elliptic. Perigee is as of today 35789.0 km and Apogee 35799.5 km. So this results in a West-East movement as part of the orbit is slower than perfect and another part is faster than perfect. And on top of this, the different heights result in a up-down movement of course, hi.

You can actually have it calculated.. just load the TLEs in any satellite tracking program. Take into account, that the beacons are generated on the ground so there is actually the sum of uplink doppler (on 2.4 GHz) plus downlink doppler (on 10.5 GHz).

pe1hzg No NDA this time: I don't know.. but for commercial transponders, they can actually do geolocation of any uplinking stations to a scary degree of precision.. maybe it's linked to that.

I had a look on the RFzero.. I think we established before that 99% of the users have LOs inferior to those used on the Bochum ground station or inside the QO-100 transverter. The RFzero is no exception here as this is a simple SI5351. Don't expect any wonders from a 1 Eur device.. GPS disciplining will only change it's long time behaviour, not short term. But even a 1kEur GPSDO might not work, this has been discussed before.

Path irregularities will be an issue but this can only be overcome with a communication mode which can handle it.