Posts by OZ2M

    Indeed accuracy and stability are two separate things. Here is a very good page that describes and shows the differences: https://www.febo.com/pages/stability/


    Yes, the loop bandwidth impacts the performance. What I wanted to highlight, with the remark, was that the role of the earth stations' LO stabilities may be greatly overvalued when not looking at the total system end-to-end performance. From where I sit the latter seems to be downplayed or overlooked. The concern, that sparked my original post "High speed data use", was an understanding that a stability of at least 1e-12 was required. Later I found out that this number was over 10 000 s =:~|


    Thus there are several things to (re-)highlight

    1. 1e-12 is very GOOD, higher is adequate
    2. looking over a period of 10 000 s is nonsense for high speed data communication in the light of both 1/Tsymbol and http://g4jnt.com/QO100_Stab.pdf The "system" and decoder should be good enough, already, to sustain such a time frame otherwise they should be scrapped
    3. short term stability is important with respect to FFT bin size, 1/Tsymbol, e.g. if Tsymbol is 10 ms -> 100 Hz am effective LO wobble of 5 Hz at 10 GHz should not be a problem for a robust modulation with more than 1/Tsymbol tone spacing and a properly designed decoder
    4. a suitable modulation has to be used for the purpose, see
      and the attached comparison of FT8 and FT4 that clearly shows that FT4 is more robust than FT8 which is relevant for QO-100 use (I am not saying that neither FT4, FT8 nor PI4 should be used for QO-100, far from actually). It is all about Link Probability
    5. don't expect to use a narrow band and shoulder-to-shoulder tone location at 10 GHz
    6. using HF designed modulation(s) in the 10 GHz range is FAR from optimal irrespective if this is PACTOR IV or WSPR. Decodes may happen but Link Probability will be low even if the signal is loud especially due to path irregularities
    7. a balanced approach will be much more cost effective that an over-engineered and over-spec'ed earth station LO
    8. it all boils down to the end-to-end performance

    I don't think moving my original message into an existing thread of LO stability discussions was a good idea. It downgrades the issue of "High speed data" use to be only a question of the earth stations' LO stabilities, but, what I want to highlight is quite opposite.

    "But even a 1kEur GPSDO might not work" and "Path irregularities will be an issue but this can only be overcome with a communication mode which can handle it." - these are exactly the points I want to emphasize.


    TELE Greenland has some 8 GHz radios links at 10 Gb/s over 50-60 km but uses 8 x 56 MHz.


    The Si5351A accuracy and stability is as good as its clock which might be anything from a sloppy crystal to an optical clock.

    Recently I was contacted about transmitting "high" speed data, PACTOR IV all SL#, over QO-100. The assumption was that if only the LO was stable enough, i.e. 1e-12, SL10 would be possible since until now it failed. So the quest for a stable LO was going on and perhaps an RFzero https://rfzero.net/ could be used.


    However, I was a bit skeptic whether the stability of the LO was the only reason. Things like the LO stability of the QO-100 and path irregularities, both up- and down-link, impact the performance. I mean there is no reason for having a super duper ultra accurate low phase noise earth station LO if the rest of the system entities are orders of magnitudes worse. A balanced approach is much more cost effective.


    So for those interested in "high" speed data communication over QO-100 I suggest running some long distance terrestrial experiments. Start at 13 cm and then move to 3 cm if 13 cm works. Please also study the article ”A Measurement of Frequency Accuracy and Doppler of the QO-100 Satellite Transponder and Beacon” by Andy. G4JNT: http://g4jnt.com/QO100_Stab.pdf The stability of the QO-100 beacon vs time is very important and, together with the path irregularities, are beyond the earth stations’ capabilities.


    In 2018 I made a presentation about digital communication at the RSGB Convention: https://www.youtube.com/watch?v=jFcCncceSN4 it is about PI4 but the principles are the same. There is no free lunch when it comes to sensitivity, speed, robustness and flexibility. So yes, you may have a somewhat "high" speed but only while sacrificing something else.


    If you want to experience the RFzero in action for QO-100 you may contact Hans, OZ2XH, http://harbeck.dk/ham-radio/qo100-satellit/ and Steen, OZ5N, from the RFzero team. They both use the RFzero QO-100 program: https://www.rfzero.net/examples/qo-100-eshail/ and a transverter designed by Steen too. Many others also use the RFzero for LO generation but I don’t know their setup.