Maintenance on the NB Transponder!

1st Official Post
    • Official Post

    In the next days we will apply some experimental changes to the transponder settings, observable by less transponder noise.


    But DON'T PANIC - this is to bring the transponder performance to optimum.


    Our goal:

    • QRP-capability shall be retained or even improve.
    • User dynamics shall increase (and thus of the possible S/N of the individual users) by approx. 8-10 dB.
    • Low probability that individual users will (by too much Uplink-Power) push the TWTA too far into saturation range.


    Please stay tuned.. the above process might need several iterations...


    73s Peter

    Presumably you are going to reduce the through gain to give more dynamic range ? That would be a good move. It would be interesting to know, as you go, by how much ?

    Dave Cawley | Ex G8EAO & G6ANG/T |
    (when you had to have a separate TV license !)

    Hi Peter, OK thanks. It is a great satellite, truly great and makes me wonder why it didn't happen before. As we say here in the UK "gilding the lily" would be better dynamic range SSB for better audio quality. Presumably the AGC only kicks in with an overpowered signal ? Hi Fi and audio archiving is partly what I do for a living, but I'm also a qualified and experienced RF engineer. Is it possible to know how the NB "through gain" and AGC works ? No hurry and I know I have no right to ask. Thanks Dave

    Dave Cawley | Ex G8EAO & G6ANG/T |
    (when you had to have a separate TV license !)

    I saw the signal drop for a short period but I think that was a little bit too much...

    At the current setting (as was used last week) I still have about 12dB of transponder noise but it dropped by more than the 6dB you indicate, I think.

    (not really measured it)

    • Official Post

    Thanks Rob, I can only observe at the moment from QRL via WebSDR.

    I saw the same, during setting change it was about 4 times for short time that the gain was reduced with maximum value and finally was set to the new level. The S/N level to the station I was listening to was at least the same or even better, as much as I can judge from the WebSDR.. The noise is definitely lower than before. Our experts will now analyse and observe... but it seems we are going into the right direction :)

    Ok!

    It is fine now here as well, but I have to leave for work so I cannot listen further.

    Be careful when judging things on the BATC WebSDR because it has (or at least, had when I last checked) AGC of its own.

    I think the level is now about the same as it was before today, but it is a bit difficult for me to judge as we have heavy rain and wind...

    Hello Peter,


    Transponder noise floor now just about 2.5 dB above LNB noise floor here (GG56tv - southern Brazil). The SNR for the BPSK400 beacon has also decreased about 2 dB.


    My setup is a 120 cm offset dish and Octagon OSLO LNBF.



    It would be nice if the transponder noise floor could be a little bit higher. This would give those of us at the edge of the footprint a little more margin.


    73, Edson PY2SDR

    • Official Post

    Hi Edson, the reduced noise level shalll not have an effect on the actual S/N of individual users. Some brief measurements seems to indicate that the beacon level actually did not change. Keep in mind also, that the beacons are currently send from AMSAT-DL HQ in Bochum and we have a lot of wind and rain at the moment ;)

    On the other side we have observed that the CW, SSB and PSK signals now sound much clearer which indicates that the transponder might have been in the non-linear regio before. Lets wait for exact measurements and more observations in the next days... Unfortunately the "tombstone" signal disappeared, which might have also been a good indicator.. hihihi 73s Peter

    Quote from DB2OS

    Hi Edson, the reduced noise level shalll not have an effect on the actual S/N of individual users. Some brief measurements seems to indicate that the beacon level actually did not change. Keep in mind also, that the beacons are currently send from AMSAT-DL HQ in Bochum and we have a lot of wind and rain at the moment ;)

    On the other side we have observed that the CW, SSB and PSK signals now sound much clearer which indicates that the transponder might have been in the non-linear regio before. Lets wait for exact measurements and more observations in the next days... Unfortunately the "tombstone" signal disappeared, which might have also been a good indicator.. hihihi 73s Peter


    Hi Peter!


    Thanks for the comments. My concern is that the new levels could impact stations with 90cm dishes at the edge of the footprint. But this concern needs to be substantiated with actual observations and measurements. I will request some amateurs here to observe.


    I will also redo the tests with my antenna at different times.


    73, Edson PY2SDR

    DB2OS Good idea to increase the dynamic range of the tpx. For me it needs some fiddling as I had the tpx noise 2 - 3 dB above the noise (after the 4 dB reduction).


    Now the tpx noise sunk into the background noise and hence .. e.g. peak of the CW beacon now is ca. 18-19 dB and it was around 24-25 dB above the noise early this morning.


    I support the remark from PY2SDR, in a sense that -given the tpx worst case requirements, as summarized in the Es'hailSat lecture in Geneva- with 95cm at the EoC, 60cm in the peak and 70-75cm dish sizes 'elsewhere' tpx noise must at least equalize with the background noise (preferably 1 - 2 dB more, but that is a matter of 'taste') .

    But I reckon that is part of the 'fine tuning' of all parameters. At least I experienced no surprises : -) Thanks for the pro-active information!

    Quote from DB2OS

    Keep in mind also, that the beacons are currently send from AMSAT-DL HQ in Bochum and we have a lot of wind and rain at the moment ;)

    It is a present we got from the guys in G-land and kindly forwarded to you :)

    At the moment it is a lot more quiet here and clear sky, so it will likely improve for you too.

    Quote from sv8qg

    I suppose that TWTA will gain a longer life and the system will waste less power…

    That depends on several factors and is always a trade off/compromise.


    From the ITU presentation the OBO (Ouput BackOff) of the TWT is already 6 dB .. to obtain 26 dB NPR (Noise Power Ratio).


    So there are quite a few knobs to turn for the 'most optimal performance'. If you want optimal NPR then there is a setting, if you want optimal efficiency (at the cost of NPR) there is a setting etc etc.

    My observation is that this last adjustment now is much worse. Lower beacon has too low S/N on my RX compared to before. SSB stations now are at the same level as the lower beacon and are now not easy to listen to anymore. I hope this is not the way the NB transponder will be set. We need at least 20 dB S/N on SSB signals for comfortable copy (in my opinion).