Posts by PE4WJ

    pe1chl

    Lock indicators for Costas Loops are a tricky thing to get right. What can be done, is to tap off the quadrature component at the loop's complex output stream, average that and check that it is below a certain threshold. In the locked state, for BPSK, the in-phase (I) component contains the demodulated data symbols, the quadrature component (Q) should be (close to) 0. This is something I plan to implement in due time.

    DL5RDI you are totally right. And indeed, as measured by EA4GPZ, the C/N of the upper beacon in 2kHz is about 12 dB considering the transponder noise as the "N" term.


    pe1chl : I amo not sure that my method works on the lower beacon, first of all the Costas loop is not really designed to lock onto single carriers (although it can), and secondly, the interruptions as a result of the keying would result in loss of lock. But as you mentioned setting the bandwidth (i.e. inertia) of the system low enough it could work. Worth an experiment. IMO the nice thing about locking to the upper beacon is that the whole system is phase coherent when locked (so not just the frequency drift is compensated, but phase (time integral of frequency).

    g7iii Iain: it sounds like the loop is not locked. When locked, there should be zero drift, and the frequency should be spot-on, since the system performs phaselocking. What dish size are you using? The system is quite sensitive to SNR and needs a certain minimum SNR of the beacon signal to lock reliably. I have been able to get it to lock on a 35x45 CM dish but that is a bit marginal.

    For me, what works is to first adjust the LNB frequency correction until the upper beacon more or less matches the expected frequency (default: +125 kHz) , and then lower the loop bandwidth until the lock is solid. This can be checked by tuning to -125800 Hz as suggested by Rob, this puts the lower CW beacon at an 800 Hz CW pitch.


    DB8TF :

    No worries, I have included a mention of your solution on my blog.

    Hi Iain,


    Yes, as Remco indicated the first experimental version works. I have not yet had the time to do a write-up and explanation so you may have to dig a bit to make it work, but I will certainly do a write-up when I have time. And it most likely needs more experimentation to improve performance. I have just committed a cleaned up version of the flowgraph. Let me know if you can get it to work!


    73,


    Wouter Jan PE4WJ

    Dear all,


    I am using GNUradio for my uplink-only setup and that seems to work well. Next step is to include downlink reception. I have been thinking about doing LNB drift correction in software based on the beacon (as used / discussed by several people here on the forum). Instead of an FFT based approach, I plan to use a Costas loop to recover the (suppressed) carrier of the PSK400 beacon. Costas loops are designed to do carrier recovery on DSB-SCsignals (PSK can also be regarded as DSB-SC), so in theory it should work. I cannot test this out in practice however as my LNB is still on its way in the mail :) :)


    Therefore, would anyone be able to send me an IQ recording of the NB transponder (including the beacons) using a PLL LNB running on its original crystal (so no drift correction or GPS disciplining used)?


    Sample rate may be 480 ksamples or anything higher (not too high to limit the file size a bit). Dropbox or something similar should work to transfer the file. Many thanks in advance!


    73,


    Wouter Jan PE4WJ

    Hi Hans-Jochen,


    I have the same PA, and although it states 24V on the casing, the power supply that comes with it is 12V 5A. According to DH2VA there is an internal DC/DC converter to 28 V so it appears to be quite flexible with respect to input voltage.


    Mine also has ordinary SMA female connectors for input / output. No reverse polarity issues :)


    The square near the output appears to be a position where a circulator / isolator can be placed, which would protect the PA in case of a mismatch. I am using mine with an external circulator + dummy load on the "isolated" port just to be safe.


    Up till now, it has performed very very well, I run it at about 10W into a 24 turn RHCP helix, and am about as strong as the upper beacon, so good enough hi :)


    Many thanks to DH2VA for spotting this one and making the community aware! It is not a cheap unit, but certainly delivers and appears good quality.


    73,


    Wouter Jan PE4WJ