Strange LNB behaviour with GPSDO

  • Hi.

    I am using the Leo Bodnar dual GPSDO on a Pluto SDR and prepared two different LNBs for external oscillators,
    Octagon green quad and GoldenMedia from Pollin.
    Output 1 from GPSDO is set to 25MHz, Output 2 to 40MHz by using the following PLL parameters :

    My SDR shows stable peaks on 25 and 40Mhz.
    When tuning to the cw beacon on QO100, it seems that the signal is oscillating.

    I connected the GPSDO directly ( 50cm cable ) to the LNB and tried my RTL SDR - same result.
    Both LNBs behave this way.
    Does anyone have an idea about this ?

  • Hello Mark, try to change the freq by 1Hz maybe, anothers tried the same and that worked. AND a LPF on the output is used here because the outputs are not clean sinwave, so i made a small 10MHZ LPF, will make some for 25/27 also for the second output.


  • Thanks a TON Jerry - the 1Hz did the trick. Changed to 1000001 GPS reference and the signal shifted up a little - and is stable as a rock. I would have NEVER tried this on my own - how the hell does that make sense ? If i only asked that question before, ruined my Green LNB when trying to change the caps again and again.

  • I also read it here on one of the topics,hi, i have the same gpsdo but actuallly i do not have that problem, i have 10MC exact as out1 and between 25 and 26 as out2(439Mhz receive).


  • The GPS reference frequency is accurate but has small variations (jitter) which are then averaged out by the following PLL synthesiser to give the final output frequencies. This works well when the jitter has a high enough frequency. I suspect what is happening is that some GPS reference frequencies have a very low jitter frequency which the PLL cannot average out. By changing the reference frequency slightly you are probably increasing the frequency of the jitter, thus allowing the PLL to work correctly.

    The behaviour may well differ between individual units as the jitter will be dependent on the GPS receivers internal oscillator calibration.