Down Converter V3d defect?

  • Hi,

    I've used the Down Converter V3d with the GPS-Modul V2 for several month now. It worked as expected.
    Today I got a problem. It seems that the some PLL isn't locked and some frequency isn't stable. You can see the effect on the screenshot. All Leds are ok and also the seriell Debug output from the converter looks normal.

    Does anyone knows this effect and maybe have some hint?

    For clarification:

    The TRX is Adalm-Pluto, external clocked with 40MHZ from the Down-Converter.

    The LNB is modified and also ext. clocked with 24MHz from Down-Converter.

    73, Hartmut

  • Hi,

    I am relatively new to RF technology.

    For starters I have set up a small ground station to receive QO100. But now I experience very similar problems.
    In the beginning the DownConverter V3d (with GPS receiver) worked just fine. I am using a PlutoSDR and the modified LNB as well.
    For better signal stability I wanted to feed the 40 MHz clock reference from the DownConverter to the Pluto. So I modified the Pluto in order to connect the clock.
    Unfortunately the readings I am getting now are not what I expected at all. So I checked the Pluto without DownConverter. I connected the signal generator of an Osci to the clock input of the Pluto. Receiving the local radio stations with this set up worked nicely. To verify that the rest of the setup works correctly I built a simple Bias Tee. With the Bias Tee (antenna, Bias Tee, Pluto) I can receive the signals again.
    The DownConverter however is not working as it should. I have observed the following:

    1) GPS receiver LEDs:
    PLL: off
    Power: blinking
    GPS sat connection: on

    Though I have to say that I think i saw the PLL LED on for a second just after switching on the power for the DownConverter. But that was not reproducible a second time. I tried several times.

    2) DownConverter LEDs:
    Power supply(A): on
    Down mixer lock(B): off

    (C) : mostly off but blinks once in a while shortly
    Central PLL sync. (D): blinks

    3) I also measured several outputs of the DownConverter. Some measurements differed from the normal values.

    The 40 MHz clock output was giving 35.08 MHz or 48 MHz (varying from test to test).
    The frequency of the wide band output is 28 MHz instead of 25 MHz.

    The frequency at the GPS receiver 8.77 MHz instead of the expected 10 MHz.

    The Jumpers for LNB reference selection are at 25 MHz selection. (10 A bridged)

    Does anybody know the diagnosis for these symptoms?
    I am grateful for every hint to locate the error or further tests I could run.

    Thank you very much.

  • Hi,

    I asked Kurt DJ0ABR and he replied:

    "what Jonas writes sounds like the PLL on the GPS module is not locked.

    I've seen something like that myself when the GPS signal was weak, but after 2 seconds the PLL was locked again.

    What is also important to know, how many GPS SATs does he receive at all.

    Most people don't know this because they don't have a display, but this information is also available on the serial port."

    I it doesn't help, please send an eMail to our office...

    73s Peter

    Peter Gülzow | DB2OS | AMSAT-DL member since 1983 | JO42VG

  • Hi,

    I had erratic and unexpected readings on my counter just as you had.

    Get a Display like this:…/1-3zoll-i2c-oled-display ,

    hook it to your Downconverter via a 4-wire connection. Amongst other Info, it will show how many satellites you receive, so you can tell THAT is or is not the cause for the trouble. I used the V3D for a while and noticed (in my qth) gps-reception is weak or completely fails now and then. I would never know if the unit was in a box where you can't watch the LEDs. OTOH a box is important because every small airflow causes the TCXO to drift and makes the correction do it's job. You can see that as drift on the downlink.

    73s , Martin