• Tag Archives gpsdo
  • Titanium C1-PLL LNB GPSDO Modification for C-Band ADSC

    You may recall from the very end conclusion of the blog entry on my ADSC C-Band ground station that I had two outstanding issues.
    Tracking the sat.
    Thermal drift on the output.

    The sat issue is the nature of physics.
    The thermal drift is like wise. The problem here is that you only need the tiniest amount of drift in the 25 mHZ crystal to make a massive difference in the output.
    The LNB is sitting out in the sun at the dish focal point and so has no choice but to undergo a major thermal cycle every 24 hours.

    The first one is solved thanks to Node-RED.
    The second has also been solved thanks to a very cool product. Here is how I did it.

    BTW, I bought another LNB so I would not have any downtime, so now I have two… But I think I found a home for one of them….

    You will need to buy a GPS disciplined oscillator from Leo.
    http://www.leobodnar.com/shop/index.php?main_page=product_info&cPath=107&products_id=301
    Shipping to California only took a few days (pre Covid), so you won’t have to wait too long.
    Here is the kit you get;

    Its one of the best presented bit of electronics that I can recall unpacking.
    You get a GPS antenna, the electronics and a USB lead.
    Jump on the website and pull down the little setup utility.
    Sorry for the screen shot of a screen shot, but here is the utility in action.

    You need to set the output to be the lowest.
    Set the frequency to 25 mHZ and that’s it.
    Takes all of about 30 seconds to set up and has been rock solid reliable the whole time I have been running it.

    Now that the easy part has been done, its time to dust off the soldering iron and mod the LNB.
    What we are doing is removing the crystal and installing a coax to inject the output from the GPSDO.

    Here is the unmodified LNB.

    Unsolder each end of the crystal and put it aside (I put mine carefully in the trash! Hey, it caused me a lot of grief over the past year chasing its drift!!).

    Here is what you are left with.

    Now is when we have some real fun.
    Go ahead and power it up on the dish and look at the output on the waterfall.
    You have a 50/50 chance of getting the coax on the right leg of the old crystal pads.
    Of course, I got it wrong and the waterfall was FULL of birdies and no ADSC signals at all.
    So, simply move the center of the coax onto the other crystal pad.

    Water prof the coax and you are done.
    Drift free!

    This little GPSDO clock is a.m.a.z.i.n.g.

    I just cant give Leo enough praise for making such a cool device.

    How you provide 5 volts to the USB input will depend on how you have your dish setup.
    I have a 12 core cable going to the dish to take the position indicator and up/down control, so have plenty of spare pairs to take the volts out to the GPSDO.
    The little GPS puck has a magnet on it, so attaches the dish frame just fine.

    We have been running with the setup since December and its been great not to have to worry about any drift at all.