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Why aren't the PGXL shields grounded ?


While troubleshooting some strong switching power supply birdies on 160 meters generated by the "B" side of the PGXL, I discovered that the PTT shields aren't grounded to the amp. There is about 50 ohms resistance between the shields and the PGXL ground lug. Why ?

BTW, connecting the PTT shield to the ground lug nearly eliminates the birdies. Before I permanently do that, it would be good to understand the design intent.

73, Steve, N2IC


  • VE7ATJ_Don
    VE7ATJ_Don Member ✭✭✭

    Hi Steve... We may not be able to help much here in the Community.

    I'd suggest you submit this as a Help Desk Ticket inquiry, as it will get directly to the 'source' that way :-)

    I have to admit though, it would be interesting to hear if there is a design consideration here. Maybe they don't engage PTT by 'grounding' a particular signal??? Maybe a little like the old days where you were actually keying high voltage :-)

  • WX7Y
    WX7Y Member ✭✭✭✭

    I would contact 4O3A about your question, they are the Designer and builder of the PGXL.

  • Mike-VA3MW
    Mike-VA3MW Administrator, FlexRadio Employee, Community Manager, Super Elmer, Moderator admin

    The PTT inputs are opto isolated by design.

    There are many reasons to do it this way, but one thing that comes to mind is to eliminate RFI issues with ground loops.


  • k0oks
    k0oks Member ✭✭

    I would be curious if the birdies go away when you simply disconnect the PTT cable from the amplifier end?

    In other words, are the cables acting as antennas radiating the birdies? It would be odd that an opto-isolator would feedback noise, but I suppose it may indeed have a separate internal ground, which could do so (when connected to a cable acting as an antenna).

    What frequencies are the birdies? Would love to see if we are getting those as well. We have all kinds of birdies around here, but I attribute most of them to the close proximity with neighbors in my temporary rental QTH.


  • Steve London
    Steve London Member ✭✭

    According to the PGXL developers, the intent of not grounding the PTT shields is to reduce the possibility of birdies caused by ground loops. They said there is no harm in grounding the shields to the PGXL ground lug. I took a compromise - a 0.1 uF capacitor between the PTT shields and the PGXL ground lug. No more birdies. Amp worked perfectly in CQWW SSB and CW SS with 2 radios.

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