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Connecting FT-991A to computer = QRM /o\

Discussion in 'General Technical Questions and Answers' started by PY2RAF, Jul 25, 2019.

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  1. PY2RAF

    PY2RAF XML Subscriber QRZ Page

    Hello there, QRZers.

    Resorting to the Community Knowledge.

    Scenario: FT-991A. Without a computer connected to it, just the radio and the antenna, IPO (no preamps), letting it gaily be, I have a nearly silent band. Less than S1 of background noise (red annotation). You can even see a AM QSO at around 3600 (green annotation).


    Good stuff, right?


    This is the story when I connect the USB cable to the computer:


    Well. Not so. Almost 6 S-units of background noise and... Where's that QSO around 3600? Engulfed by QRM.

    Life's hard, right? ¯\_(ツ)_/¯

    The 80m band is the worst case scenario, the most badly affected band. Other bands suffers mildly or not affected.

    SO. Solutions.

    The first thing I thought about was a USB isolator. But it can only handle up to 12 Mbps of traffic (see isolator). And honestly, with audio full duplex plus the CAT interface, I'm unsure it would be a good solution - I suspect that the USB traffic might surpass this 12 Mbps limit.


    Then, I thought about the good old ferrite choke trick. Actually, this cable already sports two built-in ferrite chokes, one at each end.

    After thinking a bit more, I realized that the shielding (where the choke blocks any interference), bonds together with the ground power line that runs inside the USB cable inside the radio, BEFORE a ferrite bead. See below:


    SO, yay, the ferrite might filter out whatever signal is picked by the external shield, but the QRM travels undisturbed and safely through the USB ground lead, safe and sound, protected by the external shielding.

    Another (maybe) relevant piece of information. If I remove the antenna cable, connecting or disconnecting the USB port has no effect - it stays S0.

    However, if the antenna is connected, by just touching the USB cable at the USB port, then it spews the QRM.

    Any ideas to tackle that out? Or is my understanding 100% plain wrong, wrong, wrong as Krusty Olde Kurt used to write?

    Appreciate your analysis & comment.

    - Rodrigo.
  2. KU3X

    KU3X Ham Member QRZ Page

    I have the same setup and no issues.

    Did you try another USB cable?
    Do you think it is a possible ground loop issue?

    I had a ground loop issue with my rotor control box. Turned the antenna and the computer screen had some noise in it.
    Fixed same and all ok now.
    Ferrite had no effect with this issue.

    Good luck,
    Barry, KU3X
    Last edited: Jul 25, 2019
    N7UJU likes this.
  3. PY2RAF

    PY2RAF XML Subscriber QRZ Page

    Hi Barry,

    HAH. There.

    This is a sturdy Lenovo W541 laptop. So, idea:
    Remove the power supply, let it run by its battery and see how that goes.
    Guess what: Radio silence.

    Well, now what.

    The laptop power supply is a two-lead switching power supply , it has no ground pin and... Well, now I'm puzzled on how to tackle it.


    73s de PY2RAF / Rodrigo.
  4. PY2RAF

    PY2RAF XML Subscriber QRZ Page

  5. KU3X

    KU3X Ham Member QRZ Page

    Is the radio's power supply and the computer plugged into the same outlet?
    Probabaly are. The ground loop is probably between the antenna and the radio.

    But you are on the right track.

  6. PY2RAF

    PY2RAF XML Subscriber QRZ Page


    Ugliest and oldest trick ever - DON'T TRY THIS AT HOME, KIDS!!!

    Resorted to the old hack of leaving the ground pin of the Alinco power supply floating. Now what? Radio silence.

    Life now is good, rainbows and unicorns.

    KU3X likes this.
  7. KE0CAA

    KE0CAA Ham Member QRZ Page

    I have an HP laptop and found on amazon a matched power adapter that just steps up the voltage to what the laptop needs from 12v which allows me to power the laptop off of either batteries or my switching power supply rather than needing to use 2 outlets. This might help with the issue your experiencing however I will also add that I dont have anything inside of my shack grounded. Its difficult to develop a ground loop without a ground connected to anything :p
    KU3X likes this.
  8. PY2RAF

    PY2RAF XML Subscriber QRZ Page

    Hey that's a great idea! I'll take a look, thanks for the suggestion!

    Can you share which regulator you bought? I've seen a few in Amazon but the reviews were not great.

  9. KE0CAA

    KE0CAA Ham Member QRZ Page

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