Why does my Transmitter Hum?

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Because it doesn't know the words! Or, it could be that you are suffering from one of several technical causes of hum in transmitter circuits... According to the folks at North Country Radio, there are several causes to consider. These causes include:
  • Power supply hum due to poor DC supply filtering
  • AC hum pickup in the transmitter audio due to poor shielding
  • Ground loops
  • RF induced hum modulation on the transmitted signal What's the cure? Here are several links to technical articles dealing with hum reduction in audio and RF circuits: Hum reduction info from www.northcountryradio.com Pixie II tips, including hum redux, from www.kenneke.com Hum Rejection in Unbalanced Audio Cables from www.bluejeanscable.com Ground loops from www.epanorama.net Breaking ground loops from www.epanorama.net Hum Update Listserve for Hams from lists.contesting.com Happy Hum Hunting!
  • Comments

    Carl Blare's picture

    Thanks for re-posting this forum as it supplies tips on two issues being experienced...

    The transmitter being experimentally built for 13.560mHz Part 15 shortwave does have hum on the received signal, but it's the tricky kind which goes away by simply moving a radio. It must be "polarized hum?"

    But another issue is addressed in the "Pixie II Tips," about how to shift the frequency of a crystal.

    One of the crystals we have which are marked "13.560mHz," actually comes in at exactly 13.555mHz. Therefore we need to kick it up by 5kHz.

    Any ideas about how to do this are welcome!

    Carl Blare

    scwis's picture

    Bummer about the mis-labeled xtal - did you contact the seller about that? You should get what you ordered.

    On the other hand, in your statement

    "exactly 13.555mHz... we need to kick it up by 5kHz"

    I'm wondering why you want to do that.

    As far as I know, the 13 MHZ Industrial, Scientific, and Medical (ISM) band spans 13.553 MHz to 13.556 MHz so 13.555 seems well within band.

    Is there a specific reason you want to be on 13.560, which appears to be out of band?

    Was there a recent spectrum reallocation leaving us with just 13.560?

    Experimental broadcasting for a better tomorrow!

    Carl Blare's picture

    The Rule which makes 13.560mHz special is 15.225 (a)

    (a) The field strength of any emissions within the band 13.553-13.567
    MHz shall not exceed 15,848 microvolts/meter at 30 meters.

    13.560 is dead center of that prime range.

    13.555 is too close to the lower limit and doesn't leave room for the needed signal bandwidth.

    Carl Blare