[RFI] Cable Modem Interference

[Phil Duff]

At 10:29 12/3/01 -0600, dgsvetan@rockwellcollins.com wrote:

>Phil,
>
>Finding that your problem is the wall wart is probably one of the best
>"cures" to have.  Recently, I was having problems with excessive noise on
>10m, and traced that issue to a noisy wall wart that powers a video box.

The problem cable modem supply was not a "wall wart" type supply as such but
was a small supply with separate AC input cord and DC output cables.
  See portion of my original note below.  Its a little larger than a pack of
cigarettes.

This supply came with the Terayon modem and has manufacturer logo
of "ARTESYN"   (Made in China).   It's has UL, CSA, GS?, CE
markings.

I purchased a Radio Shack 12VDC 1a. wall wart to use in place of the
above supply.   I measured the no load output of the RS wall wart at
15VDC.  The wall wart box indicates "filtered output" but who knows
what that means.    The modem seems happy so far and there is no
noise from it or the RS wall wart in my 'MP's receiver.    I may try to
measure the voltage to see how close to 12vdc it is under load.

I checked around the shack and my PC printer (HP 712C) has
a power supply similar to the modem.  The printer supply is an 18V
output unit.  I put my "sniffer" probe on it and it was radiating some
crud but a far cry from what the modem supply was generating.

73 Phil NA4M


>Last night I determined that the crud from my cable modem is actually
>coming from it's power-supply.   It's a small 12V 1.25a. output deal that
>is manufactured offshore and is supplied with the cable modem.  Obviously
>a switching supply.
>
>I made a small "sniffer probe" of a few turns of bare wire on the end of
>some coax attached to my FT1000MP and when placed next to the
>plastic case of the power supply the crud jumps to 20db/S9
>over a small 75m frequency range - i.e. 10-20khz that wanders
>up and down the band.  The crud usually drifts across the 75m DX window
>just when
>some weak DX is transmitting of course

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Phil Duff NA4M     na4m@arrl.net   Georgetown, Texas