102 KHz strong "comb"

W5UXHW5UXH
in Installation, Antennas and Interference
Recently I noticed a wavering strong "signal" just below 7000 KHz but only zoomed out today and realized it is a comb spaced at about 102 KHz intervals.  I then realized this may well have begun around the same time I installed a new Comcast modem, an Arris TG1682G.  This modem has an internal dual band router and replaces an older model that was only single band.

Does this spacing ring any bells as a likely emission from a cable system modem / router?  I suspect this must be the source of my new RFI.  

I will need to dig through the forum to study the solutions I know I have seen posts about, I think they involved shielded network cables among other things.  I do not really know if any of mine are shielded, but I have quite a few and would guess that typical off the shelf ones in which case it will be a bit of a job to replace all of my cables!  

Comments

  • jksjks
    edited March 2018
    "Wavering" carriers like that are a definite signature of a switching power supply where an unstable RC-network oscillator (possibly on-chip) is being used to control the switching frequency, as opposed to say a crystal. The switcher could be the modem's wall wart or it could be a switching supply internal to the modem. 100 kHz is a moderately high frequency for a switcher. So you might expect it to be a more modern higher efficiency chip that was used on the modem PCB as opposed to a cheap solution in the wall wart switching at 20 - 30 kHz.

    If you have a portable radio with a built-in loopstick antenna that tunes the longwave band (e.g. Sony SW-100, SW-7600 etc.) you could go sniffing around at the third harmonic to locate the source. A Kiwi antenna made out of a bit of coax terminated in some turns of wire between center conductor and shield might work too (depends on impedance issues).

    Switchers seem to fall into two categories: those whose switching harmonics fade away around 1 MHZ, and those that go blasting all the way to 30 MHz (and probably beyond).

  • W5UXHW5UXH
    Unfortunately the supply is internal.  I will attempt to verify that it is the new modem/router but feel certain that it is due to the coincidence of timing.  I have other switching noise at the 62 KHz spacing that are not as strong and start out up around 15 MHz and run all the way to 30 MHz.  


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