[clug] OT: ADSL2+ and AM radio interference

Ben Nizette bn at niasdigital.com
Sun Apr 25 23:45:45 MDT 2010

On 26/04/2010, at 12:53 AM, Adam Baxter wrote:

> Can anyone on this list confirm or deny some of the things said on this
> post:
> http://forums.whirlpool.net.au/forum-replies-archive.cfm/369495.html
> "Interference from AM radio transmitters is a known cause of noise induction
> into the copper network."
> *  I've been getting quite a bit of radio interference when listening to ABC
> on AM. Coincidentally my ADSL sync speed and reliability has been degrading
> steadily. Are these related?

As Steve mentioned, the ADSL sync rates are pretty well in the middle of the AM band so on paper this seems a pretty persuasive argument.  However, all your equipment should be C-tick certified which has really anal regulations on both emission and acceptance of radiation over the whole spectrum up to 30MHz.  I'd be pretty surprised if you found any two things in your house playing hob with each other.

An interesting note here is that there are (at least) 2 classifications for C-tick compliance, consumer and industrial.  The industrial specification allows more interference generated but also requires more internal rejection.  The upshot of this is that if you think "hells yea, got me a sweet new expensive industrial CISCO router!" or some such then hook it up to a network with commercial switches, ATAs etc you might end up with problems.  (FWIW I've pushed half-a-dozen devices through this compliance, when I say it's strict I know it!)

> * Are noisy AC-DC adapters to blame? If so, how can I isolate which one
> without unplugging my whole house?

If something has malfunctioned and has started radiating it could quite possibly be a switchmode AC/DC power supply.  The switching frequency is generally around the hundreds-of-kHz which is where your problems are.  

> * Is there a way (other than ye-olde AM receiver) to measure where/what the
> noise is?

An AM receiver is probably second-best to a proper frequency meter.  A decent trick is probably to use a low sensitivity, wideband AM receiver (a crystal set would be perfect) and have a bit of a swing around the rooms and see if/where/when the white noise volume increases.

> * Would strategically placed ferrite "lifesavers" help or is it snake-oil?

They theoretically could be made to work but I doubt you could find the right wire around which to wrap it, a ferrite which works effectively in what's quite a low frequency by ferrite standards and it wouldn't do anything about radiation off the inductors inside the offending power supply which is the most likely place for them to be coming from.

Similarly, standard ADSL filters remove lower frequency signals (like your dialup/voice/fax signals) but necessarily leave the modulation frequencies untouched.

> * Is there any way to shield the modem itself? (
> http://forums.whirlpool.net.au/forum-replies.cfm?t=369495&r=5426693#r5426693
> )

Unlikely.  The problem is that you'd need a very sharp filter in order to knock out the offending frequencies but not the actual modulation.  The modem should actually do this itself in a very effective manner, though it usually comes after the front-end amplification so if the interference is MASSIVE then the amplification might end up saturating, buggering adjacent frequencies with nothing the later filter stages can do about it.


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