[clug] OT: ADSL2+ and AM radio interference

steve jenkin sjenkin at canb.auug.org.au
Mon Apr 26 02:34:02 MDT 2010

Nice to know someone on the list works in this arcane area!
Anything "RF" is not simple nor easy.

I have interference problems with my TV at home. Shows up on my AM radio
It gets worse in summer - which makes me think part of it is a nearby
air-conditioner with an inverter. (distinctive sound on radio).

Ben - any comments on noise from powerful inverters?

With ADSL2, I had a bad experience with Optus infrastructure.
I signed on shortly after the Optus DSLAM went into Scullin (SCLN).
There was a crossed line and I could prove it (SNR went from +12db to
-0.5 within 20mins) but because my service 'ran' at 1.5Mbps, the ISP
wouldn't log a fault (!).

Point of this: I doubt that it's direct interference from just one other
ADSL modem, though there can be interference between a bunch of ADSL
modems sharing long lengths of unshielded pairs.

Ben Nizette wrote on 26/04/10 3:45 PM:

> 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.
> 	--Ben.

Steve Jenkin, Info Tech, Systems and Design Specialist.
0412 786 915 (+61 412 786 915)
PO Box 48, Kippax ACT 2615, AUSTRALIA

sjenkin at canb.auug.org.au http://members.tip.net.au/~sjenkin

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