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1-8 of 8 messages
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Keying TX will take down my ADSL modem - how to fi
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by KU7T on October 7, 2003
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Hi,
I have an RFI issue that really bothers me. Whenever I key down my TX on 40 m and some other bands (> 10 W), my DSL modem will decide to stop working.
I have to power-cycle it in order to come back up again. I am looking for any solutions or troubleshooting tips.
Here is my setup:
Station:
- K2/100 property aligned
- safety ground to AC earth (I am too high in the building to run ground wire down), no real RF ground
- stealth parallel dipole for 40 and 20 m (issue came up before I added the 2nd 20m dipole) very low hanging, about 15 feet (CCNRs)
DSL modem:
- Westell Wirespeed 516
- sitting in garage, about 15 feet from station and 25 feet from antenna
- connected to phone line, AC power (routed through UPS), WAN out (WAN out is connected to a Wireless AP, the wireless AP does not hickup at all when keying down)
Any help is appreciated.
73, Andreas, KU7T
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RE: Keying TX will take down my ADSL modem - how t
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by WB2WIK on October 7, 2003
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The problem is likely caused by your rather "low" antenna, and its close proximity to the modem, etc.
However, ferrite RF chokes added to the modem's I-O cabling, including the power cable (between UPS and modem), should help. I routinely use the Radio Shack snap-on filters, RS P/N 273-104, which are sold in packs of two for $7.95. They come with good instructions regarding installation, which is a cinch (no tools, no re-wiring).
In some cases, like RFI I experienced with my kids' Nintendo game, it takes more than two of these snap-on chokes to do the job! In that particular case, I tamed the Nintendo only after installing THREE of the snap-on chokes on the AC power cord between the wall socket and the Nintendo power supply "brick," and TWO more between the power supply "brick" and the game itself, on the DC cable. Five chokes to do the job. One, two, three, or four didn't do it.
Moral of the story: Don't give up easily.
WB2WIK/6
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RE: Keying TX will take down my ADSL modem - how t
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by KU7T on October 8, 2003
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I tried this out last night. Here are my findings:
Those RS RF chokes work pretty well. It took me awhile to find out over which cable the RF got into the DSL modem. I added one choke on the AC, then on the Ethernet and then on the phone line. I noticed that the phone line choke improved things a bit. So, I kept adding more chokes on the phone line and at some point removed the ones on the other wires (to free them up for use on the phone line as well). Currently, I have 5 RS RF chokes as close as possible together and as close as possible on the DSL modem. It worked, but I am trying to understand what I did here.
The documentation of the choke tells me that every winding after 4 windings adds 5 db of attenuation of RF in that frequency range.
Test: I used some web radio station to play while doing the transmission to tell me that the DSL modem goes down. I also started a huge file copy from another machine on the LAN to make sure the DSL modem is the culprit and not the WAP.
Without any chokes (original setup) 10 W is the level when the DSL modem starts to hickup. With 5 chokes this level is about 100 W (the modem hicks up sometimes, but seems to recover even when the transmission continues). The Wireless LAN is unaffected at all times.
This is 10 dB difference in RF output. However, all my chokes have about 12 windings, this makes it 60 windings total. It seems that I needed way too many windings to do the trick. Can someone explain this too me? 60 windings should result in >= 250 db attenuation....
73, Andreas, KU7T
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RE: Keying TX will take down my ADSL modem - how t
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by KA5S on October 9, 2003
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Andreas,
Glad to see you were able to fix things to your satisfaction. If you want to add more RF attenuation, you can take a turn or two through each of the ferrite beads.
I recently spent a few years working on RF emissions and electromagnetic compatibility and immunity of Central Office, ADSL, VDSL and other telco equipment. An ADSL modem is a full duplex transceiver that works between 100 KHz and 1.1 MHz. Its front end is basically a wide open receiver, fed through a high-pass filter, and it does not take a lot of common-mode RF to desense it. The ferrite beads cut this down.
ADSL users at the maximum range of the service are most bothered by RF and electrical noise. The original promise of ADSL service was over three miles of copper wire (telco's call this "reach"), but though it can work at that range, more than a few microvolts of transient noise (or RF) bollix it up. This is IMO pretty much inherent in the concept.
Cortland
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RE: Keying TX will take down my ADSL modem - how t
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by KA5S on October 10, 2003
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Of course I meant a turn or two MORE on the beads. But I had skipped over the number of turns already applied.
It takes double the beads (or 50 percent more turns) to double the attenuation. There's a point of diminishing returns, usually about the point your wallet diminishes beyond return!
I once saw a (futile) attempy to quiet a field RF problem where the engineer had installed 42 beads. They didn't help.
Cortland
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RE: Keying TX will take down my ADSL modem - how t
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by WA4MJF on October 18, 2003
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I just got ADSL. I'm about a quarter mile
from my slick. I provided shielded station
wire to the telco man to run from the
spilter (shield gronded at network interface
to common ground with CATV and power) to the
block and made a shielded
RJ-11 wire for block to MODEM. Shielded
CAT5 to router and from router to 'puters.
So far no problems, but I haven't needed
to run a full gallon yet.
73 de Ronnie
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RE: Keying TX will take down my ADSL modem - how t
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by N5EG on January 1, 2004
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Andreas:
The "250 dB attenuation" doesn't really mean much. The problem is that the phone line is acting like a single-long-wire antenna for RFI. The RFI path is the phone-line-as-long-wire-antenna, modem-as-undesired-detector, ground-path-of-the-modem. You are putting a lot of impedance in series with the long-wire antenna, but if the load impedance (the modem) is high already, then a lot of ferrite-choke impedance doesn't attenuate things very much.
The dB attenuation spec supplied by RS assumes that the 'filter' composed of ferrite chokes looks into a low-impedance load -- in this case it doesn't.
A better approach is to effectively short out the long-wire antenna for RFI (but not shorted out for ADSL), by providing it a low-impedance path to ground so that the ADSL-modem does not serve this purpose. IE: you want to provide a low impedance between the antenna (the phone line coming into the ADSL modem) and modem ground (common mode) thus the ADSL modem won't have any RFI voltage to 'detect'. However you don't want to screw it up for ADSL (differential mode) signals.
This is easier with unbalanced conductors - such as coaxial TV cable. Just run the cable through a grounding block (grounding only the shield) to the same point that the TV set is grounded (keep that ground loop small).
Unfortunately the ADSL modem needs frequencies up into the low megahertz range to operate properly, and it is balanced so the filter gets complicated. Secondly, any ground path for the phone line has to be able to withstand possible voltage imposed due to some faults that could happen in the outside wiring network.
At this time I do not know of any ADSL filters available that can meet all of these requirements. So you are stuck with using large amounts of ferrite. It may prove more economical to use larger ferrite cores (like the Amidon ones) with multiple turns of the phone line on the core rather than lots of the snap-on chokes (unless of course you have already purchased the RS ones..).
-- Tom, N5EG
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RE: Keying TX will take down my ADSL modem - how t
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by KC8VWM on January 15, 2004
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Had a similar problem once. I found that grounding the equipment actually worked better to bleed off the stray noise than the chokes did.
Also grounding you PC case to the same common ground as your radio equipment will significantly reduce noise levels on your radio equipment.
I fully realize that PC's already have a ground through the 3 prong plug, but for some reason when you tie in the grounds in this manner it works better in reducing noise especially on HF bands.
Try it!
73
Charles - KC8VWM
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