BCB and other RFI from Motorla SB5100 SURFBoard Cable Modem

[Todd, KA1KAQ]

Just got the trusty ol' SX-62B set back up yesterday for HiFi BCB & shortwave use, only to be greeted by a wall of noise on the BCB every 50-100 kc. Today's investigation revealed the culprit, a cable modem installed a few months ago to replace a different service provider. Though the receiver is less than 10 feet from the modem, I get it equally strong in the bedroom on the SP-100. This leads me to believe it's propagating back through the AC lines as much as radiating from the device.

Searching past threads turned up a discussion from last year about similar issues and a list of offending units but not a lot of remedies for the problem. The only ferrite beads I've ever used are a few slid over RG-213 and taped in place near the aerial. So I'm a newb to this approach basically. Never had problems before now.

I'm interested in learning what others have had the best success using for cores, number of turns on the power cable, etc. Clearly this is something that is now and will continue to be a growing problem for us going forward. Not looking for a P&M session about the ARRL, FCC, or cheap Chinese junk, just a technical solution or guidance in the best direction.

Seems there was also a list somewhere online to report offending electronics for possible action. Anyone familiar with this?

[Pete, WA2CWA]

Power to the cable modem is 12VDC.

I would first jury rig a battery DC supply to either confirm AC line propagation or eliminate it. It could be as simple as a crappy switching AC to DC adapter. Then, if necessary, with the coax connected, disconnect either the Ethernet or USB (whichever you use) cable from the box to see if noise disappears or is greatly reduced.

[Steve - K4HX]

I have the 5120 here and never noticed any interference from it.

[Todd, KA1KAQ]

Should've mentioned it, but forgot. It appears to have a one of those slim switcher supplies instead of a wallwart. All I know is that as soon as it's unplugged, the noise vanishes. I've got a spare supply I'll try but would expect the same result. Should be able to substitute a 12v wall wart in its place?

[Steve - K4HX]

Yea. Give that a try. If no joy, lememe know. I have plenty of ferrite.

[kb3ouk]

As long as the plug is wired the same polarity and the voltage and current is the same, then it shouldn't matter what it's being powered by. As long as there's nothing wacky like 60 hz square waves coming out of the supply, then it should be fine with the wall wart. You could even hook a generator to a hamster wheel and power it that way, which would have an added benefit: train the hamster to stop running when the radios were on for noise-free listening.

[n1uvi]

The consumer grade routers and modems are real rf noise makers.
They live in a plastic box with no shielding, and have long cat 5 antennas attached to them.
I have had to change over to commercial products to preserve my sanity here.
Find a quality modem housed in a metal enclosure, with processors that run
on a clock speed that is higher than 433 MHZ, and your rfi will magically disappear.

[ke7trp]

I have that modem.  I used a different wall wart that was heavy.  Then wrapped the RG45 cable in a cheap Ferrite split core.  Not sure of the material. A guy was selling them 1 each at a ham fest.  Noise gone..

One EASY way to find out of it is the wall wart todd is to unplug the modem but leave the wall wart plugged in. If the noise changes tone, You know its the wall wart.. With no Load, The cheap switchers GO "high" and the sound will follow on the receiver.


C

[Todd, KA1KAQ]

Good idea, Clark. Hadn't tried anything beyond unplugging it so far, which ended the noise. Will give that a try. Was out for the day so made no more progress. I suspect the spare ps will produce the same result. Will need to track down an 'old fashioned' wall wart. Never thought I'd see the day!

I've been very fortunate with evading/avoiding high level interference for years. With more and more plastic crap invading the house, guess it had to happen. Commercial grade gear would no doubt help, but finding it or knowing what is best proves challenging for most. I stopped keeping up when I unhooked my external 14.4 modem in a thick aluminum case and traded it for a 36.6 card modem. Everything spun out of control from there with stuff changing faster and faster each year.

[Pete, WA2CWA]

You don't have a 12 volt DC supply ( doesn't have to be a wall-wart) kicking around that you can use to see if it makes a difference??

[Todd, KA1KAQ]

Probably 3-4 of them, Pete. The only question is - where? Most of that stuff is still boxed up in one of the garages, plus a lot got left behind to be scrapped.

Should be able to find a wall wart easily enough. Plenty of those kicking around.

[W1AEX]

Todd,

I had a Motorola SB5100 at one point and found that subbing the annoying little switcher out with a linear supply resolved the noise. I use the same linear supply with my Motorola SB6120 that replaced it. As I recall, the linear supply needs to be able to supply 2.5 amps continuously. The little switchers that come with the Motorola cable modems are quite noisy at HF, but the modems themselves seem to be quiet across the bands and they don't flinch with full legal limit TX power here.

Rob W1AEX

[W1RKW]

I enclosed my SMPS on my modem in a metal box. Also grounded the metal box. It took 95% of the noise away. Added ferrite too which helped.  Still have some noise but it's S1 in the shack.

[Todd, KA1KAQ]

Finally had a few minutes this evening to get back to it. Tried what Clark suggested by unplugging the switcher from the modem instead of the strip to see if the noise/note changed. It vanished, though I think it's related to the switcher shutting down when the circuit is open - dunno.

Also got something in the mail from TWC about the modem but didn't have time to read it yet and need to find which pile I put it in.

But at least it's isolated to the modem which makes it easier to deal with as time allows.

[KB2WIG]

   " got something in the mail from TWC about the modem "

http://www.dslreports.com/shownews/Time-Warner-Cable-Modem-Lease-Fee-Expands-121453

" Time Warner Cable is expanding plans to start charging users a $3.95 modem rental fee. Users in our forums in the New York market say they've been getting cards in their bills notifying the new charge will start showing up on their bills starting October 15. Time Warner Cable has always had a very fractured structure, so it remains unclear how quickly this fee will spread to all regions. "

 The PHONE Company usta get about $1.30 a month for the wire inside yer home.....   Must have been a nice return on investment after a few years or so.


klc

[Todd, KA1KAQ]

Yep, found the card and read it today. I think this is a response to their $4.95/month fee for wireless and so many people simply getting their own router instead of paying for it. Modem fee recoups some of it for those who don't want to make the effort.Y ou have to give them the MAC number so they can activate it which makes me wonder if the approved modem list are all models that don't support wireless. I'm more than happy to get my own modem and send this RFI generator back to them in favor of my own RFI generator.

I'm going to review the list well and check reviews. My guess is that pretty much all the new stuff will have switching supplies vs old transformer wall warts and the accompanying noise.

[gerry_w1id]

In almost all cases of interference as you describe, it is almost always due to a poorly made, inexpensive switching power supply. The dead give away is the noise appearing every 50 to 100kHz. This indicates the prevalent switching frequency of those supplies. Switching power supplies are much more efficient than linear supplies and are also much smaller. At this point in time, they can also be manufactured much more cheaply than linear supplies owing to the fact the components are also smaller, less numerous and the transformer, operating at some 50 to 100kHz is also smaller and less expensive. A properly designed switching supply can be as quiet as a linear supply but obviously they would cost a little more. I have seen power supply OEM's argue over fractions of pennies on component costs so I think you can see what we're up against. The solution as you may have found out is to locate a linear power supply capable of delivering the required current at the indicated voltage.maywill see several agency approvals stamped on the case. I bet it does not have any European agency approvals because in that part of the worls, switchers are required to have a power factor correction (PFC) circuit which prevents RFI noise propagation back to the AC line.

[Todd, KA1KAQ]

Thanks Jerry. That's pretty much the assumption I had come to. Replacing it probably won't change much and I'll still have to deal with noise from a crappy switching supply. But it's good to at least have it narrowed down and to hear from folks who have far more experience in this area than I.

[WD5JKO]

This leads me to believe it's propagating back through the AC lines as much as radiating from the device.

Todd,

   I have a similar issue with a switch mode wall wort going to the cordless telephone base station. Same strong spurs up/down the band like yours. I also get RF into it, and on some bands, the phone talks aloud, starts the paging routine, and it has even removed the programming such that I need to set up the base/handsets as if new. You never mentioned RF from the ham set causing modem packet loss...

  Anyway, I only had tiny incremental luck with ferrite beads on the phone line, the DC power line, both together, common mode, or separate, etc. I was mainly fighting the ham interference issue, and the received spurrs as only a minor annoyance. To the XYL, having the phone go through fits when I key up was a big deal, and the spurrs on my receiver was of no concern to her.

  So today I made progress both ways. I unplugged that switch mode wall wort, and took a .01 uf 1KV disk ceramic capacitor, and placed the disk between the power prongs, and then wrapped each lead several times around the nearest prong.

  BINGO! The spurrs are gone, and so far running full power on 75/40m, the phone has remained calm when I am on the air...

  I need to go back and maybe solder that cap to the prongs..

So just maybe this will be an easy fix applicable to your situation.


Jim
WD5JKO