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Low RFI Cable Modem?




 
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AJ1G
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« on: August 16, 2017, 02:57:17 AM »

Trying to trim my Comcast bundled service costs here.  One thing I'm looking into is buying my own cable modem vs renting theirs.  Hoping that there also might be one out there that is less of an RFI generator than the one I gave been renting from them at 10 bucks a month, which really raises the noise floor on 75 meters especially when the ambient lightning QRN levels are  low.  Any recommendations? Need one that supports my home phone service that is part of my service package.
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Chris, AJ1G
Stonington, CT
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« Reply #1 on: August 16, 2017, 01:12:49 PM »

You may find, as I did, that most of the HF RFI generated by cable modems and routers comes from their power supplies. I've tried several and this was the case 100% of the time.

I solved this by using an analog 12V power supply (1974 Archer CB Radio Power Supply) which easily had enough current capacity to power both the cable modem and router. The latter has numerous long CAT5 tentacles reaching out throughout the house, thus the importance of RFI reduction/elimination at the source.

Another approach is to use very high permeability toroids and wrap both the power cords and 12V output leads through them. For "wall-warts", you'd need to plug them into an extension cord to accomplish this.
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W1RKW
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« Reply #2 on: August 16, 2017, 05:05:27 PM »

Chris,
I echo Clark's assessment. I don't have a cable modem but a DSL modem and I'm on my 3rd one in 12 years which I bought online to get away from the 2 provided by the phone company.  My SNET one died, not sure why. The Frontier modem was a POS. And the current one is an Actiontec which works wonderfully. That being said, they all had one thing in common and that was a SMPS. The power supplies were polluters. My solution to reducing significantly or eliminating RFI was to shield the SMPS wall wart in a metal electrical box which was grounded to the electrical ground.  For the Actiontec I got away from the shielding and simply went with a linear wall wart. The SMPSs will crap out over time too. So simplified my life and went linear. Again, RFI gone and hope this PS out lives the SMPSs, so far so good.. And a side note, all modems were in close proximity to my radio gear.
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Bob
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His fear was when I turned it on for the first time life on earth would come to a stand still.
kd1nw
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« Reply #3 on: August 16, 2017, 06:27:30 PM »

I'm using an arris / surfboard sb6182 and it seems to be pretty good. the surfboard is a cable modem only, no phone line and no battery. I just went downstairs with a portable shortwave on 3.885 and i had to get within 6 inches to hear anything from it. i got the radio a good 2 feet away and then brought the power cord from the wall wart toward it, i had to basically touch the wire to the radio to hear the rf from the power cord. placing radio near the wall wart produced the same racket when i was about 3 to 4 inches away. at least I don't have to unplug it like my previous unit which was by zyxel.. that thing was attrocious S9 to +10db broadband hash from at least BCB to 30MHz. I could walk around my yard 200 feet away from the unit and I could still hear it. the zyxel unit had a battery so I could unplug it and eliminate the noise when i wanted to operate, verifying what Clark and Bob said.. the switching power supply.

I guess I'd be careful of my "endorsement" though. I've had the sb6182 for around 5 years i think and I would suspect that arris uses different parts and suppliers for various runs of the sb6182.. pcb board revisions, different supplier of wall wart etc.. they could be noisy now. I would say make sure you know the return policy before buying in case it's unbearable. Good luck and let us know what you find

P.S. Sorry Chris, I was going to say you could come over and evaluate it yourself seeing as you are close by but I just saw you need phone support which pretty rules out my unit working out for you. Sorry I just caught that...

73 Kevin
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W6TOM
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« Reply #4 on: August 17, 2017, 10:27:46 PM »

  When I was back visiting in MA last Fall I set up a station in my sister's basement family room, trying to listen to the 3885 military net I had RFI on frequency and it was every 100 KHz up and down, indicative of a switching power supply. My brother in law has a cordless modem and his cable modem right by where I was operating. I'm pretty sure is was the wall wart but it was behind a 6 foot tall entertainment center, not easy to move. I used the filtering in my transceiver which attenuated it enough to make it tolerable.

 This year I will bring some ferrites and try that. When I did RFI complaints for the local power company I often found this type of RFI source, a switching supply.
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