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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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AJ1G
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« Reply #5 on: March 22, 2018, 07:16:33 PM »

Bringing this thread back up with GOOD NEWS Grin....

We had to have a Comcast service tech come over today to investigate a problem with intermittent one way outgoing dropouts or on-connect complete lack of outgoing packets on our landline VOIP phone service.  I had already confirmed that this problem was not due to our in-house copper circuits or phones, which the tech also confirmed. He also replaced the cable grip support wire loop on the house end of the drop from the street, which I discovered had apparently snapped during one of the nor'easters that by this month. This resulted in the cable pulling at a sharp right angle on a small clamp about 5 feet below the cable support hook.  I tried to get the tech to replace the drop cable, as it is now over 35 years old, but a signal level check showed all was OK, he showed me the spectral plot of the feed he made coming into the modem.  Since the VOIP problem has been intermittent, and was not happening during the service call, he offered to swap out our cable modem for a new one. He installed an Arris Model TG1682G, which of course supports the VOIP landline.  So far so good on the VOIP problem.  It also brought my data download data rate up to over 150Mb/sec, we were limited by our old modem to a nominal 50 Mb/sec.

As soon as he left, I went down to check on the cable modem RFI that I have been living with for a while now, which was especially bad at the low end of the HF spectrum, including the BCB, 160 and 80 meters.  What a HUGE improvement...NO modem RFI at all, I am now noise floor limited (but not much) by a neighbor's oil burner cycling, I haven't been able to hear that in years, and it's very low SNR when it's intermittently present.

The twenty-something tech seemed absolutely clueless about HF radio...when I told him about the RFI problem I was having with the old modem, he told me that the cable systems use spectrum between 2 and 52 Mhz, and that I should not be operating anywhere over that frequency range!
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Chris, AJ1G
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« Reply #6 on: March 22, 2018, 08:02:42 PM »



So, I hope you informed the ute of his error.


klc
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AJ1G
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« Reply #7 on: March 22, 2018, 08:59:30 PM »

Yes I did. 
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Chris, AJ1G
Stonington, CT
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