[RFI] Xfinity Router RFI (Xfinity router name XB3)

[Jim Brown]

On 2/13/2020 10:21 AM, David Huff wrote:
> I have a couple of suggestions based on dealing with one of these before.
> 
> This device actually does contain a switching power supply integrated inside of it, that is how they make it so thin.  It also has a 2 pin power cord for 120V AC (IEC 60320 C7/C8 polarized).  The switcher power supply is built into the modem box.  The box is mostly plastic, not steel, so it radiates lots of noise.  The ethernet jacks on the back are probably also plastic, with no shield connection.  It also contains a battery built into the bottom.
> 
> My suggestions are:
> 1. The box is mostly plastic, and does not actually have a ground pin on the 120V cord.  It will radiate signals onto any wires coming out of it. 

Most RFI is radiated as a common mode signal on wiring, and a ground pin 
does NOTHING to help.

  Therefore you need to add ferrites or filtering to keep the noise 
inside it if possible.

Yes. That means multiple turns on #31 cores, not just "add ferrites." 
See k9yc.com/RFI-Ham.pdf  and http://k9yc.com/KillingReceiveNoise.pdf

> 2. Add ferrites to the AC power, ethernet cables, and the phone lines if possible. If using USB or ethernet from it, consider adding ferrites on those cables as well.

> 3. You might also want to add clamp on ferrites to the coax cable connected to the back of it as well.  These devices have a habit of injecting the noise onto the coax shield, which makes your home coax TV cables into a radiating antenna.

> 4. You should use shielded ethernet cables if possible when you can. That will keep the ethernet noise contained within the ethernet cables, and hopefully reduce any coupling from the noise in the box getting out.
> 5. The MoCA residential gateway stuff uses the RG-6 cable TV cables as the transmission medium for some ethernet type of signals.  One problem I have seen is not all devices treat the shield of the TV coax the same.  You may need to clamp ferrites on the coax of those devices at the other end or in other rooms.

You've just described a Pin One Problem.

> 6. Check the TV coax system in the house to see if it has a ground connection for lightning.  I have done troubleshooting on a similar problem that turned out to have a problem with grounding.  Also note having multiple grounds on a cable TV system can also lead to problems.

By law, ALL earth connections must be bonded together. See N0AX's ARRL 
Book on grounding and bonding, or slides for a talk I've done at Visalia 
and Pacificon. Ward and I collaborated on it.
http://k9yc.com/GroundingAndAudio.pdf

> 7. This device has a battery built into the modem, which means testing for RF noise also needs to consider that turning the input power of does not always turn off the unit.
> 
> In theory, if the person who owns it did not need to use wifi, you could put it inside a large metal shield box, and see if that makes any difference.

Effective ONLY if every cable entering and leaving the box has its 
shield bonded to the box at the point of entry/exit, and with unshielded 
cables bypassed to the box at the point of entry. And proper chokes 
suppress what's conducted to the cables, leaving only what's radiated by 
wiring within the box itself. Probably not much on 15M.

> 
> 
> I looked up the manuals for the XB3 below.  You mentioned the residential gateway which uses MoCA.
> < https://www.xfinity.com/support/articles/broadband-gateways-userguides >
> < https://ciscodocs.technicolor.com/web/consumer/pdf/OL-29163-01.pdf >

That's useful!

73, Jim K9YC
> 
> Thanks,
> David
> W0IM
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