Rfi From Modem

Hi all,

I don't know if this is proper forum for this, but here goes.

Some time ago, I switched to Spectrum for my phone and internet service. They installed a modem and a router at my residence.

The router is no problem, but the modem, which is a CISCO DPC3208, generates a tremendous amount of radio-frequency interference. It is sufficient to make reception impossible on a AM radio.

Spectrum seems to think that this is a grounding problem. I don't see how - the modem case is plastic which makes it impossible to ground. Power is supplied to the modem with a "wall-wart", which has no ground pin.

CISCO, who apparently manufactured the modem, has proven themselves to be entirely worthless in this matter...they won't even discuss the matter unless you are a large customer and have a contract number.

I don't know where else to ask. Has anyone encountered this problem, and know of a solution?

Hopefully,

Kurt

 

EMI/RFI is frequently due to grounding problems so I agree with Spectrum. That said, AM radio is very susceptible to RF interference. Even in the best scenarios, RFI is often unavoidable. That's one of the primary reasons FM became so popular.

Grounding has nothing to do with the modem's plastic case. It is the electronics inside that matters.

Are you certain it is the modem? If you unplug the modem from the wall, does the RFI noise go away?

The first (and free) thing you can try is to make sure the modem's and the AM radio's power cords are in good repair and securely fastened. The second thing to try is to turn the modem's power supply ("wall-wart") over - if possible. That is, if its two prong connector is not "polarized" (both spades are the same shape and size), you can try reversing them. Same with your AM radio.

You should also try to position the radio as far away from the modem and other potential sources of RFI (the computer, TVs, etc.). If the radio has an external antenna, re-orient that.

Because the distance between each wall outlet to the service panel is different, the value to Earth ground at each wall outlet will be different too. This can create a "difference in potential" that can result in RFI. If the radio and modem are plugged into different outlets, try plugging them into the same. If plugged into the same outlet, try using different - preferably on different circuits.

The above all assumes you wall outlet is properly wired. Every home and every computer user should have access to a AC Outlet Tester to ensure the wall outlet is properly wired and grounded to Earth ground. I recommend one with a GFCI (ground fault circuit interrupt) indicator as it can be used to test bathroom and kitchen outlets (outlets near water) too. These testers can be found for your type and voltage outlet, foreign or domestic, (like this one for the UK) at most home improvement stores, or even the electrical department at Wal-Mart. Use it to test all the outlets in the home and if a fault is shown, have it fixed by a qualified electrician.

 

Digerati,

To first answer the questions you had...
When the modem is disconnected from its power, the interference stops instantly.
I tried reversing the wall-wart's AC connections as you suggested. There was no change.
A radio at the opposite end of the house, and at right-angles to the first one, experiences the same type of interference.

Experimenting a bit further, I have learned that...
When ONLY the power plug (15 VDC) is plugged into the modem, there is no interference.
Besides the power cable, there are three others plugged into the modem: the cable to the router, the coaxial cable to outside, a regular phone cable for the phone.
Every time one of these three cables is plugged in, the interference becomes worse.

The reference to a plastic case was made with the thinking that a metal case could be grounded and serve as a "faraday cage" to contain/absorb the interference before it could escape.

I am wondering if a metal enclosure might be the best thing. I live in an older mobile home that does not have a modern electrical system (there are no three-prong ground-type outlets).

The only ground immediately available is the outside conductor (sheathe) on the coaxial cable.

Would the cables be radiating interference, or would that be coming solely from the circuit board?

Kurt

 

Please expand on this for clarification and certainty. Are you saying if you unplug the coax cable from the modem and all other data cables, you hear the interference? If so, then the problem clearly is the modem.

I'm asking because proper shielding and grounding of the coax cable is essential too. So if you meant no output cables connection and only the power and coax, then it could be a damaged coax.

And yes, the cables - in particular the coax cable - can easily be the culprit here. This is especially true if the cable or one of its connectors or a splitter is damaged.

And note it is most likely you have at least one splitter in your coax cable, possibly more. Typically there is at least one to split between the Internet and TV. Then there may be more splitters to feed TV signals to other parts of the home. Often, the first splitter is outside the residence, possibly exposed to the elements. Even if in a J-box outside, they get exposed to extreme swings in temperature and high humidity. Each splitter should be inspected to ensure the connections are undamaged and tight. And if corroded and/or very weathered, replaced. At some point, there should be a ground connection from the cable/splitter to ground. This is often in the J-box. The only ground should NOT be via the modem.

The splitters should at least support up to 1000MHz bandwidth. Typically, these are supplied by the cable company. If less than 1000MHz, that typically indicates they are old.

As far as putting your modem in a metal box for shielding, my point is you should not have to. By design (not to mention by law in most jurisdictions - including the entire US and most, if not all of the EU) RFI is supposed to be suppressed at the source by (1) eliminating its generation in the first place and (2) shielding and blocking any excess in the immediate area of the RFI generating device. In other words, a metal case should not be necessary. If it was, necessary, it would have come in one or not be allowed to be sold in those jurisdictions.

Also, I would worry about heat build up if put in a metal box.

Are there no grounded outlets in your bathroom(s) or kitchen? You should also look at the grounding at your radio.

Also, one common problem with 2 wire outlets is they may be wired backwards. That is the hot and neutral my be reversed. Because there is no 3rd wire (the ground), the outlet can still be safely used, as long as there is no connection to a properly wired two-wire outlet. So using one of those outlet checkers with a 3 to 2 prong adapter can verify the hot and neutral are where they should be.

You might also look to see if there is a Ham/amateur radio operator club in your area. My bet is if you call them up, explain your situation, they will be happy to offer you advice, perhaps even coming by and taking a look.

While you may not have any grounded outlets, I would bet your mobile home is grounded at your service panel. You may need to consider upgrading your wiring - at least to the outlets powering your computer/network gear and your radio.

 

Digerati,

>> When ONLY the power plug (15 VDC) is plugged into the modem, there is no interference.

What I meant was this.

Modem
Connections Interference

1 Power only No interference
2 Power and coaxial cable Slight interference
3 Power, coaxial cable Slightly more than 2
and router cable
4 Power, coaxial cable, router Slightly more than 3
cable and phone cable

I did not attempt to "rank" the cables by the amount of interference they caused.

There are no splitters. The coaxial cable that comes in from the pole connects only to the modem. The cable was installed only a few months ago and is in good shape, at least in the areas above ground.

The sheath on the coaxial cable is grounded at the outside connection.

Kurt

 

So you don't have cable TV service too? If not and this is a new installation then you might not have a splitter. But you must have a coupler or splice in the cable of some sort in the box outside or else the shielding inside the cable could not be "tapped" to provide proper grounding to Earth ground.

Since you hear the noise with just the coax connected, that tells me the problem is most likely associated with the coax cable. The cable itself is probably okay - assuming it is not damaged (cut or smashed) or has any sharp bends or kinks in it.

All coax (and Ethernet) cable routing around corners should be in nice, even curves to avoid bends and kinks. And data (coax, Ethernet, phone, etc.) cables should never run parallel to power cables. The greater the distance between data and power cables the better. If they need to cross, do

So it is more likely it was improperly terminated (connectors not properly installed somewhere between pole and modem) or the Earth ground to your home is not good (should be checked by professional - power company may do it for free). The other potential suspect is the modem itself has a fault. In any case, it is not likely a design problem or other users of that model modem would be complaining too.

Do you have another AM radio you can check with - preferably a battery operated one? If you can check with a second radio and you still hear the noise with it too, that will eliminate your first radio from the equation.

If the battery operated radio is nice and clear, the problem may be on your radio's end.