[SCCC] 40M RFI from Ooma VOIP phone hub - HELP!

[Marty Woll]

Hi, Dennis and SCCC'ers.

I sent ARRL Lab manager Ed Hare W1RFI an inquiry about the problem you reported.  Despite the late hour on the East Coast, I received a very prompt and thorough reply, which appears below for the info of those concerned over the issue.

As ARRL members, you have a valuable resource in Ed and his staff.  Don't hesitate to report such matters to them if you can't resolve  a problem or are uncertain as to how regulatory limits might come into play.  Ed sits on IEEE's Electromagnetic Compatibility Standards Board, and he is well known among those in the business.  (His influence is a major reason that later-generation BPL systems - may they rest in peace -  do not interfere with Amateur HF bands.)

73,

Marty N6VI

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----- Original Message ----- 
From: Hare, Ed W1RFI 
To: Marty Woll 
Cc: Norton, Richard (Dir, SW) ; Gruber, Mike W1MG 
Sent: Thursday, March 25, 2010 7:58 PM
Subject: RE: [SCCC] 40M RFI from Ooma VOIP phone hub - HELP!


I'm not surprised by the report. FCC does not have radiated emissions limits below 30 MHz.  They control emissions by limiting the amount of noise that is conducted onto the electrical lines. There are no specific conducted or radiated emissions limits on the signals conducted onto the phone wiring. The principles behind this are:

o Small devices generally don't radiate well at HF, but the wires connected to them do.
o Telephone wiring is generally well balanced and radiates less than power lines

I don't agree completely with all of the rationale above, but that is the general reasoning of the regulators and industry standards committees that in essence set the limits.

Under the rules, the manufacturer of such a device has a further requirement to use good engineering practice, although that is a very vague requirement.   In general, a device that would meet emissions limits that apply to any other type of device would usually be considered to be using good engineering practice.  On HF, the only radiated limit that applies are the limits for intentional emitters or carrier current devices.  Over the HF range, this limit is 30 uV/m 30 meters from the source.

Under the rules, the operator of a device has a requirement to use it in a way that does not cause harmful interference to licensed services. Through the same cooperative program that ARRL has worked out with the FCC to handle power-line cases, the FCC has sent advisory letters to the neighbors of hams with RFI problems, atlhough this has yet to get to any formal enforcement. 

The emissions limits are generally of little help.  A device that meets the conducted emissions limits below 30 MHz would typically generate about S7 interference to abutting radio systems, on any frequency for which the emission was at the limit.  A device that meets the radiated limits for intentional emitters would typically create an S9+10 dB noise level 100 feet away. (This was and still is a major aspect of the concerns about BPL.)  In reality, it is the harmful interference clause that protects licensed services when interference occurs. It is the intent of the limits to control the likelihood of harmful interference by limiting the geographical area over which interference is possible. 

Having said all that as background, the only good news here is that the signals conducted onto the phone lines do NOT have to be present above the audio baseband needed by the phone. This should mean that a telephone RFI filter installed at the phone-hub device may be able to control the RFI.   If it doesn't, it may also be necessary to try to filter the ac line. I'd start with a common-mode ferrite choke made from about 10 turns of the ac line onto an FT-240-43 ferrite core. (Don't translate this to mean one ferrite bead, as on 7 MHz, inductance is needed to make a good common-mode choke). 

At the levels reported here, if the noise is radiated by the ac power wiring in the neighbor's house, it is very likely that the device is putting more noise onto those power lines than the rules require.  The ARRL Lab has the test facility to measure those conducted emissions if we have a unit in hand. (If need be, we can buy one, if it looks like there is a need and value in doing so.) 

If the device meets the emissions limits, then this becomes a matter of harmful interference, which can be addressed through the ARRL program to report and resolve RFI cases with the FCC.  Send email to Mike Gruber, W1MG, at ARRL HQ, rfi at arrl.org, with all of the details.  If the device doesn't meet the limits, then this is something that ARRL can facilitate getting reported to the FCC.  Use the same email address, but with a cc to me, w1rfi at arrl.org.

I would think that the next steps in this case should be to:

o Try a telephone RFI filter on the phone line, installed at the hub.  If you don't have one and are willing to try one and report back to Mike Gruber, he can send you one, as we have a few of them here in the Lab.  

o If the telephone RFI filter makes little difference, it is very likely that there is an emissions-limit problem here, so that would be sufficient justification for ARRL to buy and test a unit. 

o Nonetheless, if the telephone filter doesn't help, do try a common-mode choke. Mike can also supply the correct core, if you don't have one handy. 

It could be that the correct cure for this may be to add some internal bypassing and filtering inside the unit.

The best solutions to these problems are usually local, and if the local guys working on this can identify the correct cure, ARRL is pretty effective with its industry contacts at persuading manufacturers to undertake some program to resolve problems systematically.  I don't do so on behalf of the industry committees on which I serve, but when manufacturers realize they are in communication with someone who serves on the IEEE EMC Society's standards board, they usually are willing to listen.  We do have a few success stories under our belt, although there have been some issues that are still not resolved.

I do recommend, though, that in the local work to resolve this, you NOT modify a neighbor's unit. If you get to that point, let me and Mike Gruber know and we'll get one ourselves in the Lab and see what fix can be made.  This is something we can get to the manufacturer, IF this is a real US manufacturer, and not just being imported.  We have much less success with overseas manufacturers, assuming we can even figure out who some of them are.

One of this lists participants sent this thread to Marty Woll, the ARRL Southwestern Division Vice Director, who forwarded this to me.  ARRL is quite willing to try to help resolve these sorts of problems, but to do so, our staff need to be informed. Unfortunately, there are dozens and dozens of lists to monitor, and we just don't have the time to keep complete track of them all. I know that Mike Gruber does check out the rfi list from time to time, but right now, he is working on revisions to the RFI chapter in the next ARRL Handbook, so he has probably not seen the thread.

73,
Ed Hare, W1RFI
ARRL Laboratory Manager
225 Main St
Newington, CT 06111
Tel: 860-594-0318
Email: W1RFI at arrl.org