[RFI] Rectified 120Hz RFI?

[Hare, Ed W1RFI]

>>Anyone have a theory about what would cause RFI with a primary 120Hz
>>component instead of 60Hz?

> Could be an arc on both the positive and negative excursions.
 
Jim is right; that's exactly what happens most of the time with power line noise.  Rarely, the arcing can be assymetical on positive and negative peaks, usually when there is some corona involved.   Noise with a strong 120-Hz component is a diagnostic giveaway to indicate electrical noise.

This may be arcing or sparking on power lines, or other types of devices ranging from fluorescent bulbs, dimmers, LED controllers, and the like.  Some of those have a 60-Hz pattern rather than 120 Hz.
 
It is useful to look at the waveform in the time domain (oscilloscope on your receiver). Set the receiver to a wide bandwidth, in the AM mode and connect a scope to the audio output.  If it looks erratic, think sparking. If it looks very regular and clean, I would look more toward devices than power lines.
 
Those switching power supplies can also make noise, but although they can exhibit a 60- or 120-Hz component, they usually create their noise on harmonics of their free-running frequency, typically somewhere between 5 and 50 kHz, although they can run lower or higher.
 
Switching supplies are used in nearly every electronic device being made today.  Switching regulators are also used in battery chargers or solar array controllers.   The ARRL Lab gets occasional reports of interference from these types of devices.  
 
When those reports started to surface, I wanted to see whether this is a systemic problem, a handful of noisy models or the occasional defect or atypical installation. One switching regulator is found in the LV power supplies used for 12-volt lighting.  Of the ones that we found at Home Depot, Lowes and a local electronic supply house, all were operating below the FCC limits. This still poses a moderate intererence risk, but so far, the legal ones have not turned out to be the next BPL.  Unfortunately, with most of the reports we get on these devices, we typically can't obtain a model number from the complainant and or a source where the noisy unit was bought.  In a few cases, we helped a ham diagnose a defective unit. In others, we either figured out that the ham had guessed wrong on what it was, or we suspected same, but never heard back from the complainant.
 
I do know of a few import models that are not FCC legal.  In the few cases where we identified an illegal product and could point to the source, ARRL filed a formal complaint with the FCC.  There are a few things to look for.  Switching supplies that operate under Part 15 of the rules will have the "This device must not cause harmful inteference" labeling.  Devices like controllers for luminaries operate under Part 18 and they will have The Fcc logo on them.  Devices that lack both are often not tested by the manufacturer for compliance.   ARRL has built a testing room that uses the test methodology and equipment described in the ANSI C63.4 industry standard to measure radiated and conducted emissions.  When we can get a model number, and a sample of the noisy unit, sometimes purchased by the League, the first step is to measure its conducted emissions. A device that exceeds the limits is illegal irrespective of any other factors.
 
Even legal devices can cause harmful interference.  In that case, after working with the complainant and/or his or her neighbor, utility, or nearby business, Mike Gruber, W1MG in the ARRL Lab, wraps the case into the program we use with the FCC to try to address power line noises.  The results of that work can be seen in the letters sent by the FCC to the neighbors of hams who will not correct interference that results from the operation of some consumer equipment (http://www.arrl.org/part-15-radio-frequency-devices  and a similar page for power line noise also on the arrl.org site). Some of the more interesting cases have involved 1000-watt lighting controllers used for a form of indoor gardening.  
 
For the most part, ARRL has one full-time staffer who works almost exclusively on power line noise. Some of these cases can take tens of hours, so although the League has managed to do quite a bit in the RFI area, we also rely heavily on reports from Amateurs. The more specific those reports the better. I occasionally have time to prowl the lists and newgroups to find posts like this and participate a bit, but information sent to our staff is very important to the work of protecting Amateur Radio from RFI.  
 
You can contact Mike Gruber on problems using rfi at arrl.org.  Though I kept the call I worked hard to deserve, my work takes me more into industry committees and FCC arenas, so Mike is the correct point of contact.  Reporting RFI is important, as the database Mike maintains is a growing source of information about the scope of current RFI problems affecting Amateur Radio.  This is also used in various committee and FCC work.
 
,
Ed Hare, W1RFI
ARRL Laboratory Manager
225 Main St
Newington, CT 06111
Tel: 860-594-0318
Email: W1RFI at arrl.org

________________________________

From: Jim Brown [mailto:jim at audiosystemsgroup.com]
Sent: Wed 8/4/2010 11:40 AM
To: RFI List
Subject: Re: [RFI] Rectified 120Hz RFI?



On Wed, 4 Aug 2010 08:16:59 -0700, Kelly Johnson wrote:

>Anyone have a theory about what would cause RFI with a primary 120Hz
>component instead of 60Hz?

Could be an arc on both the positive and negative excursions.

73, Jim K9YC



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