Kirt's Cogitations™ #248 - AM/FM Under Siege
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Cog·i·ta·tion [koj-i-tey'-shun] – noun: Concerted
AM/FM Under Siege
The electromagnetic world sure is a noisy place and it is getting worse all the time - in every region of the spectrum. Intentional radiation is not so much of a problem because it usually falls within well-defined limits and is predictable, but sloppy engineering and, honestly, ignorance, has made life harder for just about everyone. Listeners to broadcast radio in both the AM and FM bands have really taken a hit.
AM has always been prone to interference by its very nature, so anyone listening expects the occasional pop or hiss from atmospheric phenomena or a light switch being flipped on or off. Have someone in the house run a blender or drill and you can forget hearing anything until the task is completed. It comes with the territory, so to speak. FM was and is largely immune to most forms of interference, but lately I have been noticing it coming from some of the most unusual places.
For as long as I can remember, I have preferred to have a radio on in the background whilst whiling away at work and at play. In the days before Al Gore invented the Internet and Mr. Jobs created the iPod, my favorite entertainment was provided in the form of a radio. It was almost always possible to locate at least one station that played acceptable music, news, or talk shows. Some otherwise intolerably long days at work were made better by the presence of radio's subtle diversion. Pulling in broadcast in both bands from halfway across the country in the nighttime hours with just a cheap clock radio was really great. I especially appreciated being able to listen to local news and weather from, say, Milwaukee, Wisconsin, while tuning in from Annapolis, Maryland. Keeping current on local happenings is also one of the primary reasons for my liking over-the-air broadcasts.
Most of us over forty probably have stories about the lengths we have gone to to pick up radio stations from within a steel and concrete office building. Often, it was a team effort. BTW, I now do most of my radio listening via the Internet because it is hassle-free.
About five years ago while living in North Carolina, I tuned into an AM station during my drive to and from work specifically to hear local news and weather. The broadcast crew were a couple of real wits, so the entertainment factor was enhanced considerably by their presentation. After a couple years of the same route at the same time every day, I began noticing interference as I approached a particular area. It would build to a crescendo at one point, then die out again. It did not happen all the time, so I searched for a pattern. The common factor was high humidity, or rainfall. Ah-ha.
I contacted the FCC to see whether they had gotten any other complaints about interference. No, they had not. Besides, as I found out, the FCC no longer (at least at the time) was in the business of hunting down electromagnetic interference in broadcast radio bands. I was on my own, per the representative that I spoke with. So, I called the power company to suggest that the noise might be originating from a malfunctioning piece of their equipment. I used my most authoritative electrical engineer voice to explain how there is a chance I had stumbled upon a disaster waiting to happen. They bought it.
A week later a field agent whose job it was to investigate just those types of failures called back to say he had indeed found a leaky transformer. It was located back in the woods and serviced a private garage, a couple hundred feet off the road. He went out on a rainy day based on my story and said the beast was actually throwing off sparks. When he talked to the owner about replacing the transformer, he told the agent something like, "Oh, it's been doing that for a long time." Ignorance is not necessarily bliss.
Those types of interference can be excused, because they are not anyone's "fault." The transformer was replaced, and the noise went away. Some interference should never happen, though, as with the next couple anecdotal instances.
As some of you may know, I like pendulum-driven clocks. My fascination is primarily with the mechanical movements, although I have always appreciated the fine woodworking in many clock cases. Nice clocks with high quality movements are expensive, so the two regulator models I own were purchased for a little under $100 on eBay as near-disasters. I successfully restored both two wooden cases, and one clockworks, but the mechanical movement in one of them was too worn out to be repaired easily. The holes in the metal frame where the gear axles rest were about 50% larger than the axle diameters. Well-designed clock movements require a minimum amount of energy input during each cycle of the pendulum or the balance wheel/spring. When the gears do not move easily, the main drive spring or weights cannot overcome frictional forces and the clock either does not run at all, or is does not run for long with reach rewinding or weight resetting. A video that I made of the movement that was restorable is now on YouTube (shown to the right).
Keep reading, please. There is an RF-related story here.
So, while I wait until I can stand to part with $300-$400 for a replacement pendulum movement, I installed an electronic model instead. It uses a cone speaker driven by a digitized chime sound. The sound is actually pretty good as it rings out the Westminster tones, and then a bim-bam for each hour. When listening nearby it sounds almost like the real thing, but not so back in the bedrooms.
Being electronically generated, the tones generated by the circuit are not as pure as a solid or tubular mechanical chime would be. The sound is actually comprised of the fundamental and harmonics; to what degree I do not know. However, that fact that they are not pure is made evident by the way the tones are perceived when in a bedroom or in the kitchen where the pressure fronts experience the same kinds of multipath excursions as RF do between the transmitter and receiver. Various frequency components arrive at different phases that combine to create some pretty sever distortions. Depending on where I happen to be in the room, each of the tones can sound very strange - often enough to make me cringe. Moving to another spot results in some unique sounds.
A pure tone would also experience multipath effects, but the perception at any point would be only a increase or decrease of volume depending on the overall amount of constructive or destructive interference, but the frequency is not changed.
Now here is the RF application that I mentioned. While the electronic chime is chiming, I get a very high level of noise on the FM radio (yes, FM) in the 88-90 MHz realm. Evidently, the clock oscillator that runs the microprocessor for the chimes has very high harmonics that extend into the FM band (and likely beyond). The interference is constant throughout the chiming sequence. My guess is that the movement is violating FCC regulations for unintentional radiation, but I will not bother to report them to the Feds - they probably do not care. If the RF Cafe laboratory had a spectrum analyzer to capture the entire spectrum, maybe the FCC would take an interest. There is no visible FCC or CE mark on the case. This is an example of sloppy engineering causing grief.
I wrote a while back about the compact fluorescent light (CLF) bulbs that I have deployed throughout my house, primarily as an energy saving measure. The slight delay in turn-on time and the color oddity does not bother me as much as it did initially. It is nice in the summer to turn on a couple 100 W bulbs that really only add 26 W of heat to each to the room; that is 74 W of heat that the AC system does not have to remove to keep me comfy. I have noticed no RFI issues at all with any of them. Oddly enough, the only fluorescent light that generated interference has come from the 4-foot high energy (and supposedly high-efficiency) tubes in my kitchen ceiling fixture. Those things have been around for decades, and they still mess with AM radios?
But wait, there's more.
Last June (2008), I had a new gas-fired furnace installed. It is a top-of-the-line, 94% efficient model from Trane (Model #4TXCB025BC3HCAA). The unit is small because our house is only 940 square feet. There is a compressor outside for air conditioning, but it is not configured for double duty as a heat pump because, I am told, they do not do that up north. Since, as mentioned earlier, I like to listen to over-the-air broadcasts when possible, I usually tune in one of the local AM radio stations. The furnace never ran over the summer so I never detected any issue with interference. It was not until around the end of September that when the furnace fired up, it created a lot of noise on the AM radio - across the entire 520 to 1,610 kHz band. It only occurred with a radio powered off the household AC supply, not when operated on batteries, so I deduced that the interference was being conducted through the household AC wiring, not being broadcast through the air. I was utterly amazed that a modern system would emit such a high level of conducted emissions.
Suspecting a potentially flawed unit, I called the Trane installer. He response indicated that the gas-fired models do that because of noise generated on the flame controller board. Fortunately, he is a good guy and sent a technician out to install an EMI filter on the AC lines where they exit the controller housing, free of charge. It was a significant filter, consisting of an isolation transformer, a couple big coils wound on ferrite toroids, and a couple capacitors. It completely fixed the problem.
When I contacted Trane about the interference issue, their terse response was that I can contact my installer and have a filter installed for a relatively low price. Being downright indignant at the slacker attitude, I felt compelled to rip them good in a letter to the QA department. I quoted conformance requirements provided to me by me friend and RFI/EMI/EMC expert David Guzman, of RfTek, and further chided the company for having the unmitigated gall to install such offensive products in residential structures. After all, I told them, I have in the last four years had two competitor HVAC installations completed in other houses and none of them exhibited any interference at all.
That must have really put the fear in them, by Jove. It has been several months now, and nobody at Trane has dared to contact me in response. Or, maybe they just had a good laugh around the Continuous Improvement Committee meeting table at yet another foolish customer who actually expects a quality product from them.
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