Greetings:
Here is my newest Kirt's Cogitation. Your comments are encouraged.
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.
Here is my YouTube video demonstrating how a pendulum-driven clockworks operates.
Melanie is doing the Vanna White thing.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.
