In article ,
William R. Walsh m wrote:
What I've noticed is that some stations just disappeared from the dial after
installing the signal amp. This is not a big deal, the amp works well
otherwise and I'm not missing the stations that don't come in. The user's
guide points this out as being a possible problem, and it says that the only
solution is to completely remove the amplifier. What it doesn't say is "why"
and that is what I would like to know. By the way, missing stations are not
confined to any one area of the FM band.
I suspect a phenomenon often referred to as "strong-signal
desensitization", a.k.a. "desense".
It can occur when you have a mix of strong and weak signals coming
into the system through the antenna. If you stick an amplifier into
the equation, it amplifies both the strong and the weak signals equally.
This can cause problems, if the signals involved are strong enough to
overload either the amplifier itself, or the "front end" in the FM
tuner/receiver. What happens, is that the strongest signals actually
push some stage of the tuner (or the amplifier itself) into saturation
or "clipping", and the RF or IF waveform becomes badly distorted.
When this happens, the circuitry can no longer respond to the
lower-strength signal at all, and the signal effectively "disappears".
By analogy: consider an old-style vinyl record, with the signal being
molded into the record in the form of a groove. Consider a signal
which has both a low-frequency component (bass) and a high-frequency
component (treble) at the same time. As long as they're of roughly
the same amplitude, the shape of the wiggles in the groove will be the
sum of the wiggling that you'd get from the low-frequency signal along
and the wiggling that you'd get from the high-frequency signal alone.
If the high-frequency signal is weaker (smaller wiggle amplitude),
things still work OK, as long as the total signal isn't too high.
Now, try turning up the amplitude of the whole signal a lot - say,
10:1 or so (20 dB). The low-frequency wiggle is so broad that it
actually slams the playback stylus all the way to the edge of its
travel in the cartidge much of the time... the stylus can't go any
further. For all of this time, when the stylus is jammed against its
limit stops, it can't wiggle at all in response to the weak
high-frequency signal. The playback system is now "de-sensitized" to
the weaker signal... it's not strong enough to overcome the saturation
effects of the stronger signal.
This is probably what's happening in your situation. You have some
strong local FM signals, and the amplifier is boosting them up so much
that they are "jamming the stylus" in your FM tuner.
In really severe cases, a strong local transmitter can even
saturate and desensitize the amplifier itself... clobbering *all* of
the desired FM signals. I had this sort of problem with a TV
redistribution amplifier... my 2-meter amateur radio transmissions
saturated it and wiped out all of the TV signals. My wife was
unhappy... and I had to find a specialized 2-meter notch filter to
keep the interfering signal out of the TV coax.
This sort of situation is relatively common, and it doesn't always
have to involve an amplifier. Some 800 MHz public-safety (e.g. police
and fire) radio systems have had desensitization problems, due to
strong cell-phone-tower transmissions on nearby frequencies. This has
been one reason for the "push" to digital TV... because it allowed the
700 MHz frequency band to be freed up, and some portions of it are
being allocated to public-safety users so they can move their
frequencies away from the desense-causing cellphone signals.
--
Dave Platt AE6EO
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