Ken Arck wrote:
>At 11:18 PM 4/13/2005 -0400, you wrote:
>
>You can have a repeater with no limiting and a user that is severely
>clipped and have a total deviation of only 3.5 kilohertz will sound
>considerably louder than a user running 5 kilohertz deviation not being
>clipped.
>
><---No argument about this at all, Kevin. However if I read him correctly,
>he said these dev amounts were measured on a service monitor.
>
>Regardless of whether one signal "sounds" louder than others or not, the
>amount of deviation is the amount of deviation :-)
>
I'd bet he has his deviation set to 4.5 kilohertz; where clipping of his
repeater transmitter starts to occur. In a Maggiore transmitter (could
happen in a Micor or Mastr II as well) harmonic filtering of the clipper
can add to the total deviation. Have you ever set up an exciter using a
fixed audio tone (say, 1 kHz), where you adjust the deviation control
pot to yield 5 kHz deviation. The limiter in most two-way radios is
really just a clipper; no fancy compression or AGC or anything. So
theoretically whatever tone you stick into the input should be
hard-limited at 5 kHz deviation by the clipper. But if you crank up the
audio generator some more, say increase it another 6 dB, the deviation
will creep up somewhat, maybe to 5.5 kHz. If you really slam it hard,
you might see 6 kHz deviation or more. Why? Is the clipper failing to
clip? Nope. The problem is caused by the low-pass "splatter" filter.
Here's why...
Clipping produces odd-order harmonics. The low-pass filter's job is to
scrub off those clipping harmonics to prevent the bandwidth from
exceeding limits. Occupied bandwidth in FM is a function of the
deviation AND the audio bandwidth (actual modulating frequency). The
goal is to keep both properly limited to prevent the signal from getting
to wide and "splattering" onto adjacent channels. The problem comes in
when the audio is excessively clipped, which puts more and more energy
into the harmonics. The splatter filter attenuates those harmonics --
that's its job. In order for the signal to remain perfectly limited
(clipped), all of those harmonics need to be maintained, both in
amplitude as well as phase coherence, but obviously we can't do that.
As the harmonics are filtered off, the fundamental will overshoot the
preset clipping point. And the more harmonic content is being filtered
off, the more overshoot there will be.
His original complaint doesn't point to a problem in the set-up of the
repeater, it does, however, point to users that are severely clipped.
The "fix" isn't in the repeater, but rather in the users radios that are
too hot.
Kevin Custer
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