On Tue, 10 Jul 2007 13:00:41 -0700, Telstar Electronics
wrote in
. com:

Assuming you set your processor's limiter to 100%, you still need
to set the output level to the radio so it doesn't overmodulate. Even
just 101% modulation means clipping; pump that with 100% average noise
and you have the potential for some serious bleedover. And there's no
way for the average user to determine the precise point at which his
radio is at 100% modulation. Radios are set at the factory for 95% to
98% modulation to allow for slight variations in tolerance that happen
during normal operation; for you to claim that your processor limits
modulation to 100% all of the time is not only absurd, it's impossible
because of the tolerance limitations of the radio.

I think this is the last of your comments I didn't have time to
respond to before. You are certainly correct that you need to set the
radio so it doesn't overmodulate. In the VoiceMax installation
instructions it says that for a precise modulation level, you should
use a scope. While it's true there's no perfect way to do this without
a scope... you should be able to get reasonably close by having an
assistant monitor your transmission during the adjustment. This is
also covered in the VoiceMax installation instructions.


Such a procedure is pointless unless the assistant is monitoring on
the second and third harmonics.


Even levels up
to 110% modulation will not cause excessive bleedover.


But they will cause harmonics, especially on the television.


The people you
hear on the air bleeding 20+ channels are usally running a combination
of excessive power with perhaps upward of 130% modulation. That is a
deadly splattering condition all right.


Adjacent channel bleedover is caused by audio harmonic distortion
bypassing the 3kHz filter. RF Amplifiers only amplify the bleedover.
Modulation level is not a factor because overmodulation only creates
RF harmonics (e.g, being heard on 54MHz, 81MHz, etc.), which is also
caused by non-linear linears.


However, I don't agree with your statement that factory radios are set
to 95-98% possible modulation. There is no way there is enough margin
here to assure they will not exceed the 100%. That's why they they
normally set to around 85%. Both my Cobra radios were in that
vicinity. I have never put a factory radio on the scope and seen more
than 90%!


I have a shelf filled with CB manuals, both Sams and factory manuals.
Except for some of the old tube-type radios, the modulation adjustment
for all of them is around 95 to 98% -- I didn't pull those numbers out
of a hat. If you've never seen more than 90% then either your scope is
too slow or you need to learn how to use it properly. And that's why
scopes are not recommended for modulation alignment, which is usually
done with an RF voltmeter or demod probe, and an audio source with a
fixed amplitude and frequency (a sig-gen, not someone yelling AAAUUDIO
into the mic). You should really read some of these manuals -- some of
them also describe how to adjust the mic audio AGC circuits....