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If high distortion is present at the higher modulating fre-
quenciès only,
it can usually be traced to one of three causes.
1.
High FM or AM noise.
2.
Insufficient bandwidth in the frequency multiplier
stages.
3.
Frequency and modulation monitor not corredtly tuned to
carrier frequency.
A standard FM monitor contains de- emphasis circuitry that
causes lower modulating frequencies of 30 to 1,000 cycles to
come ont of the monitor with an apparent advantage of around
15 to 17 DB over the audio that is recovered at 15,000 cycles.
If noise is down only 40 to 50 DB with respect to 100% modula-
tion at 400 cycles,
it will usually not prevent a good distor-
tion reading at a low modulating frequency.
However,
if fre-
quencies between 10,000 and 15,000 cycles are 15 DB lower in
amplitude
then 400 cycles
e
the noise with respect to these
frequencies, will only be about 30 DB down.
This would corres-
pond to the 3% distortion range on a distortion analyzer.
A
quick check to determine whether noise is causing an apparent
high distortion reading is to remove all modulation from the
input to the exciter or transmitter.
If the distortion meter
needle does not drop appreciably a noise measurement should be
made on the exciter.
If bandwidth is insufficient in frequency multiplier stages,
some of the higher frequency sidcands will be clipped causing
undue distortion.
A complete re- tune up is then recommended.
Mis - tuning of the monitor will also cause some clipping of side-
bands at higher frequencics.
In addition,
beat frequencies may
be present that show up as noise and prevent a good distortion
reading.
Once set,
the distortion controls R115,
C111, R126 and C119 may
not have to be re- set for the life of the the exciter unit.
Changing modulator tubes will probably not cause distortion
figures to change by more than . 1 or . 2%.
There are exceptions
to every rule though.
OVERLL FRWJENCY RESPONSE
If the exciter unit is used in the FM broadcast band of 88 to
108 MC or as the aural exciter unit for TV transmitters, over-
-15—
M6095 Exciter
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