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all frequency response ehould fo,llow the 75 microsecond curve
shown on drawing ES
-
0117U.
In other
frequency ranges,
it may
be desirable to have the overall frequency response flat.
o
Several methods of making frequency response measurements using
an FM monitor are available.
Two will be described, the simp-
lest is to set the audio frequency at about mid- range,
say 5000
cycles, and modulate the exciter the proper amount,
in this
case the proper modulation level would be 35%.
Keeping the in-
put audio level constant,
the frequency may then be adjusted
upward to 15,000 cycles and then downward to 30 cycles.
Using
this method the response will seldom rise above the curve and
makes it easy to calculate the percent or decibel error.
For
example,
if at 15,000 cycles modulation the modulation monitor
reads only 80% modulation,
it can be quickly seen from the
drawing ES- 6170 that the response is 2 DB below the normal curve.
The same reasoning may be applied to the low end of the curve.
If the input attenuator is calibrated in small steps,
it is
also possible to determine the amount that the audio input has
to be increased to bring the monitor up to the required per-
centage of modulation at any modulating frequency.
Another method of measuring frequency response involves keeping
the percentage of modulation constant as rend on the monitor.
To use this method the audio oscillator output must be accura-
tely calibrated.
To start with,
the carrier should be modulated
100% at 400 cyeles,
changing the audio frequency from about 30
cycles to 400 cycles should not change the percentage of modula-
tion appreciably.
If the modulating frequency is raised upward,
say to 5,000 cycles,
theinput level must be reduced to keep the
percent of modulation at 100%.
For 5,000 cycles the amount of
reduction should be 8.2 DB.
For 15,000 cycles the amount of re-
duction of input level should be 16.9 DB.
Recording the amount
of reduction of the input level versus modulating frequency and
reversing the sign of polarity, will give the curve and frequency
response.
This can then be compared to the curve of drawing
ES- 6170.
The second suggested method is particularly useful when response
measurements are being made at 25 and 50% modulation levels,
or
when a standard FM monitor is being used to measure response of
an exciter being used to generate the aural carrier for a TV
transmitter where normal 100% modulation is
25 KC.
This will
correspond to 33-1/3% modulation on a standard FM monitor for
the FM broadcast band of
88 to 108 MC.
-16-
M6095 Exciter
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