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PROOF OF PERFORMd,NCE
Center Frequency, Noise,
Distortion, Response
Proof of performance data as made by the Gates Radio Company
on FM transmitters can be likened to listening to the trans-
mitter on a high quality receiver.
This tends to " prove- out"
the transmitter since measuring and listening eouipment is
completely external to the transmitter proper and the RF sig-
nal is taken from " off-the- air".
Instead of a receiver, an FM monitor of good quality and FCC
approved is used.
Reference to drawing A-4165 will show the
general test set up for making proof of performance measure-
ments.
First off, a sample of the transmitted RF is coupled to the
modulation and frequency monitor.
This is taken from the an-
tenna,
transmission line or from the PA chamber.
The method
used is determined somewhat by the amount of power needed by
the monitor ( usually about 1 watt)
and by the output power of
the transmitter.
For low power FM tranamitters up to perhaps
250 watts, a sample of RF may be taken by " tapping" off the
output transmission line with a variable condenser in series
with the coaxial line going to the monitor.
This has the dis-
advantage though of introducing a slight mismatch back into
the transmitter.
Usually,
it is impossible to obtain enough
power to drive the monitor from the antenna without introducing
another amplifier ahead of the monitor to raise the receiver
signal
up
to the necessary level.
In higher powered trans-
mitters, a monitor loop is usually coupled to the final ampli-
fier section to sample a portion of the transmitted output.
A good quality audio oscillator of 600 ohms output impedance
is then connected to the audio input terminals.
These are
TB101-1,2 and
3
on the exciter unit with terminal # 3 being
ground.
Output level requirements are at least
7
110 DBM
S
Since the exciter itself is capable of generating a frequency
modulated carrier with distortion ranging as lów as . 2% the
audio oscillator must be in good working order.
:1 distortion analyzer or motor is connected to the audio output
terminals of the monitor,
in oscilloscope while being an op-
tional item in making measurements is very helpful in tracing
any possible difficulty.
-12-
M6095 Exciter
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