Instruction manual

TM 11-6625-3017-14
(2) Apply a test signal to the left channel at 15 kHz
modulation frequency (for example) with no signal on the
right channel.
(3) Connect the oscilloscope, switched to d. c.
coupling, to the OUTPUT terminals of the modulation
meter. If the oscilloscope has a high input resistance it is
advisable to connect a resistor of about 1 kQ (the exact
value is unimportant) in parallel with the oscilloscope
input; this is to prevent leak- age in the electrolytic output
coupling capacitor of the modulation meter from
producing a voltage which may shift the trace off the
screen. Adjust the oscilloscope to suitable time base
and input attenuator settings for a composite signal
waveform as in Fig. 2-8. Procedure here depends on the
equipment being tested, in general it consists of setting
up the system for deviation, and gain etc., in accordance
with individual system manufacturers’ instructions until a
satisfactory waveform is seen. The gain of the L - R
channel should then be adjusted for minimum amplitude
’B’. Measure this amplitude and calculate separation in
accordance with formula (1).
2.17 PHASE MODULATION AND TELEMETRY
DEVIATION
Phase modulation
To find the degree of phase modulation, measure the
deviation and divide by the modulating frequency to
obtain the answer in radians
Telemetry deviation
To measure the deviations in a modulated
telemetry signal connect the OUTPUT terminals to the
external meter via a switched series of band-pass filters
at the sub-carrier frequencies. This enables the
deviation due to each tone to be measured, having first
calibrated the meter against a signal modulated by a
single tone.
2.18 STRAY FIELDS
The oscillator uses permeability tuning and so
the oscillator frequency may be modulated by stray
magnetic fields. An internal magnetic screen is fitted but
nevertheless we recommend that the oscillator section of
the instrument should not be placed close to other
devices having strong external magnetic fields.
The instrument has internal screening which, for
most purposes, provides adequate attenuation from stray
fields. However, a signal source such as a transmitter of
greater power than about 10 W may cause stray electric
r. f. radiation which may be picked up by the deviation
meter. This means that it may be necessary to take care
in the siting of the instrument. The effect will be most
notice- able when making a.m. measurements; f.m.
measurements will be affected only if the stray field is so
great as to pass the limiters.
2-14