STRAY FIELDS

TM 11-6625-3017-14

(2) Apply a test signal to the left channel at 15 kHz

deviation and divide by the modulating frequency to

modulation frequency (for example) with no signal on the

obtain the answer in radians

right channel.

Telemetry deviation

(3)

Connect the oscilloscope, switched to d. c.

coupling, to the OUTPUT terminals of the modulation

To measure the deviations in a modulated

meter. If the oscilloscope has a high input resistance it is

telemetry signal connect the OUTPUT terminals to the

advisable to connect a resistor of about 1 kQ (the exact

external meter via a switched series of band-pass filters

value is unimportant) in parallel with the oscilloscope

at the sub-carrier frequencies.

This enables the

input; this is to prevent leak- age in the electrolytic output

deviation due to each tone to be measured, having first

coupling capacitor of the modulation meter from

calibrated the meter against a signal modulated by a

producing a voltage which may shift the trace off the

single tone.

screen. Adjust the oscilloscope to suitable time base

and input attenuator settings for a composite signal

2.18

STRAY FIELDS

waveform as in Fig. 2-8. Procedure here depends on the

equipment being tested, in general it consists of setting

The oscillator uses permeability tuning and so

up the system for deviation, and gain etc., in accordance

the oscillator frequency may be modulated by stray

with individual system manufacturers' instructions until a

magnetic fields. An internal magnetic screen is fitted but

satisfactory waveform is seen. The gain of the L - R

nevertheless we recommend that the oscillator section of

channel should then be adjusted for minimum amplitude

the instrument should not be placed close to other

'B'. Measure this amplitude and calculate separation in

devices having strong external magnetic fields.

accordance with formula (1).

The instrument has internal screening which, for

2.17 PHASE

MODULATION

AND

TELEMETRY

most purposes, provides adequate attenuation from stray

DEVIATION

fields. However, a signal source such as a transmitter of

greater power than about 10 W may cause stray electric

Phase modulation

r. f. radiation which may be picked up by the deviation

meter. This means that it may be necessary to take care

To find the degree of phase modulation, measure the

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