A.M. depth accuracy

TM 11-6625-3017-14

Function switch to SET CAL and, without disturbing the

(3)

Connect the selective level meter to the I. F. OUT

SET CAL F. M. preset, bring the meter reading to the

socket of the modulation meter, tune the level

SET mark by adjust-ing the internal f. m. preset, AORV3

meter to the carrier frequency of 1.5 MHz and set

- see Fig. 4-1. If this has insufficient range select a new

its gain to give a 0 dB reference level.

value for resistor AOR15.

(4)

Retune the level meter to one of the 3 kHz

(5)

This check can also be made at other deviations as

sidebands and adjust the l. f. oscillator out-put to

shown in Table 4.2.

give a level meter reading of 12 dB below the

reference level. If the sideband levels are different,

adjust for a mean reading of 12 dB. This

(6)

If the deviation errors show a general trend in one

corresponds to a modulation depth of 50%.

direction the presets RV1, RV3, RV5 and RV4

should be adjusted to give the best compromise on

(5)

If the modulation meter reading differs from 50%

their respective ranges. As RV3 affects all ranges it

when switched to either PEAK or TROUGH, adjust

should be adjusted first.

the internal preset A9RV1 for the best compromise

accuracy between the two readings.

Preset

Adjust for best compromise

on deviation range

Note: Adjustment of RV1 may affect the accuracy of the

(kHz)

f. m. deviation reading; therefore, after adjusting

RV1, reset the f. m. calibration as shown in Sect. 2.5

RV3

5, 15, 50

(8) (not by adjusting the internal preset).

RV4

500

RV5

150

(6)

This check can also be. made. at other modulation

RV1 (on TM8742)

1.5

depths, as shown in the following table, in order to

optimize reading errors.

(RV3 will be usually found to give the best compromise

TABLE 4.3

by adjusting on the 5 kHz deviation range.)

Modulation Depth

A.M. RANGE

Modulation

Sideband

amplitude, dB

4.3.4

A.M. depth accuracy

100

-6.94

Test equipment:- a, f and i.

100

-7.96

100

-9.12

A. M. depth accuracy, unlike f.m. deviation, cannot

100

-10.46

be checked against an internal calibrating circuit.

100

-12.04

Therefore, the a. m. Measurement accuracy can only be

100

-14.02

checked by making a normal measurement on a signal

-16.52

of accurately known modulation depth. A depth of about

-20.00

50% is recommended. It is important that this signal

-26.02

should have a very low envelope distortion as this can

cause an apparent error in depth indication.

The higher modulation depths are not recommended

unless an unusually low envelope distortion can be

The modulation depth can be checked by means of

achieved.

a selective level meter as follows:-

(1)

Set the 1. f. oscillator frequency to 3 kHz and use it

to modulate the signal generator to a depth of

4.3.5

F.M noise

nominally 50%.

Test equipment:- b, d, h, j and k.

(2)

Set up the modulation meter for measuring this

signal as described in Sect. 2.6.

This is measured by comparing the output of the

modulation meter when 5 kHz deviation is applied with

the output when deviation is zero.

4-6