MEASURING AM. DEPTH

TM 11-6625-3017-14

switch to SET CAL and check that the

deflection and then adjust LEVEL to give a meter

meter reads SET. If necessary, adjust the

deflection in the black R. F. LEVEL arc,

f.m. calibration by inserting a screwdriver in

preferably towards the lower end for best mixing

the SET CAL F.M. preset and adjusting

and hence minimum distortion.

until the meter reads SET.

(4)

Set the Function switch to A.M. - SET FREQ.

Slightly readjust the OSCILLATOR TUNE control

CAUTION

Do not attempt to SET CAL on the 1.5 kHz

until the meter deflects to the SET line. Avoid

f.m. deviation range.

Noise in f.m.

spurious settings - the correct one is the closest

measurements

to the original setting obtained in (3).

(5)

Set the Function switch to SET A.M. Adjust the

To accommodate signals with up to 100%

adjacent SET A, M. control until the meter reads

amplitude modulation the maximum r. f. input level, as

on the SET line. Note that this setting can be

indicated by full-scale deflection on the black meter arc,

done with or without modulation since the

is set at 6 dB below the maximum amplitude which the i.

adjustment is to the mean level of the i. f.

f. amplifier can handle. When making measurements

signal. However, appreciable even harmonic

on f.m. signals including f.m. noise, and f.m. on a. m.,

distortion can affect the setting accuracy and, in

this extra level capability of the i. f. amplifier can be

such circumstances, it is preferable to set up

used with advantage to reduce the internally generated

without modulation.

noise to a minimum. This is useful when measuring

(6)

Set the Function switch to A.M. PEAK. Read the

noise on signal sources or when measuring deviations in

percentage modulation depth at the peak

the 5 kHz range on the 200 kHz bandwidth so that errors

(7)

Set the Function switch to A.M. TROUGH. Read

due to noise are minimized.

the percentage modulation depth at the trough.

When the peak and trough readings are

The optimum r. f. level is set by monitoring the i.

unequal, distortion is present in the input signal.

f. signal at the I. F. OUT socket and adjusting the r. f.

input level for the maximum undistorted i. f. signal.

2.7

MEASURING F.M. ON A.M.

Alternatively, if no means of monitoring the i. f. signal is

available, it is sufficient to increase the r. f. input level

To measure the spurious f.m. on an amp-

until the noise signal being measured is at a minimum, or

litude modulated source, proceed as for f.m.

if deviation is being measured, until the deviation reading

measurement, section 2.5.

is at a minimum.

A certain amount of spurious f.m. is introduced by

the instrument itself.

When the carrier is deeply

2.6 MEASURING AM. DEPTH

modulated, at high modulating frequencies, this should

be taken into consideration, but below 40% depth in the

After carrying out the preparation procedure in

audio range of frequencies the spurious deviation is not

section 2.4:

very significant. At 80% depth and 1 kHz frequency, the

maximum spurious deviation is typically less than 150 Hz

(1)

Set the A. M./DEV RANGE switch (below the

referred to the 15 kHz bandwidth.

meter) to the 30% or 100% a.m. depth range

according to the expected modulation depth of

Tables 2.2 and 2.3 give typical results which should

the signal under test.

be used for general guidance purposes only .

(2)

Set the oscillator RANGE switch in accordance

with table 2.1. For detailed instruct- ions on

oscillator arrangements, see section 2.10. In

this section, it is assumed that the internal local

oscillator will be used.

(3)

Set the Function switch to TUNE OSCILLATOR

AND ADJUST LEVEL.

Adjust the OSCIL-

LATOR TUNE dial to a frequency of 1.5 MHz

above the input frequency. Tune for peak

2-5