Fig. 2-4. Typical modulation meter noise levels

TM 11-6625-3017-14

For noise measurements in other bandwidths

width in which the measurement is to be made

select the MAX MOD FREQUENCY 200 kHz band- width

and the carrier frequency.

setting and use a similar type of low-pass filter designed

for the required cut-off frequency.

In the 200 kHz bandwidth, there is little advantage in

using crystal control below 500 MHz.

In both

(2)

The output of the modulation meter is 0. 775 V

bandwidths, crystal control will eliminate oscillator

into 600 n for full-scale deflection on the internal

microphony and therefore may be advantageous in

meter.

conditions of vibration or high acoustic noise levels.

(3)

For measurements in the r. f. range 22 MHz to

F.M. noise

1000 MHz the local oscillator must be crystal

controlled if lowest possible internally generated

(1)

Connect an external meter to the OUTPUT

noise is required. Thus, a crystal suitable for the

terminals. Apply r. f. input at a suitable level.

appropriate frequency must be available. This is

not a requirement below 22 MHz, where the

(2)

Adjust the OSCILLATOR TUNE dial to a

internal noise of the oscillator is sufficiently low

frequency 1.5 MHz above the carrier and tune

to make crystal control unnecessary.

for peaking, as in section 2. 5(5). Then adjust

LEVEL to the top end of the black arc on internal

(4)

The curves given in Fig. 2-4 show typical noise

meter. Switch to 15 kHz or 200 kHz MAX MOD

levels produced by free-running and crystal

FREQUENCY setting, thus selecting the

controlled oscillators over the r.f. range of the

required low-pass filter in the modulation meter.

instrument. In the 15 kHz bandwidth, below 70

(See also Sect. 2.5 - Noise in f.m.

to 100 MHz, there is insignificant difference in

measurements. )

respective noise levels.

Above 100 MHz,

however, the noise level free-running increases

(3)

Switch to position F.M. SET FREQ and adjust

progressively with frequency, whereas under

the oscillator until meter reads SET.

crystal control the level remains nearly constant.

The necessity to use crystal control depends on

(4)

Switch to crystal control and check that the

the noise level of the equipment under test and,

meter still reads near to the SET mark - the

as shown by the curves, the band-

Fig. 2-4. Typical modulation meter noise levels

2-7