Instruction manual

TM 11-6625-3017-14
For noise measurements in other bandwidths
select the MAX MOD FREQUENCY 200 kHz band- width
setting and use a similar type of low-pass filter designed
for the required cut-off frequency.
(2) The output of the modulation meter is 0. 775 V
into 600 n for full-scale deflection on the internal
meter.
(3) For measurements in the r. f. range 22 MHz to
1000 MHz the local oscillator must be crystal
controlled if lowest possible internally generated
noise is required. Thus, a crystal suitable for the
appropriate frequency must be available. This is
not a requirement below 22 MHz, where the
internal noise of the oscillator is sufficiently low
to make crystal control unnecessary.
(4) The curves given in Fig. 2-4 show typical noise
levels produced by free-running and crystal
controlled oscillators over the r.f. range of the
instrument. In the 15 kHz bandwidth, below 70
to 100 MHz, there is insignificant difference in
respective noise levels. Above 100 MHz,
however, the noise level free-running increases
progressively with frequency, whereas under
crystal control the level remains nearly constant.
The necessity to use crystal control depends on
the noise level of the equipment under test and,
as shown by the curves, the band-
width in which the measurement is to be made
and the carrier frequency.
In the 200 kHz bandwidth, there is little advantage in
using crystal control below 500 MHz. In both
bandwidths, crystal control will eliminate oscillator
microphony and therefore may be advantageous in
conditions of vibration or high acoustic noise levels.
F.M. noise
(1) Connect an external meter to the OUTPUT
terminals. Apply r. f. input at a suitable level.
(2) Adjust the OSCILLATOR TUNE dial to a
frequency 1.5 MHz above the carrier and tune
for peaking, as in section 2. 5(5). Then adjust
LEVEL to the top end of the black arc on internal
meter. Switch to 15 kHz or 200 kHz MAX MOD
FREQUENCY setting, thus selecting the
required low-pass filter in the modulation meter.
(See also Sect. 2.5 - Noise in f.m.
measurements. )
(3) Switch to position F.M. SET FREQ and adjust
the oscillator until meter reads SET.
(4) Switch to crystal control and check that the
meter still reads near to the SET mark - the
Fig. 2-4. Typical modulation meter noise levels
2-7