Instruction manual

TM 11-6625-3017-14
TABLE 2.2
Spurious deviations with increasing % a.m.
1. LEVEL control set for meter reading at top end of arc.
2. A.M. frequency = 1 kHz
Spurious deviation
A.M. 15 kHz bandwidth 200 kHz bandwidth
30 75 Hz 750 Hz
80 150 Hz 2 kHz
TABLE 2.3
Spurious deviations with increasing a.m. frequency
1. LEVEL control set for meter reading at top
end of arc.
2. A.M. depth = 80%.
Spurious deviation
A.M. freq. 15 kHz bandwidth 200 kHz bandwidth
1 kHz 150 Hz 2 kHz
10 kHz 2 kHz 3 kHz
100 kHz - 7 kHz
2.8 MEASURING A.M. ON F.M.
The modulation meter may be used to indicate amplitude
modulation in a frequency modulated signal, provided
that the deviation is less than 100 kHz The procedure is
that of a.m. measurement, section 2.6.
In general, the spurious a.m. Indication will be
proportional to the deviation. The i. f. amplifier is set up
for optimum phase response for f.m. deviation
measurement rather than a maximally flat response for
measuring a.m. on f.m.
Below 100 kHz deviation, the internally generated
spurious a.m. is less than 2% approximately.
2.9 NOISE MEASUREMENTS
By connecting an external meter to the OUTPUT
terminals, noise measurements limited only by the noise
level generated within the instrument can be made. The
internal meter, being peak reading, is not suitable for
measuring noise and, ideally, an r.m.s. responding meter
should be used. However, sufficiently accurate results
are usually obtained with an average reading meter by
applying the appropriate correction factor.
Typical meters which can be used are as follows:
(a) R. M. S. valve voltmeter capable of measuring
to the necessary accuracy (1% deviation on any
range is approximately 7.75 mV). Errors due to
crest factor and zero shift can be avoided by
operating the meter at mid-scale by means of an
attenuator.
(b) Marconi Instruments Sensitive Valve Volt- meter,
type TF 2600.
(c) Marconi Instruments Distortion Factor Meter,
type TF 2331.
(b) and (c) are average reading and a correction of
+1 dB should be applied.
(1) The modulation meter has two selectable f.m.
bandwidths and the appropriate filter must be
selected when making the test.
A capacitor should be connected across the
OUTPUT terminals - 0. 014 ~F for the 15 kHz
bandwidth, or 0. 0013 ,F for 200 kHz. These corrective
capacitors are necessary because of the design of the
filters (see section 3. 9).
For optimum results in the 15 kHz bandwidth position
a low-pass filter as shown below should be used
between the OUTPUT terminals and the external meter
instead of the capacitor.
2-6