Instruction manual

TM 11-6625-3017-14
Example : To find the fundamental crystal frequency for
an r. f. input of 83.666 MHz:
1) Consulting Table 2. 4, it can be seen that this r. f.
falls within range 4 and the harmonic factor is therefore
2.
83.666+ 1.5
Crystal frequency = 2 = 42. 583 MHz
A +10% tolerance is permitted on the 1.5 MHz i.
f. (+150 kHz) and, since the normal tolerance on
crystal frequency for +20 °C to +60 O C
temperature operation is +0. 01%, a crystal with
a frequency of 42.58 MHz and normal tolerance
should be suitable, and it is not usually necessary
to specify a tighter tolerance.
Note: Where f.m. deviation approaches the full i.
f. bandwidth and the r, f. approaches 1000 MHz ,
the crystal tolerance should be as close as
possible for minimum distortion and crystals with
tighter frequency limits (+6. 003%) should be
specified.
In every case the crystal frequency selected
should be such that the lowest possible harmonic factor
gives the required operating frequency. This is
automatically given by Table 2.4 using the procedure
described.
Example : Required oscillator frequency is 176 MHz.
1) It can be seen that a crystal of 22 MHz
operating with a harmonic factor of 8 will satisfy
frequency conditions.
2) However, a crystal of 44 MHz operating with a
harmonic factor of 4 should be specified for
maximum output from the oscillator
Crystals are fully specified in Data Sheet
Q01670F, to which reference should be made
for further details if required. Both data sheet
and crystals are available from The Marconi Co.
Ltd., Chelmsford, Essex.
2.16 F.M. STEREO MEASUREMENTS
Distortion and channel separation in stereo
systems which are designed to meet U. S. Federal
Communications Commission (F. C. C. ) requirements
can be measured with the TF 2300A Modulation Meter.
The general procedures are described in the following
sections.
2.16.1 Harmonic distortion
It can be assumed that distortion introduced by
the modulation meter is small in comparison with that in
stereo transmitters, being typically better than 0.1% for
deviations up to ±75 kHz and therefore, for practical
purposes, can be ignored.
The typical frequency spectrum of a G. E. -
Zenith type of f.m. stereo signal when a 1 kHz test tone
(for example) is applied to the left channel and no signal
to the right, is shown in Fig. 2-5.
Ideally, only the fundamental 1 kHz signal in the
left + right (L + R) channel and the 37 and 39 kHz
sidebands of the suppressed sub-carrier in the L - R
channel should be present. In practice, however,
harmonic distortions will occur at 2, 3, 4 kHz in the L + R
channel and at 37 and 39, 36 and 40, 35 and 41 kHz . in
the L - R channel. These are the distortion components
which it is desired to measure.
Fig. 2-5. Frequency spectrum of f.m. stereo signal with 1kHz modulating tone.
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