F.M. STEREO MEASUREMENTS

TM 11-6625-3017-14

Example : To find the fundamental crystal frequency for

Crystals are fully specified in Data Sheet

an r. f. input of 83.666 MHz:

Q01670F, to which reference should be made

for further details if required. Both data sheet

1) Consulting Table 2. 4, it can be seen that this r. f.

and crystals are available from The Marconi Co.

falls within range 4 and the harmonic factor is therefore

Ltd., Chelmsford, Essex.

83.666+ 1.5

2.16

F.M. STEREO MEASUREMENTS

Crystal frequency =

= 42. 583 MHz

Distortion and channel separation in stereo

A +10% tolerance is permitted on the 1.5 MHz i.

systems which are designed to meet U. S. Federal

f. (+150 kHz) and, since the normal tolerance on

Communications Commission (F. C. C. ) requirements

crystal frequency for +20 C to +60 O C

can be measured with the TF 2300A Modulation Meter.

temperature operation is +0. 01%, a crystal with

The general procedures are described in the following

a frequency of 42.58 MHz and normal tolerance

sections.

should be suitable, and it is not usually necessary

to specify a tighter tolerance.

2.16.1 Harmonic distortion

Note: Where f.m. deviation approaches the full i.

It can be assumed that distortion introduced by

f. bandwidth and the r, f. approaches 1000 MHz ,

the modulation meter is small in comparison with that in

the crystal tolerance should be as close as

stereo transmitters, being typically better than 0.1% for

possible for minimum distortion and crystals with

deviations up to 75 kHz and therefore, for practical

tighter frequency limits (+6. 003%) should be

purposes, can be ignored.

specified.

The typical frequency spectrum of a G. E. -

In every case the crystal frequency selected

Zenith type of f.m. stereo signal when a 1 kHz test tone

should be such that the lowest possible harmonic factor

(for example) is applied to the left channel and no signal

gives the required operating frequency.

This is

to the right, is shown in Fig. 2-5.

automatically given by Table 2.4 using the procedure

described.

Ideally, only the fundamental 1 kHz signal in the

left + right (L + R) channel and the 37 and 39 kHz

Example : Required oscillator frequency is 176 MHz.

sidebands of the suppressed sub-carrier in the L - R

channel should be present.

In practice, however,

1) It can be seen that a crystal of 22 MHz

harmonic distortions will occur at 2, 3, 4 kHz in the L + R

operating with a harmonic factor of 8 will satisfy

channel and at 37 and 39, 36 and 40, 35 and 41 kHz . in

frequency conditions.

the L - R channel. These are the distortion components

which it is desired to measure.

2) However, a crystal of 44 MHz operating with a

harmonic factor of 4 should be specified for

maximum output from the oscillator

Fig. 2-5. Frequency spectrum of f.m. stereo signal with 1kHz modulating tone.

2-11