Instruction manual

TM 11-662 5-3014-14
The test arrangement for measuring distortion in
an f.m. stereo transmitter is shown for measurements on
the G. E. -Zenith system is given below :-
(1) Connect the transmitter output to the mod-
ulation meter in accordance with section 2.4 -
Preparation for Use.
(2) Set up the modulation meter with the trans-
mitter signal on, in accordance with section 2.5 -
Measuring F . M. Deviation. The following
notes are intended to amplify the setting
instructions and refer specifically to the G. E. -
Zenith System.
(3) Set the controls as follows :-
A. M./DEV RANGE: 150 kHz.
MAX MOD FREQ:= 200 kHz.
DE-EMPHASIS : OFF, even if pre-emphasis is
being applied.
Function: DEV + or DEV -
(4) Apply a test signal of a chosen frequency to the
left channel with no signal to the right channel.
(5) Connect Wave Analyser type TF 2330, or an
equivalent instrument for harmonic analysis of
the audio range, to the OUTPUT terminals of the
modulation meter. (Refer to the instruction
manual on the analyser for calibration and
measuring procedures. )
(6) The frequency spectrum of the demodulated
output of the modulation meter can be obtained
by tuning through the frequency range and
taking measurements at the appropriate
frequencies. The result is usually referred to the
fundamental and expressed as percentages or
dB readings, calculated by the usual method for
harmonic distortion measurement as described
in the wave analyser manual.
Fig. 2-6. Test arrangement for measuring distortion
For practical working purposes, the small
amount of distortion introduced by the modulation meter
can be neglected.
2.16.2 Channel separation
If it were possible to view the 50 Hz to 15 kHz
and the 23 to 53 kHz regions of Fig 2-5 separately, the
waveforms shown in Figs. 2-7 (a) and (b) would be seen.
The ideal composite modulating signal with the
19 kHz pilot tone removed should be the sum of these
waveforms as shown in (c). In practice, there will be
amplitude unbalance and relative phase shift (group
delay error) between the two added signals. These two
effects, which can be viewed on a direct coupled
oscilloscope, are illustrated in (d) and (e), where
amplitude ’B’ represents a signal on the right channel
due to an input on the left channel.
Channel 1 separation is given by:
Separation = 20 log
10
A/B (1)
where A and B are measurements indicated in
Figs. 2-7 (d) and (e).
An f.m. stereo transmitter or stereo modulator
must meet specified requirements as regards response
and phase shift. In the F. C. C. requirements, it is
deemed that these are met if the channel separation
exceeds 29.7 dB.
A suitable oscilloscope, such as a Marconi
Instruments type TF 2200, connected to the OUTPUT
terminals 6f the modulation meter, enables the channel
separation to be measured. Fig. 2-8 shows typical
oscillograms obtained on a TF 2200 Oscilloscope from
which such measurements would be made.
From 200 Hz to 1 5kHz. the channel separation
figure of the modulation meter is typically 13 dB below
the minimum required of f.m. stereo channels in the F.
C. C. regulations, while at 50 Hz (the worst condition), it
is at least 3 dB below.
2-12