Fig. 2-6. Test arrangement for measuring distortion

TM 11-662 5-3014-14

The test arrangement for measuring distortion in

For practical working purposes, the small

an f.m. stereo transmitter is shown for measurements on

amount of distortion introduced by the modulation meter

the G. E. -Zenith system is given below :-

can be neglected.

(1)

Connect the transmitter output to the mod-

2.16.2 Channel separation

ulation meter in accordance with section 2.4 -

Preparation for Use.

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

(2)

Set up the modulation meter with the trans-

waveforms shown in Figs. 2-7 (a) and (b) would be seen.

mitter signal on, in accordance with section 2.5 -

Measuring F . M. Deviation. The following

The ideal composite modulating signal with the

notes are intended to amplify the setting

19 kHz pilot tone removed should be the sum of these

instructions and refer specifically to the G. E. -

waveforms as shown in (c). In practice, there will be

Zenith System.

amplitude unbalance and relative phase shift (group

delay error) between the two added signals. These two

(3)

Set the controls as follows :-

effects, which can be viewed on a direct coupled

oscilloscope, are illustrated in (d) and (e), where

A. M./DEV RANGE: 150 kHz.

amplitude 'B' represents a signal on the right channel

MAX MOD FREQ:= 200 kHz.

due to an input on the left channel.

DE-EMPHASIS : OFF, even if pre-emphasis is

being applied.

Channel 1 separation is given by:

Function: DEV + or DEV -

Separation = 20 log 10 A/B (1)

(4)

Apply a test signal of a chosen frequency to the

where A and B are measurements indicated in

left channel with no signal to the right channel.

Figs. 2-7 (d) and (e).

(5)

Connect Wave Analyser type TF 2330, or an

An f.m. stereo transmitter or stereo modulator

equivalent instrument for harmonic analysis of

must meet specified requirements as regards response

the audio range, to the OUTPUT terminals of the

and phase shift. In the F. C. C. requirements, it is

modulation meter. (Refer to the instruction

deemed that these are met if the channel separation

manual on the analyser for calibration and

exceeds 29.7 dB.

measuring procedures. )

A suitable oscilloscope, such as a Marconi

(6)

The frequency spectrum of the demodulated

Instruments type TF 2200, connected to the OUTPUT

output of the modulation meter can be obtained

terminals 6f the modulation meter, enables the channel

by tuning through the frequency range and

separation to be measured. Fig. 2-8 shows typical

taking measurements at the appropriate

oscillograms obtained on a TF 2200 Oscilloscope from

frequencies. The result is usually referred to the

which such measurements would be made.

fundamental and expressed as percentages or

dB readings, calculated by the usual method for

From 200 Hz to 1 5kHz. the channel separation

harmonic distortion measurement as described

figure of the modulation meter is typically 13 dB below

in the wave analyser manual.

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.

Fig. 2-6. Test arrangement for measuring distortion

2-12