Page 299 - Satellite Communications, Fourth Edition

Analog Signals 279

Example 9.6 A satellite TV link is designed to provide a video signal-to-noise
ratio of 62 dB. The peak deviation is 9 MHz, and the highest video baseband fre-
quency is 4.2 MHz. Calculate the carrier-to-noise ratio required at the input to the
FM detector, given that the combined noise weighting, emphasis improvement,
and implementation margin is 11.8 dB.
Solution

9
D 2.143
4.2
Equation (9.23) gives:

2
G PV 12 2.143 s2.143 1d 173.2

Therefore,
[G PV ] 10 log 173.2 22.4 dB

Since the required signal-to-noise ratio is 62 dB, Eq. (9.24) can be written as

62 c C d 22.4 11.8
N

from which [C/N] 27.8 dB.

9.7 Problems and Exercises
9.1. State the frequency limits generally accepted for telephone transmission
of speech and typical signal levels encountered in the telephone network.

9.2. Show that when two sinusoids of different frequencies are multiplied
together, the resultant product contains sinusoids at the sum and difference
frequencies only. Hence show how a multiplier circuit may be used to produce
a DSBSC signal.
9.3. Explain how a DSBSC signal differs from a conventional amplitude
modulated signal such as used in the medium-wave (broadcast) radio band.
Describe one method by which an SSB signal may be obtained from a DSBSC
signal.

9.4. Explain what is meant by FDM telephony. Sketch the frequency plans for
the CCITT designations of group, supergroup, basic mastergroup, and super
mastergroup.

9.5. With the aid of a block schematic, show how 12 VF channels could be
frequency-division multiplexed.