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Wire and Wireless Communication Applications Chap. 8
60
Peak-to-peak weighted video output (S/N)(dB)
50
40
30
Threshold extension receiver
(PLL detector)
Receiver with FM discriminator
IF bandwidth=30 MHz
20
0
Figure 8–27
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
(
C
N
detector input (dB)
Output SNR as a function of FM detector input CNR.
(
discriminator for the FM detector, and the other is for a receiver that uses a PLL FM detector to
provide threshold extension. † For a CNR of 14.3 dB on the input, both receivers will give a 51-dB
output SNR, which corresponds to a high-quality picture. However, if the receive signal from the
satellite fades because of atmospheric conditions by 6 dB, the output SNR of the receiver with an
FM discriminator will be only about 36 dB, whereas the receiver with threshold extension detection
will have an output SNR of about 45 dB.
8–7 FIBER-OPTIC SYSTEMS
Since 1980, fiber-optic cable transmission systems have become commercially viable. This
is indicated in Table 8–2, which shows that common carriers have installed extensive fiberoptic
systems in the United States, Japan, Great Britain, and Canada. Fiber-optic cable is the
preferred underground transmission medium. Also, as shown in Table 8–2, transoceanic
† The threshold point is the point where the knee occurs in the plot of (SN) out vs. (CN) in . In the characteristic
for threshold extension receivers, this knee is extended to the left (improved performance) when compared with
the knee for the corresponding receiver that uses a discriminator as a detector.