Newnes Radio and RF Engineering Pocket Book, 3rd Edition

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or 0s may be consecutive in the data stream, the pulse repetition

rate will vary throughout the message although the bit rate will be

constant, and over a long period of time as many spaces will be sent as

pulses. Transmitting either a stream of spaces or of marks requires no

bandwidth; it is only when a change of state occurs that frequencies

are produced. In the duration of one cycle (Figure 8.1(a)) two bits

may be carried and the maximum fundamental frequency contained

in the wave is one-half the number of bits per second, i.e. the channel

capacity, bits/second, equals twice the bandwidth in hertz.

8.1.3 Channel capacity – Hartley – Shannon theorem

Channel capacity as stated by Hartley’s law is, in the absence of noise:

C = 2δf log

2

N

where

C =