Example 4.1
A radio receiver is limited in performance by
thermal noise in the receiver 'front-end'. It is designed to provide an S/N
ratio of better than 10 dB to the demodulator input. The channel bandwidth for the
receiver is 25 kHz. What is the minimum received signal level that can be used to achieve
this performance target, assuming the receiver is operating at a temperature of 280oK?
Solution
The average power of thermal noise for this case is
given by:
Nav = kTB = 1.38 x 1023 x 280 x 25000
= 9.66 x 1017 W or 130 dBm (dB relative to 1 mW.)
To achieve an S/N ratio of 10 dB, the received
signal power must therefore be in excess of (130 + 10) = 120 dBm.