Question

A communication system uses a superheterodyne receiver containing two mixers. The intermediate frequencies are 10.7 MHz and 470 kHz. The desired signal has a carrier frequency of 102.3 MHz. (a) What are the two local oscillator frequencies in the receiver (both mixers are superheterodyne)? (3 marks) (b) Explain what an image frequency is. (2 marks) (c) How many image frequencies are there in this communication system? Calculate their frequencies. (5 marks) (d) Explain how this receiver removes each of the following unwanted signals from its output: wideband noise; signals in adjacent transmission channels; and interfering signals at an image frequency. (3 marks) (e) Discuss one advantage and one disadvantage of using a superheterodyne receiver over a simpler receiver. (4 marks) (1) If the second mixer was changed to be subheterodyne, what would the new second local oscillator frequency be? (2 marks)

Answer 1

د Explanation : In this problem, the two inter-medrate frequencies for two mixere are given il- lo 7 MHz and 470 KHz The desired signal frequency signal frequency of 102.3 MH12. Der fe) 9) The local orcillater frequency in a super-heterodyne reciever is always elightly higher of incoming signal frequency je; foz fst fi Where, Fs - signal frequency , fi- intermediate frequency and fo- oscillater frequency. 9 There, 9 fos 102.3t 10.7 1/3 MHZ and for 102.3 +0.470 M 102. 7 70 MHZ b) The image frequency is defined as the signal frequency play twice the intermediate frequency and it is given as; fsi = fot a fil where, fsi image frequency fi intermediate frequency, c) There are two image frequencies in the reciever for different intermediate (1.p.) Pequency and can be calculated as: two fsin 102.3 + 2 (0.7) [ Taking I. f = 107M 123.70 MHz and fsiz [Taking Jif: =0480 ] 102.3 + 2 (0:470) 103.24 MHz

d) To - Hmove the inwanted signal from its output such as wide-band noite Signals, in adjacent transmission channels and interfering lignals at mage frequency; the super het has the same essential components as the TRF reciever h addition to the the mixer, local orcillater and intermediate frequency (UF) amplifier, The Rf circuits are mainly used to reject interference such as the mage frequency and (especintly at high frequencies to reduce the noise figure of the recieves. 9 Advantage of Saper het Reciever The problem of instability m sufficient adjacent Bequency rejection and band-width Variation can all be solved by use of reciever. the Disadvarstage: The superhet recievers has a number of timable cipecite and the local oscillator must be timed to a frequency precisely higher than this by the I fi frequency, which difficult to track and sometimes resulted in tracting error: