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Unformatted text preview: 2.7 Heterodyning We return now to a discussion of the AM receiver. In general, the receiver will begin by detecting a signal r ( t ), for example using an antenna. This signal will contain the information we want to receive, modulated at a carrier frequency f c , along with other broadcasts that we dont want, and noise. Fig. 62 shows an example of a received spectrum. The numbers 1-4 represent the bandwidth occupied by four simultaneous AM broadcast which we assume are transmitted with carrier frequencies f 1- f 4 . Along with these broadcasts is noise at other frequencies. 1 4 3 2 4 1 2 3 l f h f l f- h f- f ) ( f R Figure 62: We have previously studied the receiver in Fig. 63. A tuneable bandpass filter is used to remove the noise and the channels we dont want. The result is the AM signal of interest, s ( t ). For example, if we wanted only channel 3, the filter would be tuned to remove everything else, leaving only the spectrum in Fig. 64 (a). This signal in now demodulated using a tuned ) ( t s BPF ) 2 cos( ) ( t f t c RX p = l ) ( ~ t s ) ( t r tuneable tuneable Figure 63: local oscillator, RX ( t ), in this case tuned to f c = f 3 . This oscillator mixes the spectrum up and down, leaving a copy of the message at baseband. This spectrum is indicated in Fig. 64 (b). We recover m 3 ( t ) by removing the high frequency images with a low pass filter not shown. The difficulty with this method is that it is difficult to design a good, narrow, high frequency filter which can also be tuned. In practice, it is easier to design a good filter at a fixed intermediate frequency (IF) and shift the signal of interest to that frequency before 57 3 3 l f h f l f- h f- f 3 f 3 f- 3 f 3 3 3 2 f 3 2 f- ) ( 3 f S 2 ) ( 3 f M (a) (b) )...
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- Summer '09