 PICKS REMOVES ALL BUT DESIRED TURNS AM SIGNAL UP ALL RADIO STATION TO SOUND RADIO STATIONS Figure 12.14 The superheterodyne receiver The standard construction of such a receiver is shown in Figure 12.14. Let's say that you want to pick up an AM signal at 900 kHz. 1. To start, you have an antenna, which picks up all the radio signals. 2. Next, you use three components to get rid of the other radio stations and leave you only with the sounds of 900 kHz. 2a. The first component used to cut out the other radio stations is a tunable RF (radio frequency) filter. The RF filter is tuned to 900 kHz, and acts as a bandpass filter, as shown in Figure 12.14. It cuts down (but not out) the radio signals at other frequencies-the reason we don't use a very sharp bandpass filter here that cuts out all the other radio stations is because it is expensive to build a very sharp filter that is tunable. 2b. Next, we apply a mixer, which is just a fancy way to say that we multiply the signal by a cosine waveform with frequency f. Multiplication by a cosine causes the incoming signal to shift in frequency by fL. By tuning the dial to 900 MHz, the cosine waveform frequency f is set to shift the fre- quency of the 900 MHz station to 455 MHz. Analog Communications 2c. A very sharp filter, called the IF (intermediate frequency) filter, is applied to the incoming signal. This filter is a very sharp bandpass filter at 455 MHz, as shown in Figure 12.14. With the desired AM signal now at 455 MHz, it makes it through the IF filter-all the other radio frequencies are cut out. 3. Now you have the AM signal you want with all those other radio signals cut out. So you put your AM signal through an envelope detector, which extracts the sound information x (t) from the AM signal. 12.5 Summary This is it. The end of the chapter. The end of a long journey through a book. In this chapter, we briefly looked at analog communication systems. We looked at AM and FM modulation, and we saw how to detect AM and FM signals. We even "built" a receiver that allows