 signal r(t) = s(t) and turns that back into your information (t). You know that there is no information x(t) in the amplitude of r(t) = s(t)-all the information is in the instantaneous frequency w(t) = The demodulator works to get w(t), the instantaneous frequency, out of r(t) = s t ) , the received signal. A demodulator for an FM signal is shown in Figure 12.13. First, a limiter is applied. The limiter takes r(t)=s(t) = and gets rid of all amplitude fluctuations, by simply forcing all positive values to +1 and all negative values to -1. The output is called v' (t). The limiter's operation does not affect the ability to extract the information signal x(t), since the information is stored in the frequency (not in the amplitude). Then, the signal r' (t) is passed through a discriminator. The discriminator outputs a value v" (t), which is proportional to the instantaneous frequency w(t). That is, it outputs r"(t)=Kw(t) (12.34) (12.35) Once we have this output, a processor doing a simple subtraction and a scalar multiplication creates the output x (t). And that, my friends, is FM, its modulation and its demodulation. LIMITER DISCRIMINATOR Simple subtraction r(t)=s(t) r'(t) amplitude of output r"(t) scalar frequency multiplication of input Figure 12.13 FM demodulator 12.4 The Superheterodyne Receiver The last section of this book has a grand title. The superheterodyne receiver is a stan- dard AM receiver that you can use to pick up any radio station. Let me explain how this receiver works, then we'll call it a done book. Take a look at the top of Figure 12.14. This shows a part of the frequency spec- trum. Specifically, each peak in the spectrum represents the presence of a radio station transmitting its songs and sounds at that frequency. You want to build an inexpensive radio which allows you to pick up any radio station that you'd like to listen to. Chapter Twelve radio stations at different frequencies 900 kHz ANTENNA filter Envelope Detector filter 455 kHz cos2f 455 kHz this line means "adjustable" or "tuneable"