Demodulator Concepts
A demodulator is a device that recovers the original information from a modulated signal (a signal that is modulated onto a carrier frequency). The block diagram of an FSK (frequency shift keyed) demodulator appears below.
Figure 1: Demodulator Block Diagram. (NOTE: All signals in the above diagram are differential).
The demodulator works by mixing (multiplying) the original modulated signal with a copy that is shifted in phase (hence the need for the phase shift network). This produces a voltage (after lowpass filtering) that is proportional to the amount of phase shift provided by the phase shift network. This makes it important that the phase shift network provide a linear phase shift over the band of interest. In our system, the band of interest has a center frequency of 5.5MHz (the 2nd IF frequency) and a bandwidth of about 1MHz.
However, the mixer does introduce higher-frequency signals as well as the desired frequency (due to its nature, it produces at least the sum and the difference of its inputs, and we only want the difference frequency). In order to remove these unwanted components and get a clean output signal, a low-pass filter (LPF) is necessary at the output of the demodulator. The LPF has a cutoff frequency of approximately 500 kHz and provides 60db/decade of attenuation.
Finally, another block (the loop filter) is shown, but grayed out, on this diagram. This block would provide automatic frequency control (AFC) functionality to the 2nd IF filter, and the demodulator phase shift network. However, it was not implemented in this iteration of the design. We can implement it with external components when testing.