FSK PLL demodulator

00-11 220n R2 • sin CIE MIN R9 IC2 LM 311 4 Computer 0 O O Ra O+12V ® 5V Al FSK (Frequency Shift Keying) signals can be demodulated in a simple manner with the aid of a PLL (Phase Locked Loop). Frequency shift keying is used regularly for data transmission, where a carrier wave is switched between two predetermined frequencies. The frequency shift is obtained by controlling a VCO with the binary data signal, so that the two frequencies are determined by the "0" and "1" logic states. When a signal is present at the input of ICI the VCO is locked in synchronisation with the input fre- quency. This involves an equal change in voltage at the output of the IC (pin 7). The loop filter capacitance (C6) is smaller than usual to eliminate spikes from the output pulse. At the same time, a ladder network of three RC sections is used to filter out the remains of the carrier wave from the output signal. The free-running fre- quency of the VCO can be preset with potentiometer PI between abou...