Frequency Shift Demodulator Using Active Filter

As shown in the figure, this is a frequency shift demodulation circuit composed of an active filter. Using an active filter instead of an LC tuning circuit can avoid using a large and expensive inductor coil in the frequency shift keying demodulator. It is not only small in size, but also can improve the performance of the demodulator. This circuit is used to demodulate a 110-bit frequency shift keying signal, whose sign bit frequency is 2225Hz and the empty bit frequency is 2025Hz. Filter A allows the empty bit frequency of 2025Hz to pass. Filter B allows the sign bit frequency of 2225Hz to pass. Therefore, when the sign bit frequency appears at the input end, the transistor is saturated and the output is short-circuited. When the empty bit frequency appears at the input end, the transistor is cut off and the output is open. When adjusting the circuit, input the sign bit frequency and the empty bit frequency into the circuit respectively, and adjust the corresponding 50kΩ potentiometer to make the outputs of the two filters maximum.