Demodulator of frequency-shift keyed signals on active filters. Scheme, description

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Demodulator of frequency-shift keyed signals on active filters. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Radio amateur designer

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If active type filters are used instead of LC filters in an FSK demodulator, then it is possible to improve the performance of the demodulator and reduce its size. Active filters eliminate the need for bulky and expensive inductors. This circuit was designed to demodulate 110 baud FSK signals. It is designed to work in normal mode, when a character corresponds to a frequency of 2225 Hz, and a pause - to a frequency of 2025 Hz.

(click to enlarge)

When a symbol is received, filter B, set to 2225 Hz, passes the signal, while filter A rejects it. The outputs of these two filters are converted to DC voltages and compared by an op amp that operates in open loop mode. Since the output of filter B is applied to the non-inverting input of the op amp, the output transistor of the circuit is kept in saturation and the feedback loop is closed as a result. When the input frequency is changed to the blank frequency, the signal starts to pass through filter A and is rejected by filter B, leaving the feedback loop open.

To adjust the circuit, a signal with a symbol frequency and a pause frequency is alternately applied to its input, and by adjusting two trimming variable resistors with a resistance of 50 kΩ, the required level of the output peak voltage is set. To set up the circuit, a low-impedance audio frequency oscillator is required, providing a total output voltage size of about 1,2 V.

Author: M. I. Gordon; Publication: N. Bolshakov, rf.atnn.ru

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