Electronic Circuits and Tutorials
Electronic Schematics - F

Frequency Shift (FSK) Circuits

68HC11 Function Generator:

Circuit makes simple FSK modulator: 10/11/2001 EDN - Design Ideas / The need for a compact telemetry system poses a challenge for designing a small, light, low-component-count system. Interfacing serial data from the microprocessor is also difficult because most low-cost RF transmitters do not accept dc levels at the input. Commercial FSK (frequency-shift-keying) modulators are bulky and need many passive components.

FSK (Frequency Shift Keying) Demodulator using NE-SE 565 IC - A very useful application of the 565 PLL is as a FSK demodulator. In the 565 PLL the frequency shift is usually accomplished by driving a VCO with the binary data signal so that the two resulting frequencies corre­spond to the logic 0 and logic 1 states of the binary data signal. The frequencies corresponding to logic 1 and logic 0 states are commonly called the mark and space frequencies. Several standards are used to set the mark and space frequencies.

IC function generator connected for FSK (frequency shift keying) - This circuit is similar to that of Fig. 5-31, except that the XR-2206 is connected for sine FSK outputs. Mark and space frequencies f1 and f2 are selected by the choice of R1, R2, and C, as shown by the equation. The output is phase-continuous during transitions. The keying signal is applied at pin 9. The circuit can be converted to split-supply operation by replacing ground with V-.R3 sets the FSK output amplitude.

FSK modem (modulator and demodulator) - This circuit implements a simple FSK modem, consisting of both the frequency shift keying modulator a corresponding demodulator. In this example, the demodulator consists of a shift register and a two AND-gates that detect the 0011 or 0101 patterns for the two frequencies generated by the modulator.

FSK Demodulator - This FSK demodulator convert the FSK signal to serial digital signal. FSK modulation is used to transmit digital serial data, and we need to demodulate it to get back the digital data at the receiver. Here is the schematic diagram of the circuit

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