Circuit makes simple FSK modulator

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. The circuit in Figure 1 uses a single NOT gate (inverter), an On Semiconductor NL27WZ14 in a surface-mount package, to generate continuous FSK data from TTL-level signals. The outputs from this circuit are compatible with available transmitters. When the TTL input has a low level, the circuit is a continuously running oscillator, producing approximately 2400 Hz (adjustable with R₁). When the input assumes a high level, the oscillator's frequency reduces by one- half with the introduction of a capacitor in the timing circuit via Q₁. The inverter IC can accommodate an operating frequency of approximately 80 kHz. You can easily operate the FSK modulator at higher frequencies, such as 4800 and 9600 Hz, by reducing the values of the timing capacitors C₁ and C₂.

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