High frequency oscillator based on digital gates

Integrated circuits of SN74LS series can operate at switching frequencies up 30 MHz. By using logic gates of the SN74LS series we can build a high frequency oscillator, tunable with a double variable capacitor C, the oscillator covering the range 0.5...12 MHz (the range can be changed by changing the values of the variable capacitors C1). This oscillator can be used in miscellaneous applications, for example, it can work as a local oscillator of a radio receiver.

Digital RC oscillator for high frequency

Fig. 1. D1 - SN74LS10, D2 - SN74LS00, D1, D2 - 1N34 (Any germanium diode will work).

The schematic diagram is shown in Fig. 1. The oscillator is a symmetrical multivibrator, based on the typical multivibrator circuit with capacitive coupling. The logic gates D1.1, D1.2, D2.1 and D2.2 is the core of the oscillator. The logic gate D2.3 is intended for starting the oscillator.

The output signal of the oscillator having the form of rectangular pulses. The pulses at the outputs of D2.1 and D2.2 are in opposite phases.

oscillogram of the RC oscillator

Fig. 2. The signal at the upper output terminal and across the capacitor C1.

The frequency of oscillation depends on the capacity of the dual variable capacitor. The dependence is almost linear, in some cases it may be be important, for example, the oscillator can work as a proportional capacitive sensor.

This oscillator also can work as a voltage-controlled oscillator. In this case the variable capacitors are replaced with capacitors of constant capacitance, and the control voltage fed to the resistors R1 and R2 (instead of the voltage +5V).

The LTSpice model of this oscillator is here, and the necessary files of the 74HC library is here. If you'll need all the 74HC library, you can get it here: 74HC.zip.

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