UNIVERSAL LC OSCILLATOR

Radio, 1979, №5

This oscillator is designed to be used in measurement circuits. The circuit diagram is shown in the figure 1. An important advantage of this oscillator is that in this circuit can be used a resonant circuit with almost any ratio of L/C. So, this oscillator operates stable even if the inductance of the coil L1 changes from 50 uH to 100 mH and the capacitance of the capacitor C1 changes from 50 pF to 5 uF. For example, the inductance L1 = 50 uH and the capacitance C1 = 5 uF, the oscillation frequency is about 10 kHz, while with the same inductance and C1 = 50 pF we get the oscillation frequency about 3.2 MHz. In addition, the advantage of this oscillator is the low voltage across the LC circuit - it is approximately 100 mV. In some cases it is important, for example, when measuring parameters of varicaps.

schematic diagram of the universal LC oscillator

Fig. 1.
V1, V2 - BC252; V3 - BC172C; V4, V5 - 1N4148; V6, V7 - BC172B; V8 - BF241.

The oscillator is based on transistors V1 and V2. The transistor V3 is a pre-amplifier, the signal from this stage is fed to an output amplifier (the transistor V8) and to the AGC circuit. The AGC automatically adjusts the amplitude level of the signal. Since the pre-amplified signal is fed directly from the LC resonant tank circuit of the oscillator, the AGC maintains a constant voltage across this resonant tank circuit. The AGC circuit is composed of diodes V4 and V5, this is the doubling rectifier circuit, the DC amplifier with transistor V7 and the regulating transistor V6. If the voltage across the output of the oscillator changes, for example, the voltage became higher, it increases the bias voltage at the base of the transistor V7. This will reduce the current through the transistor V6 (hence, the current through the transistors V1, V2), and the voltage across the output of the oscillator will be reduced to the initial value.

The output voltage remains almost constant when the supply voltage changes from 3.5 to 15 V. With the supply voltage of 5 V the amplitude of the output voltage of the oscillator will be compatible with transistor-transistor logic.

In this oscillator you can use any silicon high-frequency transistors. The transistors V1-V3 should have a sufficiently large current gain (not less than 150). In this circuit the parasitic oscillations can arise, in this case increase slightly the resistance of the resistor R2 to suppress them.

«Funkshau» (FRG), 1978, № 18

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