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LF Crystal Controlled Oscillator

LF Crystal Controlled Oscillator

This is a low frequency sine wave oscillator featuring low distortion, wideband operation and crystal control.The circuit, originally developed for laboratory use, employs low cost AF bipolar transistors for the oscillator and amplifier sections and a JFET for loop-gain control.
Operation of the oscillator in the 10 kHz to 500 kHz frequency range has been found to be excellent, while measured distortion is kept under 0.1 percent. Q1, Q2 and associated circuitry form a modified astable multivibrator in which the loop gain is automatically adjusted to the threshold of oscillation by means of field effect transistor Q3. Q4 linearly amplifies the signal present at the collector of Q2 and isolates the oscillator section of the circuit from the output. This stage features wideband operation and delivers a clean 2.5 Volt amplitude sine wave into a resistive load greater than or equal to 20 kohms. The stage comprising Q5 has a voltage gain of 1 and its sole purpose is to isolate the non-linear effects of rectifier D1 from the output. Transistor Q4 also amplifies the minor changes in amplitude of the oscillator’s waveform due to temperature effects and/or power supply variations, so a magnified version of the perturbance is fedback to rectifier D1 producing a corresponding change in Q3’s gate voltage. This action modifies the FET’s drain-source resistance and hence adjusts the loop gain to a new value slightly above unity, just enough to maintain a constant amplitude in the output.

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