Solid State Circuits 2

Posted on Feb 7, 2014

This circuit can be used from low frequencies to at least 120MHz using series resonant crystals on their fundamental or overtone mode. The output can be taken from the feedback tap as shown, a low impedance winding on L2 or from the drain of the J310 via a low value capacitor. The following stage should be used a a buffer to isolate subsequent sta

Solid State Circuits 2
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ges from the oscillator. The combined series value of C1 and C2 resonates with inductor L1 at the desired frequency and crystal X1 must have a fundamental or overtone series resonance as appropriate at the same frequency. C2 is about five times the value of C1 Inductor L2 is generally only required at frequencies above about 60MHz and must be chosen to resonate with the total parallel capacitance of X1 at the frequency of oscillation. If the circuit shows signs of free running oscillation controlled by L1 then L2 will be required. The parallel capacitance of X1 will typically be 4 - This circuit is primarily intended for use as an overtone oscillator and will work from 20MHz to 120Mhz depending on the crystal and active device. Operation is extremely stable but the output level is fairly low - pk. L1, C1/C2 and the input capacitance of the transistor act as a quarter wave network, converting the low impedance series resonance of the crystal into a high impedance parallel resonance at the base of the transistor. C1 may not be needed at higher frequencies depending on the internal base/emitter capacitance. This circuit is a Colpitts fundamental mode parallel resonant oscillator for crystals designed for use with a 30pF load capacitance. Values shown are suitable for 8 - 12Mhz and will require changing for significantly different frequencies. Transistors TR1 and TR2 should be characterised for the required frequency range....

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