Crystal Sine Wave Oscillators

  
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Where good frequency stability is required, in applications such radio transmitters, basic LC oscillators cannot guarantee to hold their frequency without some drifting, which can be caused by quite small changes in supply voltage (although stabilised power supplies help avoid this) and changes in temperature. The effects of resistance and stray
Crystal Sine Wave Oscillators - schematic

capacitance within the circuit can also cause the oscillator to operate at a slightly different frequency from that calculated using just the values of L and C. In most cases this can be overcome by making the tuned tank` circuit have as high a Q factor as possible. With ordinary inductors and capacitors, Q factors more than a few hundred are not possible, but by using quartz crystals Q factors well in excess of 10, 000 can be achieved. Crystals may be used increase frequency stability in RF oscillators such as Hartley and Colpitts. The crystal may be used either in parallel mode` e. g. as an inductor operating at a frequency between ’1 and ’2 as part of the resonating tuned circuit, as shown in the crystal controlled Colpitts oscillator in Fig 2. 5. 1, or in series mode` where the crystal is acting as a highly selective low impedance at ’1 in the feedback path as shown in a Hartley oscillator in Fig. 2. 5. 2. The quartz crystal is a piezo-electric device, and will both produce a voltage across it when it is subjected to some mechanical distortion such as slight bending, or will distort slightly when a voltage is applied across it. Therefore applying regular voltage pulses will cause the crystal to bend, and the bending will in turn create voltage pulses in phase with the applied pulses, that will reinforce them and cause oscillation. The frequency at which this reinforcing effect occurs is the resonant frequency of the...



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