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20MHz voltage controlled oscillator

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  • Nicolae Sfetcu
    20MHz voltage controlled oscillator This circuit uses a voltage-varactor to form a voltage controlled oscillator. The voltage controlled crystal oscillator (
    Message 1 of 1 , Feb 22 7:11 AM
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      20MHz voltage controlled oscillator

      This circuit uses a voltage-varactor to form a voltage controlled oscillator. The voltage controlled crystal oscillator ( VXCO ) has a clean 20MHz sine wave output suitable for communications applications. The output frequency can be shifted by 7kHz over a 10 volt tuning range. The 25pF trimmer sets the 20MHz zero bias frequency. In many applications, such as phase-locking and narrow bandwidth FM secure communications, the non-linear response is irrelevant. Improved linearity will require conditioning the tuning voltage or the varactor network's response. In circuits of this type it is important to remember that the limit on pulling frequency is set by the crystals Q, which is high. Achieving wide dynamic "pull" range without stopping the oscillator or forcing it into abnormal modes is difficult. Typical circuits, such as this one, offer pull ranges of several hundred ppm. Larger shifts ( e.g., 2000-3000ppm ) are possible without losing crystal lock, although clock output frequency stability suffers somewhat.

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