The circuit starts with a standard Collpits crystal oscillator. In order to provide a 600 Hz frequency shift, a trimmer cap and 2N7000 FET is added in series with the crystal. During transmit, the 2N7000 is turned on, effectively shorting out the trimmer cap and lowering the oscillator frequency. During receive, the ’7000 is turned off, allowing the trimmer cap to raise the frequency. The trimmer cap is adjusted to give a shift of about 600 Hz.
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