In this work, we present a variable-frequency quartz crystal oscillator that is able to oscillate at LC resonance under frequency locking of a quartz crystal resonance, with the frequency tuning realized by variable-capacitance diodes. This circuit shows a steep transition between LC oscillation modes to quartz crystal double-resonance, which shows a characteristic change in the oscillation frequency. Control voltage of this diode is precisely adjusted from the low side to higher values and conversely in the vicinity of the oscillation mode transition. The transition of the oscillation modes is experimentally demonstrated and compared with an algebraic analysis.
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