A common problem in crystal sinusoidal oscillators is the excitation of unwanted modes of the quartz crystal that degrade the spectral purity of the oscillator. This problem is significant in overtone crystals, particularly if the oscillator is intended for low-voltage applications. In this case, there’s a compromise between the operating point of the transistor and the drive level of the crystal. If the operating point of the transistor produces a low current level in the crystal, it can’t reach the minimum energy for correct vibration startup. The proposed circuit is a simple modification of a Colpitts oscillator, in which the transistor’s emitter is grounded through an inductor (see the figure).
The inductor L1 increases the voltage gradient in crystal plates during the transient response of the oscillator (after power is applied), facilitating the crystal vibration startup. A detailed analysis reveals that, in overtone crystals, the impedance is inductive at the crystal fundamental frequency, whereas this impedance becomes capacitative at the desired overtone. Therefore, the desired overtone oscillation will be easily started and the undesired fundamental frequency perturbations are avoided. In addition, the overall transfer function has a low-pass behavior that eliminates higher harmonics in the oscillator output.
This circuit has been used in the production of a 27-MHz oscillator for an industrial communication application. Without the proposed modification, 23% of the oscillators failed, with the failure rate noticeably dependent on the production series (manufacturer reference) of the crystal. With the modification shown, practically all of the oscillators run correctly. On the other hand, the total harmonic distortion (THD) of the oscillator shown, when L1 is replaced by a 4.7k resistor (classical Colpitts oscillator), is 28%. With the proposed modification (inductor L1), the distortion is reduced to 0.32%. In all of these measurements, the power supply was 3.3 V.