Stable quartz oscillator. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Radio amateur designer

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It is known that the stabilizing properties of a quartz resonator are most fully realized if it is excited at a series resonance frequency. The author succeeded in developing a quartz frequency stabilized oscillator, which is self-excited at a series resonance frequency and practically does not require adjustment.

The well-known generator was taken as a basis, in which a quartz resonator is connected between the emitters of connected transistor stages. Its simplified circuit (without transistor bias circuits) is shown in fig. 1. Transistor VT1 is connected according to the scheme with a common base, and VT2 - with a common collector (emitter follower). As you know, the output resistance of the emitter follower and the input - cascade with a common base are very small, so the quartz resonator is connected between two small and almost active resistances. Under these conditions, it passes the signal, closing the feedback loop, only at the frequency of its series resonance, where its resistance is minimal.

For self-excitation of the generator, it is necessary to maintain amplitude and phase balances. The first is to ensure that the product of the gains of all links in the feedback ring is slightly greater than one. If it is less than this value, the generator will not self-excite, and if it is much larger, it will overexcite, as a result of which the frequency stability and signal shape will deteriorate (due to entering the nonlinear region of transistor characteristics).

The phase balance is that the phase incursion in the ring is either 0 or 360°. Otherwise, the additional phase incursion will have to compensate for the resonator, and in accordance with the phase characteristic of the latter, the generator will self-excite not exactly at the resonant frequency, but a little to the side. And what is most unpleasant, this frequency shift will depend on the mode, temperature and other destabilizing factors. By the way, the frequency shift due to skew is the smaller, the higher the quality factor of the resonator and, accordingly, the steeper its phase characteristic. That is why it is recommended to use high-quality resonators.

The R3C1 circuit, which serves to connect the amplifying stage (VT1) with the emitter follower (VT2), helps to fulfill the described conditions in the proposed generator. The reluctance to use an oscillatory circuit as a load of the amplifier led to the fact that the amplified voltage is released rather than on the active resistance of the load R1, but on the capacitance of the collector junction. The place of inclusion of the collector capacitance and installation of Sk is shown in fig. 1 dashed line. These capacitances act like an integrating circuit, creating a phase lag of almost 90°. The differentiating circuit R3C1 creates a phase advance by the same angle, as a result, the total phase shift in the ring is close to zero. By reducing the capacitance C1, the transmission coefficient can also be reduced, thus eliminating overexcitation. The output signal is conveniently removed from the resistor R4 included in the collector circuit of the emitter follower transistor VT2. Due to its high output impedance, the effect of subsequent stages on the operation of the generator is negligible.

Experiments with the described generator showed that it is very easily self-excited, it practically does not require adjustment, it is completely uncritical to the ratings of parts and the type of transistors. Naturally, it generates at the fundamental frequency of the series resonance of the quartz resonator. However, for VHF local oscillators and transmitters, harmonic generators are good, generating oscillations of a triple or even five times the frequency of the resonator (by the way, the quality factor of the latter on harmonics is much higher). The question arose, is it possible to force this generator to be excited at the third harmonic of quartz? It turned out that, even despite the absence of oscillatory circuits, it is possible! To do this, you need to take microwave transistors with a high cut-off frequency (not lower than 300 ... 500 MHz) and reduce the load resistance R1 and the capacitance of the coupling capacitor C1 to a minimum. The self-excitation conditions for the third harmonic are obtained in this case better than for the first.

A practical generator circuit is shown in fig. 2

DC transistor modes determine the dividers R1R2 and R8R9 in the base circuits. The collector current depends on the resistances of the resistors R4 and R11 and in this case is approximately 4 mA, and the total current consumed by the generator is 8 mA. Mode selection is not required. It is desirable to stabilize the supply voltage of 9 V.

The author used in the generator widely used small-sized quartz resonators in a metal case from 27 MHz CB radio stations. The frequency of their main resonance is about 9 MHz, but the frequency of the third harmonic is indicated on the case. Of a fairly large batch of resonators, only a few (about 5%) had insufficient activity for self-excitation of this generator. To adjust the frequency within a small range, a varicap matrix VD1 is connected in series with the BQ1 quartz resonator. When the control voltage Ucontrol changed from 0 to 9 V, the frequency changed by 700 Hz. Taking into account the subsequent frequency multiplication (and the deviation is multiplied by the same number of times), this is quite enough for narrow-band FM in the VHF bands. If modulation or remote adjustment of the generator frequency by the control voltage is not required, the elements VD1, R5-R7, C4 and C5 can be excluded (the right - according to the diagram - the output of the resonator in this case is connected directly to the emitter VT2). Within small limits, the frequency can also be adjusted with a trimmer capacitor connected in series with the BQ1 quartz resonator.

Setting up the generator is reduced to changing the capacitance of the tuning capacitor C2 to obtain stable self-excitation at the third harmonic. With its insufficient capacity, the generation breaks down completely, and with excessive capacity, a "jump" to the first harmonic may occur. The output voltage is about 0,5 V. It is convenient to control it with a high-frequency oscilloscope connected to the generator output.

Author: Vladimir Polyakov (RA3AAE)

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