Quartz oscillator for low active resonators. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Radio amateur designer
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Crystal oscillators provide the greatest frequency stability. It would seem, what is simpler, he included quartz in the circuit, for example, of a capacitive three-point and received the necessary vibrations. But it doesn't always work out that way. Previously, the term "activity" of a quartz resonator was even used. By "activity" was meant the ability of a quartz resonator to be excited in one or another circuit and the level of the signal amplitude at the output. To check the activity, various quartzes were alternately included in the capacitive three-point circuit, and a conclusion was made based on the signal level at the output of the circuit - which of the quartz is more active.
An analysis of the capacitive three-point circuits given in amateur radio literature showed that the capacitance ratings are unreasonably high in them. This leads to the fact that for quartz with low loss resistance, the power dissipation significantly exceeds that recommended for resonators. Exceeding the dissipated power (2 mW) leads to frequency drift and accelerated aging of the resonator
It has been experimentally established that for quartz resonators of thickness-shear oscillations of the cutoff AT in the frequency range of 1 ... 25 MHz with a dissipated power of 1 ... 2)*0,5 µW-2. Therefore, it is desirable to reduce the power value by an order of magnitude, using the smallest possible capacity for this. But in this case, not all quartzes are excited.
Well, there is no way out of this situation? Eat! In one of the copyright certificates, an oscillator circuit was proposed that allows excitation of low-activity quartz. Based on this author's certificate, I propose my generator. As can be seen from the diagram, this is a modified capacitive three-point circuit according to a common collector circuit. Transistors VT1 and VT2 are connected in series with direct current, and in parallel with alternating current. By selecting the resistor R1, the same voltage drops are set on R1 and R2. In this case, the transistors operate in the most optimal mode. Theoretically, the calculated capacitance C2 should be within 0,3 ... 1,0 of the capacitances C3, C4. Then the shape of the oscillations at the output is closest to harmonic. For the most "oak" quartzes, C2 can be increased to 10C3. Of course, in this case the shape of the oscillations is far from ideal.
The circuit was tested with low-active quartz (with URC marking) at frequencies from 4.43 to 16 MHz, which have a high loss resistance and low quality factor So, for quartz at a frequency of 10,230 MHz C2=33 pF, C3=C4=100 pF. For lower frequency quartz, it is recommended to increase C3 and C4 to 220 pF. Depending on what you need to get: the highest frequency stability or a higher output signal level, the value of C2 is selected experimentally.
Author: O. Belousov, Cherkasy.
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