Upon excitation of quartz and mechanical harmonics should be considered harmful influence of the static capacitance (cartegories and circuit elements), shunt quartz. Through this capacity, the activity of the crystal decreases rapidly with increasing harmonic number. Therefore, the excitation of the harmonics above the fifth is only possible when compensating or neutralizing the static capacitance.
Compensation schemes of generators because of the tendency to self-excitation and complexity of adjustment when changing the number of harmonics for radio Amateurs do not represent a special interest. More appropriate to apply the neutralization of the static capacitance due to the placement of quartz in one of the arms of a balanced bridge. Bridge harmonique quartz oscillators have a number of interesting features. They have a range of properties - allow excitation at various odd mechanical harmonics. For such adjustment is enough to change the frequency of the circuit. By neutralizing the static capacitance q factor of the quartz with the increase of the number of harmonics increases, reaches a maximum, .and then gradually decreases. Power generated oscillations varies little with increasing the ordinal number of the harmonic. Overhead generators have a high stability of frequency oscillations generated in relation to the change of supply voltage.
The oscillator frequency on the mechanical harmonics are not exact multiples of the fundamental frequency of the quartz, so if the exact frequency of the oscillation generator design .the crystal frequency must be specially selected.
Quartz crystals are made of artificial materials have a low q-factor and therefore are excited (in bridge circuits) on the harmonics of no higher than seventh. High-q resonators can be excited to the higher harmonics of the fifteenth.
The figure shows a practical circuit garmonichnogo oscillator transistor. With equal arm of the bridge and the tuning circuit at frequencies different from the fundamental or one of the odd mechanical harmonic quartz, base and collector circuit of the generator to be unleashed. When you configure a circuit to one of an odd mechanical harmonic balance of the bridge due to the sharply increasing conductivity of the piezoelectric quartz violated, and to the base of transistor will be filed with the feedback voltage.
When using the quartz frequency of 5.6 to 5.7 MHz oscillator can be tuned to the frequency of the 10-meter Amateur band, wherein the quartz is excited at the fifth mechanical harmonica. Coil wound on the frame from the TV if filter "ruby" and contains 12 coils with taps on every third round. Winding - single layer round wire PEL of 0.5.
Control over the operation of the generator can be realized using a milliammeter in the circuit of the power source, which every time when tuned to one of the harmonics will show the current change.
Author: V. Lensky, Krasnodar; Publication: N. Bolshakov, rf.atnn.ru