RF generator on low-frequency quartz. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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RF generator on low-frequency quartz. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Knots of amateur radio equipment. Generators, heterodynes

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Due to the lack of high-frequency quartz, in order to obtain highly stable oscillations in the HF and VHF bands, radio amateurs often resort to multiplying the low-frequency oscillations of the exciter. This complicates the scheme of the device, reduces its efficiency, increases the size and weight. These shortcomings can be eliminated by direct excitation of low-frequency quartz at odd mechanical harmonics.

When exciting quartz on mechanical harmonics, one should take into account the harmful effect of static capacitance (quartz holder and circuit elements) shunting quartz. Due to this capacitance, the activity of the quartz resonator rapidly decreases with increasing harmonic number. Therefore, excitation at harmonics above the fifth is possible only with compensation or neutralization of static capacitance.

RF generator on low-frequency quartz

Due to the tendency to self-excitation and the difficulty of restructuring when changing the harmonic number, the compensation circuits of generators are not of particular interest to radio amateurs. It is more expedient to apply the neutralization of static capacitance by placing quartz in one of the arms of a balanced bridge. Bridge harmonic crystal oscillators have a number of interesting features. They have range properties - they allow excitation at various odd mechanical harmonics. For such a tuning, it is enough to change the frequency of the circuit. When neutralizing the static capacitance, the quality factor of quartz increases with increasing harmonic number, reaches a maximum, and then gradually decreases. The power of the generated oscillations changes slightly with an increase in the harmonic serial number. Bridge generators have a high frequency stability of the generated oscillations in relation to changes in supply voltages.

The frequencies of the generator on mechanical harmonics are not exactly a multiple of the fundamental frequency of the quartz, therefore, in the case of a precisely given oscillation frequency of the generator being designed, the frequency of the quartz must be specially selected.

Quartz resonators made of artificial raw materials have a low quality factor and therefore are excited (in bridge circuits) at harmonics no higher than the seventh. High-quality quartz can be excited at harmonics above the fifteenth.

The figure shows a practical diagram of a harmonic crystal oscillator on a transistor. When the bridge arms are equal and the circuit is tuned to frequencies other than the main or one of the odd mechanical harmonics of quartz, the base and collector circuits of the generator are decoupled. When the circuit is tuned to one of the odd mechanical harmonics, the balance of the bridge is disturbed due to the sharply increasing piezoelectric conductivity of the quartz, and a feedback voltage will be applied to the base of the transistor.

When using quartz and a frequency of 5,6-5,7 MHz, the generator can be tuned to the frequency of the 10-meter amateur band, while the quartz is excited at the fifth mechanical harmonic. The coil is wound on a frame from the IF filter of the Rubin TV and contains 12 turns with taps from every third turn. Winding - single-layer, turn to turn, with PEL 0,5 wire.

The operation of the generator can be monitored using a milliammeter included in the power supply circuit, which, whenever tuned to one of the harmonics, will show a change in current.

Author: V. Lensky, Krasnodar; Publication: N. Bolshakov, rf.atnn.ru

See other articles Section Knots of amateur radio equipment. Generators, heterodynes.

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