Stable generator with FM. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Stable generator with FM. 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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To tune receivers with narrow-band FM modulation with an intermediate frequency from 5 to 21,4 MHz, an oscillator with high frequency stability and internal (external) frequency modulation with low non-linear distortion is required. Not everyone has industrial generators such as G4-107 or G4-116. The proposed FM modulated crystal oscillator can replace them in some cases. The generator circuit is shown in Fig.1. At first glance, the master quartz oscillator is made on the transistor VT3 according to the capacitive three-point circuit (if there were no coil L1). However, the inclusion of an expansion coil L1 gives the circuit new properties - the ability to generate at a frequency below the frequency of the series resonance of quartz. In this case, this circuit can be attributed to the oscillator circuit with a quartz resonator in the circuit.

In such a scheme, parasitic generation is possible through the parallel capacitance of the resonator. Resistor R12 is designed to dampen this generation. The frequency control of the oscillator is not quite usual. Vari-caps are included as a capacitive divider emitter-base and emitter-collector of the transistor VT3. With this inclusion of semiconductor capacitances, the frequency control occurs according to a law close to linear, which is especially important for a frequency-modulated generator with low non-linear distortions.

The generator of low-frequency oscillations is made according to the scheme with phase rotation by a four-link chain four-terminal circuit, phasing the voltage, and with a single-stage amplifier based on the transistor VT1. The generation frequency is determined by the time constant of the quadripole and is approximately equal to 1 kHz. Low-frequency oscillations through the emitter follower on VT2 and the deviation depth controller are fed to the varicaps. When using the BQ1 resonator at a frequency of 5 MHz, the frequency deviation is small, and with 10,7 MHz quartz, it is quite sufficient for narrow-band FM. From a quartz oscillator through a broadband emitter follower, high-frequency oscillations are fed to the divider R23, R24.

(click to enlarge)

When using a quartz resonator at a frequency of 10700 kHz with a dynamic resistance of not more than 30 ohms, the voltage of high-frequency oscillations at the output is 100 mV. Then they can be further weakened by an attenuator or fed to the frequency multiplication stage. When multiplying the frequency, the generator can be used as a calibrator for FM radio receivers. So, with a quartz resonator at a frequency of 12 MHz, when the frequency is multiplied by 010, the signal is well audible on a broadcasting receiver (for example, "Leningrad XNUMX").

The scheme is going to printed circuit board from one-sided foil fiberglass with a thickness of 1,5 mm. The board is designed to install resistors of the MLT-0,125 type, ceramic capacitors of the K10-17 type, and electrolytic capacitors of the K50-35 type. Coil L1 is wound on a frame with a diameter of 5,5 mm with a wire with a diameter of 0,1 mm and contains 20 turns. Trimmer core - type 30 VN PR4xO, 7x8,0. Quartz resonator - type RK 169.

The location of the radio elements on the printed circuit board is shown in Fig. 2, the layout of the conductive tracks on the reverse side of the printed circuit board is shown in Fig. 3.

Establishing a stable generator consists in accurately setting the oscillation frequency with the L1 coil trimmer. If at the same time parasitic oscillations occur that are not stabilized by a quartz resonator, the resistor R12 is turned on and its value is selected from 470 ohms and higher until harmonic oscillations of the required frequency are obtained.

With a quartz resonator at a frequency of 10700 kHz with a parallel capacitance of 4,2 pF, a dynamic capacitance of 0,0172 pF and a dynamic inductance of 0,0128 H, stable generation was achieved with a change in the supply voltage by 10%, and the inclusion of the antiparasitic resistor R12 was not required.

Authors: O.Belousov, I.Chudayeva, Cherkasy region, Vatutino; Publication: N. Bolshakov, rf.atnn.ru

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

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