Radio transmitter with quartz frequency stabilization of the master oscillator. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Radio transmitter with quartz frequency stabilization of the master oscillator. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Transmitters

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Most amateur radio transmitters are made according to schemes with an oscillating circuit in the frequency setting circuit. However, they usually have one high-frequency cascade. This cascade simultaneously performs the role of a master oscillator and a power amplifier. Such schemes, with their simplicity and diminutiveness of the finished design, have their drawbacks. This is a large instability of high frequency oscillations and a small output power.

The circuit shown in the figure is devoid of these shortcomings, since the frequency of the master oscillator of the proposed device is stabilized by a quartz resonator and has a separate power amplifier. This device operates in the VHF FM band with frequency modulation, i.e. its signal can be received on any receiver in the 65-108 MHz band. The range is about 300 m.

(click to enlarge)

The master oscillator is made on a transistor VT1 type KT368. In this circuit, quartz resonators at frequencies of 22-36 MHz can be used. The circuit consisting of coil L1 and capacitor C7 is tuned to the third harmonic of the quartz resonator. It is preferable to use a resonator tuned to the third harmonic of the series resonance, since in this case it is easier to obtain the frequency deviation of 50 kHz necessary for the normal operation of the receiver. The quartz resonator is connected to the base of the transistor VT1 and the varicap VD1 and operates according to an oscillator circuit, making up a "capacitive three-point" with the capacitance C5 and the circuit, which ensures high frequency stability.

The modulating amplifier is made on the operational amplifier DA1 type KR1407UD2. Its input receives a low-frequency sound signal from an M1 electret microphone with a built-in MKE-3 type amplifier. The operational amplifier provides an undistorted audio frequency voltage with an amplitude of about 3 V at the output, which is sufficient when using a KV104A varicap as a modulating element to achieve a frequency deviation of about 4050 kHz. The operation mode of the operational amplifier is set by resistors K1, R4, and short circuit. The DA1 chip can be replaced with K140UD1208, KR140UD608 - in the latter case, the resistor R3 can be excluded from the circuit. The frequency-modulated signal from the circuit of the master oscillator L1, C7 through the coupling coil L2 is fed to the input of a power amplifier made on a VT2 transistor of the KT610A type. The power amplifier operates with high efficiency in class "C" mode. It amplifies the signal up to 150 mW. Therefore, when using instead of a hanging or whip antenna with a length of 1 m, an L3 coil with a diameter of 3 cm, a length of 5 cm, containing 7 turns of PEV 0,8 wire, the efficiency is not worse than the standard version with a wire 1 m long and a power of about 30 mW. This power is quite enough for stable reception at a distance of up to 150 m. The winding length of the L3 coil is 5 cm.

The design used resistors MLT-0,125, capacitors such as KT, KD, K50-35. Inductor Dr1 is wound on a resistor MLT-0,25 with a resistance of more than 100 kOhm. It contains 60 turns of 0,1 mm PEV wire. Coils L1 and L2 are wound on a polystyrene frame with a diameter of 5 mm with a brass trimmer. Coil L1 (Fig. 2.16) contains 10 turns of PEV wire 0,31 mm, coil L2 - 5 turns of the same wire. The design of the L3 coil is shown in fig. 2.17.

Setting the low-frequency part of the transmitter has no special features. The transmitter is tuned according to the generally accepted technique using a field strength indicator and a control radio. The circuit C7, L1 is adjusted in such a way as to ensure the stability of the generation of the master stage. When connecting a piece of wire about 1 m long to point A of the antenna, having previously disconnected the capacitances C10, C11 and L3, the transmitter output power of about 150 mW is achieved using measuring instruments. This power is sufficient to receive a signal to a receiver with a sensitivity of 5 μV/m at a distance of up to 500 m in urban areas.

Author: V.Adrianov

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