High power radio transmitter with quartz frequency stabilization. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Transmitters
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Main technical characteristics:
- transmitter output power - 0,5 W;
- range of sound frequencies at the level of -3 dB - 300..3000 Hz;
- emission band width at the level of -30 dB - no more than 11 kHz;
- frequency deviation at maximum modulation of about 2,5 kHz;
- consumption current, no more than - 90 mA;
- power supply voltage - 9 V.
The signal from a condenser microphone with a built-in amplifier (M1) is fed to the direct input of the operational amplifier DA1. Connected to this input is a voltage divider across resistors R2 and R3, which creates half the supply voltage at this input, and thus allows the op-amp to operate with a single supply. A circuit R7, C5, C6 is connected between the inverting input and output, which creates the desired gain and frequency response of the amplifier. This amplifier works as a speech signal compressor, compressing its dynamic range due to the VT1 transistor cascade. The output voltage of the AF amplifier is detected by the diodes VD1 and VD2 into a constant voltage, negative, which acts on the gate of the transistor VT1 and increases the channel resistance of this transistor with an increase in the sound signal level.
As a result of shunting the inverting input with capacitor C6, the negative feedback coefficient changes, which leads to a change in the gain of the op-amp. The output voltage of the op-amp, equal to half the supply voltage, is supplied through resistors R1 and R12 to the cathodes of the VD3 varicaps.
The modulating voltage of the AF changes at the cathode of the varicaps relative to this bias voltage. The VD3 varicap matrix is connected between the quartz resonator and the common wire. A change in the capacitance of the varicap leads to some change in the frequency of the resonator. In this process, the inductance of the coil L1 also plays a role. The master oscillator is made on the transistor VT2, the frequency in the collector circuit of which is determined by the included resonator, inductance L1 and capacitance VD3. The circuit L2, C13 in the collector circuit of this transistor is tuned to the middle of the selected range, and a frequency-modulated RF voltage with a frequency of the resonator Q1 is allocated on it. This voltage is fed through the coupling coil L3 to the output stage, made on the transistor VT3. The coil is included in the base bias circuit of this transistor - K17, K18, which creates the operating point of the output stage.
The amplified and frequency-modulated RF voltage is released on the VT3 collector. Then, through a low-pass filter and an extension coil, this voltage enters the antenna. The low-pass filter on the L4 coil and capacitors C16 and C17 serves to suppress harmonics and match the output impedance of the cascade on the VT3 transistor with the input impedance of the antenna, the L5 coil introduces additional inductance into the antenna circuit and thus increases its equivalent length, approaching a quarter wave. As a result, the signal return to the antenna increases. Capacitor C19 eliminates the failure of the transistor VT3 from accidental shorting of the antenna with a common wire or power circuit.
All high-frequency transmitter coils are made on the same frames with a diameter of 7 mm with cores of ferrite 100 VCh with a diameter of 2,8 mm. The transmitter coil L2 has 6 turns, L3 - 3 turns, L4 - 8 turns, L5 - 20 turns of PEV 0,2 wire. Coil L1 - choke DM-0,06 16 μH. The transmitter is tuned in the traditional way, controlling the field strength generated by it using a wavemeter or an RF oscilloscope with a wire loop at the input.
See other articles Section Transmitters.
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