Two transmitters at 144 MHz. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Two transmitters at 144 MHz. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Civil radio communications

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The transmitters brought to the attention of readers are designed to work with frequency modulation in the frequency range 145,5 ... 145,85 MHz of the two-meter range. They can be used both as independent devices and as an integral part of a two-meter radio station.

A schematic diagram of a transmitter with a power of 1 W is shown in Figure 1. A microphone amplifier - a frequency modulator - is made on the operational amplifier A1. An electret microphone with a built-in amplifier from a foreign-made telephone set is used as a microphone. Power is supplied to the microphone through resistor R1, which also acts as a load for the built-in amplifier of this microphone. From its output, the sound voltage through the coupling capacitor C1 is fed to the modulation amplifier at the op-amp A1. The undistorted output voltage swing of this amplifier reaches 70% of the supply voltage. This output voltage, through the resistor R7, which acts as a decoupling element between the HF and LF paths, enters the VD1 varicap and changes its capacitance in accordance with the shape of the low-frequency signal.

Fig.1 (click to enlarge)

The master oscillator is made on the VT1 transistor, it operates on the third mechanical harmonic of the Q1 quartz resonator at 16,2 MHz (you can also use a 16 MHz resonator, but the frequency range in this case will drop to 144 MHz). The collector circuit L2C9 is tuned to a frequency of 48,6 MHz. To obtain the required frequency, after the master oscillator, a cascade is connected on the transistor VT2, which acts as a frequency tripler. The signal is fed to it through an inductive coupling between the circuits L2C9 and L3C11, the axes of the coils of these circuits are located at a distance of 7 mm from each other, which provides the necessary connection between them. The current in the collector circuit of this transistor has a pulsed character, and the circuit included in its collector circuit and tuned to a resonance frequency of 145,7 MHz is excited at the third harmonic of the input pulse signal. As a result, there is a sinusoidal high-frequency voltage in the L4C12 circuit, which is fed through the L5 coupling coil to a two-stage power amplifier built on transistors VT3 and VT4. Moreover, the transistor VT3 operates with a bias voltage at the base, which provides the necessary preliminary amplification of this RF signal before it enters the output power amplification stage, made on the transistor VT4, operating without an initial bias. The output circuit of the L9C21 is set to work with an antenna having a 75 ohm impedance.

Frequency modulation, as well as tuning within the selected frequency section, is performed in the first stage of the master oscillator on the transistor VT1. In series with the quartz resonator, an LC circuit is connected, consisting of a coil L1 and a complex capacitance of the elements VD1, C4, C5. This circuit produces a small frequency shift of the resonator resonance, and the degree of this shift depends on both the inductive and capacitive components. By adjusting L1, such an inductive component is selected at which, with the rotor of the variable capacitor C5 in the middle position, the transmitter emits a signal with a frequency of 145,7 MHz. Tuning within 145,5 ... 145,85 MHz is carried out by changing the capacitive component using capacitor C5. Frequency modulation is carried out by an additional change in the capacitive component using the V01 varicap.

Trimmer capacitors - of the PDA type with a ceramic dielectric, for a capacitance from 4 ... 15 pF to 6 ... 25 pF, but it is better if there are trimmer capacitors with an air dielectric, however, in this case, to prevent the transmitter stages from failing - due to a possible short circuit between the plates, it will be necessary to turn on permanent ceramic ones for several thousand pF in series with these capacitors. Transistor VT4 can be KT904 or KT907, transistor VT3 - KT606 or KT904. If you use a pair of KT904 (VT3) and KT907 (VT4) and increase the supply voltage of these stages to 20V, you can get a power of about 2-3 W, but you will need to select the value of R13 and the number of turns L5 so as to get the maximum undistorted output power.

Capacitor C5 - with an air dielectric, type KPV, its minimum capacitance can be 5-15 pF, and the maximum, respectively, 70-150 pF.

KT368 transistors can be replaced with KG 316, but the result will be worse.

Coils L1-L3 are wound on polystyrene frames with a diameter of 4-5 mm with tuning cores MP-100 (from high-frequency ferrite). L1 has 7 turns, L2 has 10 turns, and L3 also has 10 turns, but L3 has a tap from the second turn, counting from above (according to the diagram). Winding with wire PEV 0,2-0,3.

Coils L4 and L5 have the same frames, but the ferrite core in them is replaced with a piece of thick aluminum wire (from electrical wiring) or a brass rod. L4 contains 4 turns of wire with a diameter of 0,6-1 mm, and L5 is wound over L4 and contains 2-3 turns of PEV wire 0,2-0,3.

The power amplifier coils are wound on ceramic frames with a diameter of 10 mm without cores (they can also be made without a cascade). Winding is carried out with silver-plated (or tinned, which is worse) wire with a diameter of about 0,6-1 mm. L6 and L8 are the same, they contain 4 turns, distributed over a length of 15 mm. L7 and L9 are also the same, and contain 3 turns distributed along the length of 10 mm. The DL4 inductor is wound on a resistor R15, it contains 35 turns of PEV 0,12 wire. Inductors DL1-DL3 are wound on K7X4XZ rings made of 400NN ferrite (or on other rings of similar size made of 100NN-600NN ferrite), they contain 15 turns of PEV wire 0,2-0,3.

The transmitter is mounted in a volumetric way in a box with compartments according to the number of stages, soldered from tin or brass. The box is mounted on a massive aluminum plate, which acts as a heat sink for transistors VT4 and VT3. All installation is carried out on contact petals and mounting panels, as well as on the outputs of powerful transistors. Coils L2 and L3 are fixed on two common getinax plates with holes along the diameter of the coil frames. The distance between the centers of the hole in the plate is 7 mm. Thus, when these plates are put on the coil frames, they rigidly fix the coils relative to each other at a distance between the axes of 7 mm, providing the necessary inductive coupling.

The diagram of the second transmitter is shown in Figure 2. It develops power at a 75-ohm load of about 3-4 watts. Its main difference is that a high-frequency quartz resonator is used at a frequency of 48,4 MHz.

Fig.2 (click to enlarge)

The microphone amplifier and the modulation and tuning system do not differ from that of the previous transmitter. The master oscillator is made on a transistor VT1, in its base circuit a quartz resonator is included, the resonance frequency of which is three times lower than the frequency of the transmitted signal.

The power amplifier is two-stage on transistors VT2 and VT3, they both work without initial bias. The L4C9 and L7C11 circuits are tuned to a frequency equal to the third harmonic of the quartz resonator - 145,2, this frequency is the middle frequency of the range. It is possible to use a resonator at 48,6 MHz, while the frequency will be equal to 145,8 MHz.

Coil L1 is wound on the same frame as the coils of the transmitter master oscillator, the circuit of which is shown in Figure 1. It contains 5 turns of SEW 0,2-0,3. All other coils are frameless, wound with a silver-plated wire with a diameter of 0,7-1 mm. L3 has a diameter of 6 mm, the winding length is 20 mm and the number of turns is 8, L4 has a diameter of 8 mm. winding length 7 mm and number of turns 3, L6 has a diameter of 6 mm winding length b mm and number of turns 3, L9 - diameter 10 mm, length 12 mm, number of turns 3. L9 - diameter 6 mm, length 5 mm, number of turns 1,5 ,10, L10 - diameter 80 mm, length 4 mm. number of turns XNUMX.

Coils L5, L2 and L8 are chokes wound on fixed resistors MLT-0,5 with a resistance of at least 100 kOhm, they contain 30 turns of PEV 0,12 wire.

The design of the transmitter is the same as that made according to the previous scheme. Mounting volume in a shielded box. The details are similar.

Author: Andreev S.; Publication: N. Bolshakov, rf.atnn.ru

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