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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING 1260 MHz converter. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / radio reception The advent of low-noise microwave transistors and diodes made it possible to significantly simplify the design of a converter designed to transfer amateur radio signals from 1260 MHz to 28 MHz. The converter consists (Fig. 1) of an RF amplifier, a mixer, a microwave heterodyne with quartz frequency stabilization and a supply voltage stabilizer. The RF amplifier is resonant, made on a transistor VT1. The input circuit consists of a coupling loop L1 inductively connected to a quarter-wave strip line L2, which forms an oscillatory circuit with capacitor C1. The signal is fed to the base of the transistor VT1 through a tuning constructive capacitor C3. This makes it easy to change the coupling depth when tuning the amplifier and optimize its noise performance.
To thermally stabilize the operating mode of the amplifier, a direct current feedback was introduced through the resistor R2. It is chosen so that the voltage across the capacitor C2 is about 5 V. Transistor VT1 is loaded with a circuit formed by a quarter-wave strip line L5 and capacitor C4. The mixer is made on two back-to-back diodes VD1, VD2 [L]. It is inductively coupled to both the local oscillator and the RF amplifier. The mixer load is an IF filter C8L9C9. Its quality factor is low, about four (with a load resistance of 75 ohms). Mixers of this type are supplied with a voltage of the "half" frequency of the local oscillator, which made it possible to simplify the latter. The local oscillator consists of a master oscillator on a VT4 transistor, a tripler (VT5) and a frequency doubler (VT7), an amplifier (VT6). The master oscillator is made according to a typical circuit on a transistor with a base grounded in alternating current through a quartz resonator. Quartz is excited at the 7th mechanical harmonic. From the output of the generator, a voltage with a frequency of 105,666 MHz is fed through the capacitor C14 to the frequency tripler. The transistor in this stage operates in class C mode. Its load is a partially connected L13C16 parallel circuit tuned to a frequency of 317 MHz. To get a power margin, an amplifier based on a VT6 transistor was introduced into the local oscillator. A frequency doubler on a VT7 transistor is connected to the output of this amplifier. Oscillations with a frequency of 7 MHz selected by the L6C634 circuit are fed to the mixer through a communication loop 1.8. The converted signal through the P-filter is fed to the output of the converter. On transistors VT2, VT3, a supply voltage stabilizer is assembled. Its advantages include the ability to work with small voltage drops between the input and output (0,2 ... 0,3 V) and the presence of automatic protection against short circuits in the load.
The converter is mounted (see Fig. 2) in an aluminum case measuring 115X60X X 23 mm. The parts are placed on three boards made of foil fiberglass with a thickness of 1,5 mm.On one board (Fig. 3) elements of the local oscillator are placed (excluding the quartz resonator). On fig. 3, and the tracks on the board are shown from the side of the parts. On the second board (it is attached to the converter cover, it is not in Fig. 2) there are parts of the stabilizer. On the third board (it is located with the foil up), the elements of the RF amplifier and mixer are soldered (Fig. 4; dots are not placed at the points of connection of parts above the board). Partitions (about 20 mm high) made of copper or brass foil 0,1 ... 0,2 mm thick are soldered to it, and the local oscillator board is also fixed to it. Glass insulators are soldered into the partitions in the right places from worn-out zener diodes of the D814 series or capacitors K53-1, K53-4. Mixing diodes and communication loops are attached to the same insulators. Power is supplied through a bypass capacitor. The converter uses MLT-0,125 resistors, constant capacitors KM-4, K53-1, trimmers (except C1, C3, C4) - any, for example, KT4-21, C1, C3, C4 - constructive. The rigid design of the strip line is shown in fig. 5. It is made of copper tube 2 or wire with a diameter of 3 mm. The workpiece is flattened to a certain length and bent in the shape of the letter U, as shown in the figure. The end of the line, not connected to a common wire (board foil 1), is supported by a resistor 6 (MLT-0,125 with a resistance of at least 510 kOhm) inserted into the gap between the ends. The "base" of the line is soldered to the board along its entire length.
The length of the horizontal part of the lines L2, L5 - 32 mm, L7-70 mm. To the ends of the tubes that are not connected to a common wire, a curved strip of 5 dimensions 5x10 mm made of copper foil 0,1 ... 0,15 mm thick is soldered, which with a mating strip attached through a glass insulator 4 (for other structural capacitors) to the partition 3, form trimmer capacitors. Communication loops L1, L4, L6, L8 are made of copper wire PEV-2 0,8. The gap between the loop and the line is about 2 mm. The length of the "active" part of the loops LI-16, L4-10, L6-12 and L8-28 mm. Coil L10 contains 6 turns of PEV-2 0,31 wire, wound turn to turn on a frame with an outer diameter of 4 mm, having an MZ internal thread. The trimmer is made of brass. Coils L13 and 114-frameless (wound on a mandrel with a diameter of 4 mm), have 3 turns of bare wire with a diameter of 0,8 mm. Winding length - 8 mm. The tap is made from the 1st turn, counting from the end, which is connected to a common wire. As a choke L9, you can use any high-frequency inductance of 4 μH. Chokes 13, L12 and Sh-DM-0,1. The blocking capacitor C2 is installed on the board without leads (they are removed in advance, and the place where the leads are soldered to the capacitor plate is cleaned of paint). A communication loop L4 and resistors R), R2 are soldered to it. There must be a partition between lines L2 and L5. Resistor R13 and capacitor C22 are installed on the printed circuit board from the side of the foil (they are not shown in Fig. 3). We will replace the KT3101A-2 transistor with KT3115A-2, KT391A-2. Instead of the transistor KT363B (VT7), it is desirable to use a transistor with a cutoff frequency of at least 1,5.-.2 GHz, for example, KT3123. Diodes VD1, VD2 - any mixing microwave diodes. After assembling the converter and checking the installation, power is supplied and the value of the stabilized voltage is checked. If necessary, one of the resistors R3 or R4 is selected so that the stabilized voltage is in the range of 9,5 ... 9,7 V. The current consumption should not exceed 40 mA. Then, by tuning the L10C12C13 circuit, the generation of a crystal oscillator is achieved. This can be determined by the voltage drop across the resistor R11 of the frequency tripler (it should be within 0,8 ... 1 V). By connecting (through a capacitor with a capacity of 1 ... 2 pF) a frequency meter to the emitter of the transistor VT4, you should control the generation frequency. If necessary, the frequency of the quartz resonator is adjusted according to the generally accepted method. After that, by successively tuning the circuits L13C16 and L14C20, the maximum (0,8 ... 1 V) voltage drop across the resistors R14 and R17 is achieved, which corresponds to tuning the circuits of the previous stages to resonance. The fine tuning of the L7C6 circuit is determined by some decrease in the voltage across the resistor R17. After that, by selecting the resistor R2, they make sure that the voltage across the capacitor C2 is equal to 5 V. By connecting a microammeter to the "IF Output" socket and unsoldering one of the mixer diodes, by adjusting the connection of the mixer with the local oscillator (by moving the L8 loop closer or further away), the current in the circuit is in the range of 50 ... 100 μA. After that, the diode is soldered in place. Having connected the converter to a receiver having a range of 28 MHz, insert a short piece of wire into its input jack. Then they install it side by side and turn on the transmitter in the 144 or 430 MHz range, and try to receive the 9th harmonic of the first or the 3rd of the second. At the same time, it is desirable to know exactly these frequencies in order to calculate the frequency of the converted signal and tune the receiver to it. By adjusting the circuits in the RF amplifier and changing the connections in it, they achieve the best audibility of the signal. It is desirable to carry out the final adjustment according to the generally accepted method using a noise generator - industrial or home-made. An adjusted converter should have a gain of 6 ... 8 dB, and a noise figure of 4 ... 5 kT. It should be noted that due to the small gain of the converter, the main receiver can make a significant contribution to the noise level of the system, so it is desirable that its noise figure does not exceed 4...6 kT. If the radio amateur is going to connect the converter to a receiver with a range of 144 MHz, then the local oscillator frequency should be changed, making it equal to 576 MHz. At the same time, there should be a frequency of 288 MHz in front of the frequency doubler in the local oscillator, which can be "branched" and used in the converter for the 430 MHz band, which will minimize the mass and volume of the VHF equipment complex, as well as the cost of its manufacture. Literature
Authors: A. Ermak (RB5LFS), G. Chuin (UB5LER), Kharkov Publication: N. Bolshakov, rf.atnn.ru
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