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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Reversible broadband cascade. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Civil radio communications The proposed article considers the design of a reversible stage on broadband RF amplifiers that have the same value of the transmission coefficient in both directions of the signal passage. It can be installed in an amateur radio transceiver between the first mixer and the main selection filter (FOS). The reversing cascade consists of two non-reversing amplifiers based on transistors VT1 and VT2 (Fig. 1), each of them works only for its own signal flow direction. For example, when the amplifier is running on VT1 (+12 V is applied to its power port C), the RF signals are amplified in the direction from port A to port B.
The amplifiers are made according to the scheme with a common base and negative feedback on non-noise reactive elements (the so-called X-type feedback), which allows for an optimal dynamic range and high sensitivity [1]. Amplifiers of this type, with gains of 4,5 ... 9,5 dB, are practically not prone to self-excitation even when connected to ports A and B of loads (transceiver nodes) with an active resistance significantly different from 50 Ohms, and the presence of a large reactive component. Amplifiers are interconnected by short lengths of coaxial cable. To eliminate the influence of a currently idle amplifier (for example, on VT2) on a working one (on VT1) and thereby eliminate the possibility of self-excitation of the reverse stage as a whole in the circuit, key diodes VD3VD4 and VD7VD8 are installed. A chain of diodes VD1VD2 and resistor R2 sets the quiescent current of transistor VT1, and a chain of VD5VD6 and resistor R6 sets the quiescent current VT2. Resistors R1, R3, R5, R7 and chokes L2, L4 are antiparasitic, but if their inductance is too high, the frequency response in the HF region is blocked. The connection of the transceiver nodes to ports A and B must also be done with short lengths of cable. Structurally, the reversing cascade is made on two printed circuit boards (each amplifier separately) from one-sided foil fiberglass. The maximum possible area of the common wire track should be provided. To each amplifier board around the entire perimeter is soldered a screen strip of tinned sheet 20 mm wide, symmetrically protruding with its edges above the upper and lower surfaces of the board. Widespread radio components are used in amplifiers: resistors - MLT-0,25, capacitors - KM, K10-17. Diodes KD522A can be replaced with any silicon. Inductors L1 and L4 are wound with PEV-2 0,2 wire in one layer turn to turn, until filled, on ring ferrite magnetic cores with a permeability of 1000-2000NM, size K10x6x4 mm. Their inductance should be in the range of 100 ... 220 μH. Inductors L2 and L3 are wound on ring ferrite magnetic cores with a permeability of 1000 NM, size K7x4x2 mm. Their windings contain two turns of PEV-2 0,25 wire. Chokes are mounted directly on the terminals of transistors. Transformers T1 and T2 are wound with PEV-2 0,25 wire on M2000NM-A ring ferrite magnetic cores of size K16x10x4,5 mm. Windings I and II of the transformer contain 10 turns each, and winding III - 2 turns. In order not to damage the insulation of the wire, before winding the transformers, at the magnetic circuits, with the help of an abrasive stone, we cut (fill up) the sharp edges outside and inside. Further, twisting two pieces of wire together into a "twisted pair" with the calculation of 3 ... 4 twists per centimeter, we wind 10 turns evenly around the circumference of the ring. These will be the windings I and II of the transformers. To add winding III, we wind two turns of the "twisted pair" from either side of the ring and wrap the coiled part with a third wire of the same diameter and in the same direction. At the same time, we make two turns of the winding wire III per centimeter of the "twisted pair". Next, we restore the full winding of the transformer. When installing transformers on amplifier boards, it remains only to dissolve the winding leads and correctly phase them when desoldering. The general requirement for the installation of amplifiers is that the leads of the radio elements must have a minimum length. Transistors VT1 and VT2 have heat sinks with an area of about 50 cm2. We set up the assembled amplifiers separately according to the following method (we will consider the example of an amplifier on VT1). On the board, we unsolder one of the terminals of the capacitor C2, and close the windings of the chokes and transformers with wire jumpers. We apply a supply voltage of +12 V to port C. Selecting resistor R2, we set the current through transistor VT1 to 45 ... 50 mA. After 10 minutes from the moment the voltage was applied, we re-monitor this parameter and, if necessary, correct it. Turn off the power supply. We remove the jumpers from the inductances and solder C2. Re-serving food. We are convinced that there is no self-excitation of the amplifier due to the absence of an increase in the consumed current. Self-excitation should not occur both in the presence of load resistances at the input and output, and in their absence. Otherwise, you should slightly increase the number of turns of the inductor L2 and / or reduce the resistance of resistors R1 and R3. In practice, self-excitation most often occurs when the phasing of the transformer windings is incorrect. To port A we connect the GSS with Rout = 50 Ohm, and to port B - an RF voltmeter with Rin = 50 Ohm. We remove the frequency response of the amplifier. Then, on the contrary, we connect the GSS to port B, and an RF voltmeter to port A. The characteristics of the amplifiers in both directions and among themselves should be as identical as possible. After completing the setup, the shielded amplifier boards are soldered together. Their inputs and outputs are interconnected by segments of a coaxial cable according to Fig. 1 and external circuits (transceiver nodes). The frequency response of the author's version of the amplifiers is shown in the upper part of fig. 2.
Two curves are shown at the bottom. One of them Ku (rev.) shows the attenuation coefficient introduced by the amplifier in the absence of supply voltage (that is, it can also be used as an attenuator), the second Ku (razv.) - the decoupling coefficient between the ports depending on the frequency. In "real" transceivers, it is desirable to have a higher stage gain in the receive mode than in the transmit mode, since the signal is formed at higher levels during transmission. The scheme of this reversing stage allows you to implement the necessary gains for different directions of signal passage by selecting only the number of turns of the OOS (III) winding of the corresponding transformer. Literature
Author: V.Artemenko
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