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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING About the Radio-76 transceiver. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Civil radio communications More than five years have passed since the day when the laboratory of the journal "Radio" completed the development of a single-band shortwave transceiver, called "Radio-76". During this time, it was repeated by many shortwave and ultrashortwave radios, the design of the transceiver formed the basis of the "Electronics - Kontur-80" set, the serial production of which was started at one of the enterprises in Ulyanovsk. It can be expected that the serial production of these sets will cause a second wave of mass production of Radio-76 transceivers, in particular by novice radio amateurs (for operation on the 160-meter band). That is why it seems relevant to talk about some improvements. which it is advisable to add to the main board and the local oscillator board of the Radio-76 transceiver in order to improve its main technical characteristics. Improvements. which are described in this article, a transceiver that was already in operation, made from the "Electronics - Circuit-80" kit, was subjected to. Most of the additional parts were installed on the side of the printed conductors of the finished boards. In the local oscillator board, it was also necessary to remove (completely or partially) some printed conductors and lay new ones - hinged ones. As noted by radio amateurs who repeated the Radio-76 transceiver, most often difficulties arise when establishing a smooth range generator. In some instances of the transceiver, when switching from reception to transmission, a jump in frequency is observed, reaching 200 ... 300 Hz. This defect, which is often encountered in equipment with heterodynes that are more complex than that of Radio-76, can either be due to a change in the supply voltage of the local oscillator. or by changing its load at high frequency. In the radio-76 transceiver. having a very simple smooth range generator (GVD), as a rule, both of these reasons "work", which causes certain difficulties in eliminating the frequency shift when switching from reception to transmission. There are two options for modifying the GPA board of the transceiver. One of them is simple, with minimal PCB rework, and the other is more complex, but gives better results. We note right away that in order to completely eliminate the frequency shift, the selection of one of the resistors on the main board of the transceiver is also required. A simple alteration of the GPA essentially boils down to the fact that the emitter follower of the GPA and the quartz oscillator at a frequency of 500 kHz are read directly from the +12 V power source, and from the parametric stabilizer on diode D2 (see Fig. 2 in the description of the transceiver [1] ) feed only the GPA generator itself on transistor T1. Upper. according to the diagram, the terminals of the resistors R6 and R10, as well as the collector terminal of the transistor T2, are connected directly to the + 12 V power bus, i.e., to terminal 8 of the local oscillator board. Resistor R8 should be replaced with a new one, with a resistance of 100 ... ... 120 Ohm; resistor R9 - to a new one, with a resistance of 150 ... 200 Ohms, and select resistor R7 such that the voltage at the emitter terminal of transistor T2 is +3 ... 4 V. This transistor must have a high (preferably not lower than 150) static transfer coefficient current h21e, at a collector current of 10 ... 15 mA. Significant power is dissipated on the T2 transistor, so it is better if it has a metal case (like transistors of the KT301. KT312. KT316 series, etc.), to which a simple heat sink should be attached or soldered in the form of brass, copper or, in extreme case, tin plate. After such an alteration, the generator board is installed on the transceiver and the GPA generator is temporarily powered from a separate +12 V source (best of all, from three 3336L batteries). This source is connected to the right, according to the diagram, output of the resistor R8, having previously disconnected it from the output D of the board. Powering the GPA generator from a separate source allows you to avoid the influence of the other stages of the transceiver on the generator through the power circuits and makes it possible to consistently identify and eliminate the causes that cause a frequency shift during the transition from reception to transmission. Translating the transceiver from the receive mode to the transmit mode and vice versa, the GPA frequency shift is controlled by a digital frequency meter or an auxiliary receiver. If it exceeds 100 Hz. then you should equalize the load of the GPA in various operating modes. The fact is that. although the ring mixers on the main board are very similar to each other, their input impedance can differ significantly (by a factor of 2...3). This is due to the presence in one of them (left, according to the diagram, in Fig. 1 in the description of the transceiver) a tuning resistor R2, which balances this mixer. The input impedances of the mixers are equalized by selecting the resistor R13 (usually within 100 ... 150 Ohms) according to the minimum frequency shift. After that, the GPA generator is powered from a common power source. If at the same time the frequency shift changes due to the effect on the GPA in the power circuits, it is eliminated by known methods. By selecting a resistor R13, the frequency shift can be reduced to almost zero. but at the same time, the reason that gives rise to it is the insufficient decoupling of the GPA from the mixers. naturally not removed. That is why, with a large initial frequency shift, it is advisable to carry out a more complex modification of the local oscillator, but before proceeding to the story about it, a few words about the main board of the transceiver. It is advisable to install two additional high-frequency chokes on this board. One of them is connected between the connection point of diodes D1, D2 and capacitor C2 and a common wire, and the other between the connection point of diodes D9, C10 and capacitor C19 and a common wire. These chokes must have exactly the same inductance as Dr1 and Dr2. The introduction of a choke in the first mixer improves the suppression of the carrier frequency when working in transmission (balancing the mixer with the trimmer resistor R2 becomes very clear). The inductor in the second mixer improves its frequency response when detecting a signal. In addition, the resistor R14 should be taken with a lower value (360 ... 500 Ohms), and even better, instead of this resistor, install a coil with an inductance of 40 ... 50 mH. It can be performed, for example, on a ring of size K20X12X6 made of ferrite 3000NM-1, wound with PELSHO wire 0.1 162 turns. If the radio amateur has other rings at his disposal, then the required number of turns I is calculated by the formula
where L is the inductance in mH; D, d and h - respectively, the outer and inner diameters of the ring and its height in cm; m is the magnetic permeability of the ring material. The diameter and brand of the wire are not critical - as long as the winding fits on the selected ring. Together with capacitors C12 and C22, this coil forms a low-pass filter with a cutoff frequency of about 3 kHz. The introduction of such a filter significantly improves the signal-to-noise ratio. By the way, if a radio amateur has such an opportunity, then in order to improve the signal-to-noise ratio, it is advisable to select an MC2 chip with minimal noise, since sometimes very "noisy" specimens come across. You can significantly improve the operation of the GPA if you assemble it according to the scheme shown in the figure. Despite the noticeable difference in the schemes with the original version of the GPA and the presence of additional details, the new GPA, as already noted. easily placed on the local oscillator board. The ratings of the frequency-setting elements shown in the diagram correspond to the version of the Radio-76 transceiver for the range of 160 m with an overlap of the 1840 ... 1960 kHz section.
Let us note some schematic features of this GPA. The influence of the load - ring diode mixers of the transceiver - on the generator frequency and the amplitude of the output signal is minimized here by an emitter follower on a V5V6 composite transistor. The capacitive divider C6C7 provides additional decoupling between the oscillator itself on transistor V2 and the output of the GPA. To improve the shape of the generated oscillations and increase the frequency stability in the generator, the supply voltage is reduced, the positive feedback through the C4C5 capacitive divider is optimized (weakened), and two varicaps V3, V4 are introduced, connected in anti-series. In addition, only the generator is now powered from the parametric stabilizer on the zener diode V1. And finally, the L2C10 filter is introduced at the GPA output, which not only matches the GPA with the load, but also effectively filters out harmonics in the GPA output signal. thereby attenuating possible spurious channels during reception and spurious emissions during transmission. Transistors V2, V5 and V6 can be any silicon high-frequency npn structures (KT315. KT312. KT316, etc.). The static current transfer coefficient for transistors V2 and V5 must be at least 80 (at a collector current of 1 mA), and for transistor V6 - at least 30 (at a collector current of 20 mA). Since a current of 6 ... 15 mA flows through the transistor V20, it is advisable to equip it with a simple radiator. If the radio amateur does not have KV104 varicaps (or others with a capacitance of at least 100 pF at a mixing voltage of 4 V), then a variable capacitor will have to be introduced to tune the transceiver, since with the more common D901, KB 102 varicaps, etc., get the required overlap in frequency in the range of 160 m is not possible. Coil L1 has an inductance of 12 μH. It can be performed, for example, in the magnetic wire SB-12a (25 turns with wire PEV-2 0,15). The calculated value of the inductance of the coil L2 is 8,2 μH. but it is not critical (the author successfully used a standard D-2 choke with an inductance of 0,1 μH as L10). For a transceiver for the 8U m range, the GPA circuit remains the same. Coil L1 should have an inductance of approximately 3 μH (12 turns with PEV-2 0.15 wire in the SB-12a magnetic circuit), coil L3 - about 4 μH (a standard D-0.1 choke with an inductance of 5 μH will do). Capacitor C10 should have a capacitance of 240 pF. The establishment of the GPA begins with checking the modes of the transistors for direct current, having previously disrupted the oscillations of the generator (for example, by short-circuiting the coil L1). The voltage at the emitter terminal of transistor V2 should be approximately +1 V, and at the emitter terminal of transistor V6 - +4 ... 5 V. These modes, with serviceable parts and installation, are set automatically and may differ by 20% from those given above due to the spread resistor values and stabilization voltage of zener diodes. Then the jumper is removed from the L1 coil, an MLT-0,47 resistor with a resistance of about 0.1 Ohms (non-critical) is connected to the output of the GPA through a capacitor with a capacity of 0,25 ... 500]). If the generator is not excited (the RF voltmeter does not register the voltage at the output of the GPA), then capacitor C2 should be installed with a slightly lower capacitance (but the maximum possible for stable operation of the GPA in the entire frequency range). Having achieved stable generation, a control voltage of +5 V is applied to the varicaps and, by adjusting the nickname of the LI coil, the generation frequency is set slightly below 3,2 kHz (by 2350 ... 5 kHz). Then a control voltage close to zero is applied. The operating frequency should be slightly higher than 10 kHz. If the overlap is less than 2450 ... 110 kHz, then you can install a capacitor C120 of a smaller capacity or slightly raise the upper limit of the control voltage on the varicaps (up to + 4 ... 2,5 V). However, the latter should be done with caution: at these voltages, varicaps can be opened by the RF voltage on the GPA circuit, and frequency stability in the low-frequency range may deteriorate. At the last stage of establishing the GPA, a capacitor C4 is selected with such a capacity at which the RF voltage at the GPA output was 6 ... 0,7 V (effective value). Since the capacitance of this capacitor, albeit weakly, but still affects the frequency of the generated oscillations, after setting the output voltage, you should check the GPA frequency overlap and, if necessary, adjust the L0,9 coil. At the GPA made by the author according to the scheme of fig. 2, the initial frequency overshoot (no special measures for thermal compensation were applied) was approximately 1,5 kHz and occurred within 20 minutes after switching on. Subsequently, the GPA frequency changed from the nominal value by ±100 Hz. The frequency shift during the transition from reception to transmission was approximately 10 ... 20 Hz. The modifications of the local oscillator board described in this article are alternative measures, due to the desire to use the board already available to the radio amateur. A more radical measure is the manufacture of the GPA according to some more complex scheme that provides higher parameters (for example, according to the GPA scheme of the Radio-77 transceiver [3]). Literature
Author: B. Stepanov (UW3AX), Moscow; Publication: N. Bolshakov, rf.atnn.ru
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