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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Design features of VHF equipment. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Nodes of amateur radio equipment Ultrashortwave equipment, in comparison with equipment designed to operate at longer wavelengths, has its own characteristics that the designer must take into account. These features are determined by the fact that at high and especially ultrahigh frequencies, energy losses in lamps, oscillatory circuits and various types of dielectrics increase sharply. Ordinary lamps that work well at low and not particularly high frequencies (up to 30 MHz), work poorly at high frequencies or even do not work at all. Such dielectrics as paraffin, textolite, carbolite, getinaks, cardboard, rubber cause such large losses in the circuits that their use in ultrashort-wave equipment should be considered completely unacceptable. For this and a number of other reasons (which are discussed below), a beginner ultrashortwave should never test one or another design, resorting to the so-called "flying" montages, which the amateur often calls trial. As a rule, almost any ultra-short-wave equipment assembled according to a very good scheme, but hastily, sloppily, with a disorderly arrangement of parts, with long and tangled mounting wires in poor-quality insulation, using low-quality dielectrics, always gives unsatisfactory results or does not work at all. That is why, before proceeding with the manufacture of the intended design, it is recommended that you familiarize yourself with the following notes and tips, which can be very useful for the amateur. In the oscillatory circuits of ultrashort-wave equipment, one has to deal with coils of very low inductance and capacitors of insignificant capacitance. The higher the frequency for which the receivers or transmitters are calculated, the smaller: the working inductances and capacitances. So. at frequencies of 40, 144, and even more so at 420 MHz, these values turn out to be comparable with the interelectrode capacitances of the lamps, the inductance of the lead wires, the parasitic capacitances of the installation, and the inductance of the connecting wires. Therefore, it is always necessary to strive to ensure that the mounting capacitance of high-frequency circuits is minimal, and the connecting wires are straight and as short as possible. At the frequencies indicated above, a conductor 5-10 cm long has an inductance of the same order as the inductance of the loop coil. And if this conductor is bent, i.e., has the shape of a half-turn, then its inductance will be even greater. Failure to comply with the rules of ultrashort-wave installation leads. firstly, to a sharp change in the frequency of natural oscillations, its deviation from the calculated one, and secondly, to a deterioration in the quality factor, the circuit, and an increase in attenuation in it. From this point of view, the rational arrangement of lamps and high-frequency parts on the chassis is of decisive importance for the good performance of ultrashort-wave equipment. When choosing a place to place parts and lamps and their relative position, you must be guided by the following rules: a) Loop coils should be placed near the lamps to which they belong. b) The lamps of the stages of amplifying high-frequency oscillations, the local oscillator and the mixer should be located near the block of variable capacitors. c) Place the lamps of the stages of amplifying the oscillations of the intermediate frequency next to the intermediate frequency transformers. The designer of ultrashortwave equipment should also keep in mind. that as the operating frequency increases, the gain of conventional, non-special lamps quickly drops, approaching unity already at frequencies of the order of 80 MHz. In this case, improving the quality of oscillatory circuits, the use of silver and high-quality ceramics do not give any positive result. For this reason, the designer should always strive to use special, baseless lamps that have small interelectrode capacitances and are designed for operation in the VHF range. These lamps include all lamps of the "acorn" type, lamps 6N15P, 6S1P, 6S2P, 6NZP, 6Zh1P, 6ZhZP, 6Zh4P, GU-32. GU-29 and others. But even special lamps have a reduced input impedance at ultra-high frequencies. The main reason for the decrease in the input resistance of the lamp, depending on the increase in the operating frequency, is the inertia of the electrons. The inertia of the electron flow causes the grid current to appear. which means the appearance of the active component of the input conductivity. (At the same time, the grid current increases the noise floor.) The inductance of the lamp leads also reduces the input impedance of the lamp. As a result of the fact that the inductance of the coil at high frequencies is small, and the losses in the lamp are large, the resonant resistance of the circuit is small (1500 ohms or less). Given this, for VHF generators it is necessary to use circuits with a high quality factor. To reduce losses in the circuit, the use of a large number of dielectrics should always be avoided. Dielectrics should be used only of high quality, designed for operation at high frequencies. Getinaks, carbolite, textolite at frequencies above 30 MHz should not be used due to excessive losses in them. The best coil for oscillator circuits is a coil, which is a frame made of high-frequency ceramics, along the helical groove of which a layer of silver is deposited. Such a coil has low losses, is durable and provides an almost constant value of inductance over a wide temperature range. The use of such coils in self-excited transmitters guarantees sufficient frequency stability. The insignificant frequency drift during heating, caused by a change in the geometric dimensions of the connecting conductors, can be easily compensated by using capacitors with a negative temperature coefficient in the circuits. In amateur conditions, such coils are practically impossible to manufacture. However, a coil with increased stability, which is necessary primarily for the master oscillator, can be wound from copper (preferably silver-plated) wire, preheated to a temperature of 100-120 ° C, laying it with some tension in the grooves of the ceramic frame. It is clear that simpler, frameless coils can be used in doublers and output stages in which no frequency generation occurs. However, in all cases, it is necessary to strive to ensure that the contours are mechanically strong. Very often, radio amateurs, wanting to increase the quality factor of the circuit, make coils of an unnecessarily large diameter. In generators, this leads to large radiation losses. Coils with a diameter of 15-20 mm should be recommended, in the output stage - 30-35 mm. Coils should be placed away from metal masses to avoid eddy current losses. The minimum distance of the coil from large metal surfaces must be at least the diameter of the coil. At frequencies of 400-450 MHz and above, it is convenient to use oscillatory circuits made in the form of quarter-wave short-circuited lines. If the quality factor of ordinary circuits is several tens of units, then the quality factor of the circuit-line can be increased to several thousand. In the transmission structures described in this collection, designed to operate in the range of 420-425 MHz, instead of conventional coils, lines consisting of silver-plated copper tubes are used. The designer should pay special attention to the quality of variable capacitors, to the reliability of the rubbing contact in it. Whenever possible, the condenser rotor should be "earthed", i.e. connected to the chassis. This will prevent the operator's hand from affecting the circuit setting. In transmitters, it is best to build an exciter according to a circuit with electronic communication. This facilitates the fastening of the capacitor and eliminates the influence of hands on the frequency of the generated oscillations. Typically, the anode circuit of such an exciter is tuned to the second harmonic, and thus, using one lamp, the frequency is doubled. Lowering the frequency of the master oscillator increases its stability. The advantage of this scheme is that the generator with two lamps will have parameters no worse than a three-lamp generator. When building a transmitter, the designer must take into account that each oscillatory circuit in a multi-stage transmitter must have a tuning element (variable capacitor knob). The constant tuning of the anode circuits of the doubler and the output stage to the middle frequency of the range leads to a significant decrease in the vibrational power transferred to the antenna when the transmitter is tuned to frequencies other than the middle one. When setting up generators, the lamps of subsequent stages should never be removed; lamps should be left in sockets, and in order for them not to fail, it is necessary to remove the anode voltage from them. If the designer, when adjusting the operation of the master oscillator and setting the desired range of generated frequencies, removes the doubler lamp, and then, after completing the doubler tuning, puts it back in its place, then due to the capacitive coupling between these stages, the master oscillator will be so detuned that in the doubler circuit they cannot be fluctuations are found. For the same reason, you can't. for example, to select one or another harmonic in the anode circuit of the doubler when the coupling capacitor is turned off. When designing VHF receivers, all efforts of the designer should be directed to obtaining the highest sensitivity, which is possible only if high-frequency amplifiers with a minimum level of intrinsic noise are used. It is best to use triodes for this purpose, which are switched on but in the "grounded cathode - grounded grids" scheme. As already mentioned, at ultrashort waves, the input and output resistances of the lamps are greatly reduced. Therefore, the losses of vibrational energy in the lamp itself greatly exceed the losses in the circuit; in addition, the lamp shunts the circuit sharply, reducing its quality factor. In order to weaken the shunting effect of the lamp, not the entire circuit, but only part of it, should be connected to the lamp grid. For the same purposes, the connection of the amplifier circuit with the grid of the subsequent lamp must be made autotransformer. This reduces the attenuation introduced by the lamp into the circuit and allows you to get the highest stage gain. High-capacity capacitors cannot be used in decoupling circuits and cathode circuits of VHF receivers, since they have a noticeable inductance, the value of which at high frequencies can no longer be neglected, If, nevertheless, high-capacity capacitors are used in the circuit, for example, electrolytic ones, which, as is known, have a noticeable - inductance, then in this case it is necessary to connect a small-capacity mica capacitor with low inductance in parallel with such a capacitor. Thus, both ultra-high and lower frequencies will be filtered at the same time. It is clear that long connecting wires and a common ground wire in high-frequency paths create noticeable parasitic inductances and capacitances. Therefore, it is necessary to use straight and short connecting conductors and without any insulation, since the dielectric will cause additional energy losses. Each point of the circuit must be grounded with a separate wire, and all ground conductors related to the same lamp and cascade must be connected to the chassis at one point. Structurally, an amateur station can be designed in different ways. Undoubted advantages have a block design, in which the modulator and generator are made in the form of independent blocks enclosed in a common transmitter frame. Block design facilitates adjustment, repair and replacement in case of failure. The receiver, for many reasons, must be made separately, without rigidly connecting it to the transmitter. This expands the possibilities for experimentation in cases where the receiver must be removed from the transmitter. The rectifier is recommended to be made as an independent unit connected to the transmitter by a power hose. It is useful to duplicate the output of the rectifier, made in the form of a chip, with a socket with clamps. The use of duplicate clamps is very convenient when connecting to the rectifier any other designs that require power and have chips or connectors of a different type than those used to connect the rectifier to this transmitter. This brief introduction does not address other issues of interest to the ultrashortwave radio amateur. However, he will find answers to many of them directly in the descriptions of individual structures. Literature:
Publication: N. Bolshakov, rf.atnn.ru
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