|
Arabic
Bengali
Chinese
English
French
German
Hebrew
Hindi
Italian
Japanese
Korean
Malay
Polish
Portuguese
Spanish
Turkish
Ukrainian
Vietnamese|
ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Surprise feedback. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Audio equipment The speaker comes to the microphone and begins to speak. But instead of words, a loud ringing and whistling sound is heard in the hall. Why do such "surprises" happen? The reason is clear. The sound vibrations reproduced by the loudspeakers, propagating through the hall, return to the microphone. Once again converted into electrical signals and amplified many times, they "rock" the amplifier more and more, which quickly enters the self-excitation mode. The whistle is growing. To restore the normal operation of the equipment, it is necessary to reduce the amplification level of signals from the microphone or change the orientation of the latter relative to the sound emitters. This will weaken the effect of the so-called reverse (from the "output" to the "input") connection. Showing herself sometimes at the most unfavorable moment for others, she persistently tries to dictate her harsh conditions. And it would be okay in acoustics, low-frequency circuits ... The influence of feedback (FB) is fatal, one might say, radio-frequency devices are subject to. For example, radio receivers - with a close and parallel location of the inductive load with respect to the magnetic antenna. However, one should not think that feedback can only bring evil, which must be fought without fail. It also happens vice versa. Proper use of the "secrets" of the OS in some cases can improve the quality of the equipment. So, in a low (sound) frequency amplifier, the circuit diagram of which is shown in fig. 1, from the transformer T2, the feedback chain R6C4 is "forwarded" (according to the prevailing terminology, this is a negative OS) to the emitter of the transistor VT1. By limiting excessive amplification, such a technical solution can significantly improve the sound quality. Collect yourself such a practical (and not complicated) design - you will not regret it!
In homemade and industrial radios of the 30s and 40s, an adjustable positive feedback was widely used. And - in the radio cascades. The name regenerators was assigned to such receivers. With a minimum of radio tubes and simplicity of design, they made it possible to obtain a "range" of reception no less (and in some cases even greater) than multi-tube devices without feedback. The regenerators revealed their capabilities to the maximum only for those who are not alien to sports interest - to get high results by affordable means and painstakingly "fishing out" distant radio stations in the waves of the ether. We hope that inquisitive, hardworking, persistent people have not died out in our time. We give everything you need, including a circuit diagram (Fig. 2) and other data for the manufacture of a fairly simple (but not a tube, but a transistor) regenerator operating on short waves.
Reception is carried out on an external antenna WA1, from where the signals enter the tuned resonant circuit L1C1C2. After the broadband amplification stage on the transistor VT1, the signal is subjected to additional selection in the second circuit L4C8...C10. The latter is inductively connected to a sensitive triode detector assembled on a transistor VT2, in the collector circuit of which a feedback coil L5 is included. The magnetic flux at L5 coincides in direction with the flux of the contour coil L4. As a result, the feedback here is adjustable. It is the stronger, the greater the current flowing through the coil L5, which can be easily changed by applying one or another "bias" to the base VT2 resistor R7. The audio component of the detected signal is fed to a low-frequency amplifier based on transistors VT3...VT5. The load of the amplifier is the headphone capsule BF1. Here you can see another example of the beneficial effect of negative feedback - for direct current (between galvanically connected cascades). The OS stabilizes their modes of operation, which is easy to verify, say, when trying to "unauthorized" increase the current through the transistor VT5. Such a "surprise", of course, will cause an increase in the voltage drop across the resistor R11. Then a corresponding change will appear on the basis of the first ULF cascade due to the "bias" resistance R9. Additionally, opening slightly, the composite transistor VT3-VT4 will slightly reduce the voltage at the base of VT5 and, consequently, the amount of current passing through it. The result of this will be the restoration of the original mode of operation of the regenerator. The design of the regenerator proposed for self-production is designed to receive radio transmissions in the range from 20 to 50 m. But if desired, it can easily be adapted to work both on longer and shorter wavelengths. This is one of the advantages of the receiver of direct (at the frequency of the received signal) amplification - after all, the coils of both circuits (as well as themselves as a whole) are exactly the same. It is enough to unwind or add an equal number of turns of wire to them in order to immediately find yourself in new frequency limits. One of the advantages of our regenerator is that its circuit also provides for a positive feedback between the detector output and the second circuit, the mechanism of action of which affects the operation of the entire structure in the most favorable way. As you know, when using any real oscillatory circuit, losses are inevitable. They depend on many factors. In particular, from the electrical resistance of the coil, the scattering of the magnetic flux in the frame material, etc. Deteriorating the resonant properties of the circuit, these losses lead to signal attenuation. The introduction of positive feedback (not passing a certain threshold "called critical) allows you to compensate for the lion's share of losses and thereby multiply the efficiency of the circuit. As a result, it becomes possible to select the signal you need among the many received transmissions (often super-weak due to the great remoteness of the receiving place) The art of controlling the regenerator is precisely to maintain feedback at the "critical threshold" all the time, after which the amplifier self-excites, leading to the whistle-ringing, which was mentioned at the beginning of the material. From the analysis of the circuit diagram of the receiver, it can be seen that its tuning is carried out using a two-section block of capacitors of variable capacitance C1C8. And this is quite understandable: there are two interconnected circuits. But the purpose of another "variable" C9 is not immediately caught. But in essence, this is a tuned capacitor, similar to the other two - C2 and C10. Only the control of the C9 is displayed on the front panel of the receiver. In lamp designs, such a capacitor was called a "corrector". In our case, it performs the same function - it allows you to get an accurate pairing of both circuits anywhere in the range, which, in turn, can significantly increase the level of the selected signal. Now for the details. Many of their types will do, as long as the transistors VT1 and VT2 are sufficiently high-frequency. But in order for all this element base to be conveniently located on the circuit board (it will be discussed later), it is advisable to stop the choice on the following details. It is better to take fixed resistors of the MLT-0,25 type (except for R33, for which BC-0,25 is suitable). And as a potentiometer - SP-0,4. Now capacitors. For the KPI block, it is desirable to take KP4-5, the C9 will serve as a corrector KPVM. The rest of the "riggers" are the KPKM. Capacitors C3, C5 - type KT-1, other constants - KLS and K50-6. Loop inductors are self-made, placed on frames with a diameter of 6 mm with trimming cores made of 100NN ferrite. Moreover, the windings L1 and L4 each have twenty-one, and L2 and L6 each have three turns of wire. The tap for connecting the antenna at L1 is made from the 16th turn, counting from the grounded end. Coil L5 contains (experimentally specified) from three to six turns. It is located (in relation to 14) on the side opposite to the placement of L6. For winding, wire PEV-2 0,23 is used. Inductor L3 is wound over resistor R3, has 70 turns of wire PV-2 0,1. Headphone capsule - high-resistance (type TON-2M). The regenerator can be powered by two batteries 336 connected in series. They connect with a toggle switch. And for a vernier - a tuning retarder - it is better to take a ready-made disk (from KPI portable receivers) with a tension spiral spring and a cable to it. As a drive axle carrying a tuning knob, use a substandard variable resistor of types SP-0,4, SPO-0,5 and the like. Moreover, the case of such a resistor must be sawn across, leaving the front wall intact along with the attachment point, in which the "native" axis will rotate without restriction. The details of the receiver are mainly assembled on a circuit board made of foil-coated getinax (textolite). The configuration of printed conductors, as well as the location (on the reverse side) of the parts are shown in fig. 3. To reduce the possibility of parasitic feedback between the loop coils, one of them is located on the board "lying". In this case, the geometric axes of the inductances are mutually perpendicular. The frame of the coil L1 can be rotated at a certain angle relative to L3, L4.
The receiver is designed as a desktop device-type structure (Fig. 4). 8 mm plywood is suitable for the walls of the case. The front panel and the removable rear wall are expediently made of plastic sheet with a thickness of about 3 mm. Moreover, holes are provided in advance: on the front panel - for the axes of the tuning vernier, the feedback controller and the scale; on the side walls - under the sockets of the antenna, telephone and power switch. A somewhat "recessed" underscale is attached from the inside to the front panel. An axis is passed through it, connected with the KPI rotor and carrying an arrow - a tuning indicator. The scale is calibrated independently, after which the windows are closed with a Plexiglas plate.
The mounting plate is vertical. Attached to the bars (corresponding to the position of the KPI), it connects the walls of the case and the front panel into a single structure. Behind it (closer to the removable rear wall) are batteries. For the receiver to work flawlessly, it needs to be adjusted. First of all, the operating modes of transistors for direct current are checked and, if necessary, adjusted to the optimum. This is done with the antenna turned off. By selecting the value of the resistor R1, the voltage on the VT1 collector (relative to the common wire) is set close to 3 V. At the same time, they ensure that the collector quiescent current of the transistor VT5 is 2 ... 3 mA. Feedback here should be minimal! The contours are matched with an external antenna connected. It is necessary to make sure that the feedback occurs (when turning the knob of the resistor R7) within the entire range. If, at some positions of R7, it is not possible to force the receiver to regenerate, the number of turns at the L5 coil should be increased. If, on the contrary, generation occurs on a section of the scale, regardless of the position of the regulator, the number of turns should be somewhat reduced. Finally, it happens that the generation does not appear at all. In this case, it is recommended to swap the leads of the coil L5. Pairing starts from the high-frequency end of the range, tuning into some broadcast radio station with a long wavelength of about 25 m. With capacitor C9 in the approximate middle position, by adjusting C10, the best pairing is obtained (by maximum signal with unchanged feedback). The same is done at the other end of the range with the L4 coil core. It is better not to touch the found positions of the tuned elements in the future, and during the adjustment within the scale, correct the pairing with the C9 corrector. It is better to engage in conjugation of contours in the early evening hours, when there are still quite a lot of radio stations on the "daytime" 25-meter sub-band, but transmissions are already appearing on typically "evening" sections - 41 and 49 m. At this time, the 31-meter broadcast sub-band is also well heard - here sometimes you can "catch" voices from the island of Ceylon and even from Australia. Of course, in many places on the scale there are scatterings of departmental transmitters. And not everyone conducts radio communications over the phone. The work of the telegraph can be heard by going slightly beyond the threshold of generation. In this case, instead of unintelligible clicks, a melodic "morse code" will sound. In urban areas, radio reception is usually carried out on an indoor antenna. In buildings made of concrete and steel, the efficiency of such antennas, as a rule, is low, which can be easily seen by switching to a "pin" or "whisk" attached to the window frame from the outside. Even better is radio reception on an "inclined beam" - a piece of insulated wire thrown to the top of the nearest tree. In all cases of using outdoor antennas, it is imperative to provide for the possibility of disconnecting them from the entrance to the room with simultaneous connection to metal objects buried in the ground. Such a measure will save you from trouble during a thunderstorm. It would not be superfluous to keep a log of radio observations, where to record the name (belonging) of the stations, the approximate frequency, date and time of reception, as well as its quality. It is likely that it will be possible to "catch" the stations that are hunted by "DX-ists" - lovers of receiving distant and rare transmitters. Author: Yu.Prokoptsev
Machine for thinning flowers in gardens
02.05.2024 Advanced Infrared Microscope
02.05.2024 Air trap for insects
01.05.2024
▪ Benefit from biodegradable plastic questioned
▪ section of the site Mobile communications. Article selection ▪ article What artist dies! Popular expression ▪ article Chocolate tree. Legends, cultivation, methods of application
Home page | Library | Articles | Website map | Site Reviews
www.diagram.com.ua |
See other articles Section 
Latest news of science and technology, new electronics:
All languages of this page