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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Regenerator on NE and 160 m. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / radio reception A regenerator is a special kind of radio receiver, and the simplest one at that. It was invented by an American radio amateur, and later by the famous radio specialist Edwin Armstrong WA2XMN, while still a student, in 1914. You can read about him in the CQ-QRP themed issue #21 (Winter 2008). Until the mid-1930s, the regenerator remained the most common type of radio receiver, but is now thoroughly forgotten. In those years, it contained one or two lamps, the first worked as a detector, the second as an audio frequency amplifier. The unique radio receiving qualities of the regenerator, high sensitivity and selectivity, are explained by positive feedback, which compensates for losses in the input circuit and antenna circuit, i.e., as it were, regenerates the received signal, where the name comes from. Regeneration in the antenna circuit has been shown by recent studies to be particularly useful as it not only amplifies the signal but also causes the antenna to extract more power from the incoming field (The Secret of the Simple Regenerators of the 20s, CQ-QRP #11, April 2006). Of course, the regenerator has its drawbacks. Its selectivity curve corresponds to the frequency response of a single resonant circuit, albeit with a very high quality factor. Consequently, the selectivity at significant detunings is insufficient, and cannot be compared with the frequency response of multicavity quartz or electromechanical filters. Strong out-of-band signals can be detected, or peak off a lamp or regenerator transistor, causing cross modulation. This is the price of simplicity. How to make a bad CB receiver good. Now we have cheap field-effect transistors at our disposal, which allow us to assemble a very simple, and very economical regenerator in the form of a prefix to any broadcast receiver you have with a medium wave range and significantly improve its parameters, sensitivity and noise immunity. The receiver itself will not require absolutely any alteration, you don’t even need to open the case! The prefix has its own magnetic antenna, which is placed at a distance of 10 ... 20 cm and parallel to the magnetic antenna of the receiver. Communication between antennas is quite enough. A weak signal, received and amplified by the set-top box, enters the receiver, and, as usual, is amplified, detected and reproduced in it. Since the functions of the set-top box are reduced only to compensating for losses in the magnetic antenna, and increasing its quality factor (and therefore efficiency), the set-top box is often called a Q-multiplier. The attachment diagram is shown in fig. 1.
The coil of the magnetic antenna L1 and the variable capacitor C1 form an oscillatory circuit that covers, with some margin, all frequencies of the MW range (525 .... 1605 kHz). The signal of the desired radio station, received by the antenna and isolated by this circuit, enters the gate of the transistor and modulates the current passing from the power battery through the transistor channel (drain-source gap). This current also passes through the feedback coil L2, replenishing the losses in the circuit. A variable resistor R1 serves to adjust the feedback, a decrease in its resistance increases the feedback, and with it the sensitivity, up to the occurrence of self-excitation - the generation of natural oscillations in the circuit, which is easy to detect by the whistle that changes during tuning - the beats of natural oscillations with the carrier oscillations of the adopted signal. For a magnetic antenna, it is desirable to choose a large size 400HN or 600HN ferrite rod. Of the common ones, 400NN with a diameter of 10 and a length of 200 mm is well suited (from the Leningrad receiver, for example). In the middle of the rod, it is necessary to wind a paper tube, and on it - a coil L1 of 60 turns of PELSHO wire with a diameter of 0,2 ... 0,3 mm. Then, without breaking the wire, make a tap, and wind another 5 turns in the same direction - the L2 coil. After manufacturing, to protect against moisture, it is desirable to impregnate the coils with paraffin. A ready-made coil of a MW range magnetic antenna from the same or similar receiver is also quite suitable. On it, as a rule, there is also a communication coil, which will serve as L2. KPI can also be taken from any old transistor receiver by connecting two of its sections in parallel if the capacitance of one is not enough to tune to the lowest frequencies of the MW range. For the feedback regulator, any type of variable resistor with a rating of 33 to 68 kΩ is suitable, preferably with a power switch S1. The capacitance of the blocking ceramic capacitor C2 is completely uncritical and can be from several thousand picofarads to fractions of a microfarad. Any battery is suitable for power, for example, two finger cells (3 V), an old cell phone battery (3,6 V) or a flat battery from a used Polaroid camera cassette (6 V). This battery with a low current consumption (and for the set-top box it is much less than 1 mA) works for years. The prefix together with the battery is assembled in any suitable plastic box, the installation method is not important. And now we make a complete receiver. Using the set-top box requires skill and a certain art - in addition to tuning the receiver itself, you also need to tune the regenerator to the same frequency (by increasing the receive volume) and adjust the feedback, achieving quality and purity of reception. It is useful to choose the relative position of the set-top box and receiver. There is a huge field for experiments!
It turned out to be very simple to enter the range of 160 m: without changing the coils of the magnetic antenna, in series with the main KPI C1, turn on the tensile C1a, which has a much lower capacity. If with the main KPI the receiver covered the MW range of 540 ... 1600 kHz, then with a decrease in the loop capacitance, the tuning range moves higher, by 1800 ... 2000 kHz. The tuning is still carried out by the main KPI C1, but it becomes much smoother due to less frequency overlap. For reception of telegraph (CW) and single-sideband (SSB) amateur stations, the feedback should be set slightly above the generation threshold. The signal to the detector is taken from the source of the Q-multiplier transistor VT1 and fed to the base of the composite transistor VT2, VT3. This is the so-called emitter detector, the load of which R4 and the high-frequency filtering capacitor C4 are connected to the emitter circuit of a transistor operating at a very low current (at the lower bend of the characteristic). The emitter load provides deep negative feedback (NFB) for direct current and audio frequencies, which results in high-quality detection of weak signals. The composite transistor is used in order to load the Q-multiplier less and not disrupt its operation. For the same purpose, a resistor R3 is added, it is selected, achieving a smooth approach to the generation threshold. There is no other RF gain, except for regenerative, in the receiver! Amateurs who are in a place unfavorable for reception, if desired, can add an RF cascade in front of the detector. After additional filtering by the R6C5C6 circuit, the audio frequency signal is fed to a two-stage ultrasonic frequency converter. It is assembled on transistors VT4, VT5 according to a scheme with a direct connection between the cascades. Its amplification is quite large and can reach several thousand. The transistor mode is stabilized by the OOS circuit through the resistor R7, which creates a bias based on VT4. When receiving strong stations, it may be necessary to reduce the gain (volume). This is achieved by moving the slider of the resistor R9 down, closer to the output connected to the common wire. At the same time, OOS also increases at audio frequencies, which reduces gain, but improves playback quality. The UZCH load is high-impedance phones (headphones). The resistance of phones is indicated on their case, it is suitable from 1600 to 2200 ohms. The total resistance of the two phones will be, respectively, 3,2 ... 4,4 kOhm. It is advisable to observe the polarity indicated on the plug, then the constant collector current of the transistor VT5 will enhance the action of the permanent magnets of the phones. If the polarity is not indicated, select it experimentally, rearranging the plug and focusing on the volume and sound quality. Now low-impedance phones (from players, etc.) are more common. They can also be connected, but through a step-down transformer with a winding ratio of 10:1 to 30:1. Suitable transformers from old transistor receivers, TVK and TVZ from old tube TVs, small network transformers from power supplies plugged into a socket, and, finally, transformers from broadcast speakers. Such a loudspeaker can also be directly connected to the receiver - the volume will be small, but quite sufficient for comfortable listening to programs. Like any analog piece of equipment, this radio will work well if you take the time to tune it carefully. All you need is a simple multimeter (tester), pointer or digital. First of all, check the UCH mode by measuring the voltage UR9 across resistor R9. It should be within 0,7 ... 1 V. When working with telephones, also check the voltage on the VT5 collector (3 ... 4 V). Its optimal value is equal to (Upit + UR9)/2, while the signal limitation during overloads will be symmetrical, and the amplitude of the undistorted signal will be maximum. All voltage values are given for a 6-volt supply. For other voltages, all values must be proportionally changed. The emitter detector does not require tuning, and it is also useful to check the mode of the Q-multiplier. The voltage at the source VT1 must be 2 ... 3 V, and at the drain - at least 5 V. The mode can be selected by resistor R3. The tuning range of the magnetic antenna circuit is estimated by listening to radio stations with known frequencies. So, for example, tuning to Mayak (549 kHz) should be at the beginning of the range, with almost maximum KPI capacity, and on Radio Russia (873 kHz) - in the middle of the range. If necessary, change the number of turns of the coil L1. Having set the boundaries of the CB range, the capacitor C1a achieves the reception of amateur stations. It is better to do this in the evening, when there is a passage in the 160 m range, and many stations are working. The finest part of tuning is the selection of feedback loop parameters so that the generation approach is soft and smooth. The generation should disappear at the same position of the OS adjustment knob, at which the generation occurred. Useful selection of resistors R1 and R2, as well as the number of turns and position on the antenna rod coil L2. After properly adjusting to the described receiver, in the evening it was possible to listen to the work of radio stations in most European capitals, as well as a number of Arab and Central Asian stations on the NE. At 160 m, many stations of the European part of Russia, Western Siberia, Ukraine and the Baltic states were received, moreover, only on the magnetic antenna of the receiver itself, without any external antennas. The tests were carried out in a suburb of Moscow, in a wooden house. In difficult conditions (reinforced concrete house, lower floors), I recommend placing the receiver's magnetic antenna near the window. Do not try to surround it with other details, this reduces the quality factor. It is better if there is 10 ... 20 cm of free space around the antenna. Author: V.Polyakov, RA3AAE
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