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The last of the Mohicans. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / radio reception It seemed that the time of regenerative receivers had sunk into oblivion, and sunk a very, very long time ago, somewhere in the late sixties. That is why it was completely unexpected for many that a few years ago a factory-made regenerative receiver appeared on the American market. It was, apparently, "the last of the Mohicans ...", which spurred interest in such devices for some time. For several decades after the war, direct-amplification regenerative receivers were the first design for many radio amateurs. Despite the well-known shortcomings (in particular, not very stable operation), the "regenerator" made it possible, with a minimum of parts, to create an apparatus on which it was possible to "hunt" for distant stations. The advent of direct conversion receivers in the late sixties, which allowed stable reception of CW (telegraph) and SSB (single sideband modulation) radio stations, put an end to the era of regenerators. The triumph of direct conversion was quick and seemingly final - amateur radio literature was literally filled with descriptions of the most diverse designs of receivers and transceivers. The reasons for this triumph are clear: simplicity of design (no more complicated than a "regenerator"), good repeatability (if you "do not plow", then it works from the first start), stable operation. In fairness, it is necessary to drop honey and a fly in the ointment into this barrel. Direct conversion receivers do not work well near powerful stations (the reason is the direct detection of broadcasting and television signals), there are problems with all sorts of interference (due to the very high sensitivity of the audio frequency amplifier). However, it would probably be unfair to demand some very high characteristics from the simplest. Another disadvantage of direct conversion receivers is the fundamental impossibility of stable reception of radio stations with amplitude modulation (AM). That is why they were primarily of interest to shortwavers, who today practically do not use AM. One can only assume that the revival of interest in "regenerators" was due to this reason. But be that as it may, the American company MFJ released a regenerative KB receiver a few years ago, as well as a kit for making it yourself. The use of a modern component base allowed MFJ to create a simple device with relatively stable characteristics. This receiver (model "MFJ-8100") allows you to receive AM, SSB and CW radio stations in the frequency band from 3,5 to 22 MHz. It is divided into five ranges: 3,5...4,3. 5,9...7,4, 9,5...12, 13,2...16,4 and 17,5...22 MHz. This choice of working areas made it possible to cover most of the broadcasting and amateur bands without impairing the smoothness of tuning. It is made on three field-effect transistors with a pn junction and on one microcircuit.
On fig. 1 shows a schematic diagram of a high frequency amplifier and a regenerative detector. The use of field-effect transistors with a high input resistance made it possible to find a circuit solution for these cascades that is very simple for a multi-range design. As you know, the range switch generates a lot of design problems in a multi-range apparatus, increases the risk of parasitic feedback and, consequently, self-excitation. The creators of the "MFJ-8100" receiver managed to get by with a switch in only one direction to select the operating range, which completely removed all these problems. The radio frequency amplifier is made on a transistor VT1 according to a common gate circuit. A tuning resistor R2 is introduced between the antenna and the source circuit of the transistor, which allows you to select the optimal connection with the antenna. This resistor is slotted into the back of the receiver, as it only needs to be adjusted when changing antennas. The choice of the operating range is carried out by the switch SA1, which switches the coils LI-15 in the drain circuit of the transistor VT1. The oscillatory circuit formed by these coils and capacitors C2-C4 is both the output for the UFC and the input for the regenerative detector on transistors VT2 and VT3. Coil 11, which has a high quality factor, is shunted by resistor R1 to stabilize the operation of the radio frequency path. The combination of cascades with a common drain (this is how the transistor VT3 is switched on at a high frequency) and with a common gate (VT2) provides the necessary phase relationships in the detector. The regenerative detector could, of course, be assembled on a single transistor, but this would inevitably lead to the need to additionally switch the feedback circuits with all the ensuing consequences. The use of an additional transistor made it possible to bypass these problems completely. The optimal operating mode (regeneration threshold) is set with a variable resistor R8, and the trimming resistor R10 is used to select the working area of the detector when adjusting the receiver, which ensures a smooth approach to this threshold. The detected audio frequency signal is taken from the load resistor R9 in the drain circuit of the transistor VT3. Through the low-pass filter C12R11C14, it is fed to the audio amplifier. The UZCH circuit is not shown here, since it is made on the LM386 chip, which has no analogue of domestic production. But in fact, this is the most common ultrasonic frequency converter for transistor receivers, and it can be replaced by a cascade on the K174UN7 chip in a typical inclusion, or even a simpler one if you are supposed to listen only to headphones. Transistors VT1-VT3 can be replaced with KPZOZE. The inductors have the following values: 11-10 uH, L2 - 3,3 uH, L3 - 1 uH, 14 - 0,47 uH. The inductance of the coil L5 is not indicated in the description of the receiver. It is frameless, has eight turns of wire with a diameter of 0,7 mm. The inner diameter of the coil is 12 mm. The variable capacitor is equipped with a 1:6 retardation vernier. The recommended antenna is a wire 8 ... 10 m long. The appearance on the market of the regenerative HF receiver "MFJ-8100" has also activated radio amateurs. In a number of publications, descriptions of simple amateur designs of regenerators appeared. The most popular of them, apparently, was the single-band receiver, the circuit of which is shown in Fig. 2. Strictly speaking, in this receiver the detector is something ordinary (when receiving AM stations, when receiving CW and SSB, it becomes a mixing one). Regenerative is the input stage on the transistor VT1, which is a popular "quality factor" in the sixties. The detector is made on the VD1 diode. This diode must be germanium - this is a fundamental limitation (a small "step" in the forward direction and a relatively small reverse resistance are needed). The supply voltage of the high-frequency stage is stabilized by three silicon diodes VD2-VD4 connected in the forward direction.
The audio frequency amplifier is the most common one (transistors VT2 and VT3). Headphones must be high-impedance. Here you can use any high-frequency transistors (VT1) and low-frequency (VT2 and VT3). For an operating range of 5 ... 15 MHz, the L1 coil must have 12 turns of wire with a diameter of 0,8 mm on a frame with a diameter of 25 mm. The tap must be made from the fourth turn, counting from the bottom one according to the coil output scheme. The "boom" in amateur radio literature about shortwave regenerative receivers has led to a resurgence of interest in super-regenerative VHF receivers. The scheme of one of them is shown in fig. 3. Like all super regenerators, it can receive AM and FM signals.
Here, as in the "MFJ-8100" receiver, the input stage is made on a field-effect transistor VT1 according to a common gate circuit. The presence of RF in both receivers eliminates the radiation of a regenerative or super-regenerative detector into the antenna. The superregenerative detector is assembled on a field-effect transistor (VT2) connected according to a common gate circuit. The trimmer capacitor C8 sets the optimal feedback (super-regeneration zone), which provides a smooth approach to the threshold (adjusted by a variable resistor R4). The audio frequency amplifier on the VT3 transistor is the most common. It is designed to work with high-impedance headphones. This receiver operates in the 100...150 MHz band. His sensitivity - not worse than 1 μV. Coils L1 and L2 are frameless and have, respectively, two and four turns of wire with a diameter of 1 mm. The diameter of both coils is 12 mm, the length of the coil L2 is 18 mm. Inductor L3 is wound on a dielectric frame with a diameter of 8 mm and has 35 turns (wire with a diameter of 0,8 mm). Transistors VT1 and VT2 can be replaced by KP303E, and VT3 - on KT3102. Of course, regenerators and super-regenerators are not the future of amateur radio. But they still have a place under the Sun - in amateur design. Based on materials from the magazines "SO ham radio", "Technium" and "Electron" Literature
Publication: N. Bolshakov, rf.atnn.ru
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