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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Improvement of the detector receiver. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / radio reception A detector radio receiver ... For many decades it has been one of the first independent designs performed by novice radio amateurs. It begins with an acquaintance with the interesting world of radio receivers. It allows young radio enthusiasts to conduct various and exciting experiments on receiving signals from local radio stations. It would seem that what can be improved in this long-known device? Nevertheless, according to the author of the proposed article, the reserves for improving the operation of the detector receiver have not yet been exhausted. In the simplest receivers (Fig. 1a), the oscillatory circuit is heavily loaded by the detector. Although the loudness and sensitivity remain quite acceptable, the selectivity (selectivity) is insufficient. Due to the low quality factor of the circuit, two or three stations are often listened to at the same time. Assume that the receiver is tuned to the center frequency of the MW band (1 MHz). The inductance of the coil L1 is 200 μH, the capacitance of the capacitor C1 is 120 pF (typical values). Their reactances are approximately 1,2 kOhm and the resonant resistance of the entire circuit is Q times greater. With a constructive (without load) quality factor Q = 200, we get 240 kOhm. For the DV range, the resonant resistance of the circuit approaches megaohm! At the same time, the input impedance of the detector is considered to be equal to half the load resistance, which is used as high-resistance headphones with an impedance at audio frequencies of only 10 ... . It is easy to see how the circuit is significantly shunted, and its real quality factor turns out to be less than 10 (the ratio of the load resistance to the reactance of the circuit elements). By weakening the connection of the circuit with the detector, it is possible to increase the quality factor and, consequently, the selectivity. In this case, the volume will practically not change, since the signal voltage also increases in the circuit with a higher quality factor, which largely compensates for the decrease in the signal at the detector. Communication is usually regulated by connecting the detector to the coil tap (Fig. 1b) and selecting the tap position.
Since we are adjusting the connection, it is advisable to optimize the circuit as well. In [1-3], it was shown that the maximum efficiency of the antenna circuit is achieved when the antenna is fully included in the circuit and there is no loop capacitor. The tuning is carried out by changing the inductance of the coil, and the loop capacitance in this case is the capacitance of the antenna. If the antenna is large and its capacitance is significant, the tuning capacitor must be connected in series with the antenna (Fig. 1, b). Such a receiver works better than the previous one and has greater selectivity, but ... it is not very convenient to regulate the connection of the detector with the circuit, since this will require making a coil with many taps. And yes, adjustments still happen. A known method of resistance matching using capacitive coupling, in which the capacitance of the capacitor must be equal to the geometric mean of the matched. In our example (240 and 6 kΩ are consistent), it will be about 40 kΩ, and the corresponding capacitance will be only 4 pF! It turns out that the connection can be smoothly adjusted with an ordinary trimmer capacitor of the PDA or KPM type.
But the coupling capacitor breaks the DC circuit of the detector diode. To eliminate this drawback, you can put a second diode (Fig. 2). At first glance, we get a voltage doubling detector. In fact, due to the small capacitance of the capacitor C2, there is no doubling. During a negative half-cycle of oscillations in the circuit, this capacitor is charged through the VD1 diode, and when it is positive, it gives its charge through the VD2 diode to the load, i.e. BF1 telephones shunted by blocking capacitor C3 to smooth out ripples. The smaller the capacitance of the capacitor C2, the smaller the charge and, accordingly, the energy taken from the circuit. The communication circuit also introduces a small reactive (capacitive) resistance into the circuit, which is automatically compensated when the circuit is tuned to resonance with the received signal fluctuations. As L1 in the experimental design of this receiver, a long-wavelength magnetic antenna coil was used, containing 240 turns of PEL 0,2 wire, wound in one layer turn to turn on a frame with a diameter of 12 mm. When tuning, a rod with a diameter of 10 mm made of 400NN ferrite from the same antenna was inserted into the coil frame. The tuning range turned out to be from 200 kHz (with the capacitor C1 closed and the rod fully retracted) to 1400 kHz (with the rod removed and the capacitance of the capacitor C1 reduced). At home, with a small antenna (about 7 m) and grounding on the heating pipes, the receiver showed excellent results, accepting all Moscow LW and MW radio stations without exception. By adjusting the connection with a tuning capacitor C2, it was possible to obtain sufficient selectivity at a normal sound volume. Another advantage of the receiver was found out - due to the current supply of the detector through the large capacitance of the coupling capacitor C2, the "step" on the current-voltage characteristic of the diodes is smoothed out. By the way, the usefulness of the current supply of the detector was reported in [4]. In our receiver, silicon diodes (with a threshold of 0,5 V) work almost as well as germanium (with a threshold of 0,15 V). Moreover, it turned out to be possible to connect low-resistance (50-70 Ohm) headphones to the receiver, which is completely unacceptable in the traditional version. In this case, the capacitance of the coupling capacitor is required to be somewhat larger - up to 40 ... 50 pF. True, the sound volume will be less due to significant losses in the forward resistance of the diodes.
The high sensitivity of the described detector to weak signals led me to the idea of testing the simplest loopless version of the receiver (Fig. 3). It turned out to be a matter of several minutes to assemble it - all the details were soldered to the terminals of the telephones, and a one and a half meter piece of mounting wire with a crocodile clip at the end for hanging the wire to tree branches or other tall objects served as an antenna. The counterweight (instead of grounding) was the cord of telephones, which had some capacitance Spar to the listener and further to the ground. Even in such a primitive version, it was possible to listen to the work of a number of the most powerful radio stations. This receiver practically does not perceive low-frequency pickups, for example, from the wires of the mains - they are prevented by the small capacitance of the coupling capacitor C1, through which the radio frequency signal enters. The same audio frequency current is completely closed in an isolated circuit of telephones BF1 and diodes VD1, VD2. It cannot be said that the circuit of such a receiver is something new. The half-bridge rectifier used in it has long been well known - it was used in the field indicator [5]. By the way, nothing prevents the use of a full bridge on four diodes, connecting it with a circuit or with an antenna with a small capacitor.
A similar receiver has already been described in [6], but, unfortunately, its author incorrectly interpreted the principle of the receiver operation. The correct receiver circuit is shown in this article in fig. 4. It differs from the author's only in the presence of a parasitic capacitance Spar between telephones and ground, which plays the role of a coupling capacitor and matches the circuit with the detector. By a happy coincidence, the Spar capacity turned out to be close to optimal. But the author did not take it into account! As for the experimental results, as follows from the publication in [6], they turned out to be excellent. In conclusion, I would like to return to the scheme in Fig. 2 and draw the attention of radio amateurs to it. This detector receiver showed excellent results. Experiments with it are no less interesting and exciting than with more complex electronic devices. Literature
Author: V.Polyakov, Moscow
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