Encyclopedia of radio electronics and electrical engineering / radio reception

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Detector receivers are usually made to receive broadcasting stations operating with AM in the LW, MW [1, 2] and less often HF bands. In the VHF range, they are practically not used. This is due, firstly, to the fact that it is necessary to obtain a signal level sufficient for its detection. In the LW and MW bands, this is achieved by increasing the length of the antenna, in the VHF band it is almost useless to do this, since the wavelength is only a few meters. Secondly, it is necessary to ensure the selection of the received signal. If in the LW and MW ranges this requires a quality factor of a loaded circuit of 25 ... 100 and the circuit can be implemented on ordinary LC elements, then in the VHF range a quality factor of more than 100 is required and it is not so easy to obtain it.

There is another problem - a simple diode detector can only demodulate AM signals. Therefore, to demodulate FM signals, it is necessary to first convert FM to AM. This can be done on the slope of the amplitude-frequency characteristic (resonance curve) of the oscillatory circuit, as shown in Fig. 1. With this setting, changes in the frequency of the received signal lead to a change in its amplitude. The signal can then be demodulated with a simple diode detector. It is clear that for a good conversion, a large steepness of the characteristic is required, i.e. again, a large quality factor of the circuit.

Fig. 1

The spiral cavity resonator has a high quality factor (Fig. 2). It contains a round or rectangular screen, inside which is placed a single-layer coil. One of its ends is closed to the screen, and the other is open. To tune the resonator in frequency, a metal core or plate is brought to the side of the open output of the spiral, and the capacitance of the resonator changes. The quality factor of unloaded spiral resonators, depending on their design and tuning frequency, can be in the range of 200...5000.

Fig. 2

The scheme of the detector VHF FM receiver is shown in fig. 3. Its basis is a spiral cavity resonator. An external antenna is connected to the spiral through the XS1 connector. The frequency of the receiver is tuned by a variable capacitor C1. A half-bridge rectifier (detector) is assembled on diodes VD1, VD2, to which a signal from the resonator is fed through the capacitor C2. A load is connected to the output of the detector with a shielded wire (its capacitance smooths out the RF ripples of the detected signal) - high-resistance telephones or ultrasonic frequencies with a large input impedance. The higher the load resistance, the greater the quality factor of the resonator, which means that a larger signal will go to the diodes and the level of the AF signal will increase.

Fig. 3

To manufacture such a receiver, it is first necessary to make a spiral resonator. A cylindrical tin-plated metal can, preferably with a metal lid, is suitable for it. The design of the receiver is shown in fig. 4, it is designed for the range of 88 ... 108 MHz. Can 1 was used from coffee "Nescafe" with a diameter of 75 and a height of 70 mm. Spiral 2 is wound with PEV-2 wire with a diameter of 2 mm, it contains 6 turns. The winding is frameless, with a diameter of 35 mm and a length of 36...40 mm. It is desirable to make the number of turns a little more, so that if necessary, further adjustments can be made by shortening the spiral. The lower end of the wire is passed through a hole in the side wall, bent and soldered to the outer side. An XS1 connector is installed on the lower or side side and the central contact is connected to the spiral at a distance of approximately 0,1 ... 0,15 turns from the beginning of the winding (not counting the straight piece of wire). On the inside of the can, closer to the end of the spiral, the diodes are soldered, and one of the leads is brought out through the insulating sleeve.

Fig. 4

Capacitor C2 is a piece of wire PEV-2 0,4 ... 0,5 20 ... 30 mm long, placed next to the turns of the spiral. The movable part of the capacitor C1 is made in the form of a metal disk 3, which is attached to the screw 4. This screw moves in the nut or sleeve 5, which is soldered to the cover 6. The disk 3 can be made of tin, its diameter is equal to the diameter of the spiral, to reduce losses in it it is necessary to cut 1...3 sectors with an angle of several degrees.

For the manufacture of a spiral resonator, metal cans of a different diameter can be used, and the larger the diameter, the greater the quality factor can be obtained. It is possible to calculate a resonator with a jar of a different diameter or for a different range using a simplified method [3], which gives quite satisfactory results.

First of all, one should strive to choose a jar (see Fig. 2) with the ratio H/D = 1,2...1,3, where H is the height of the jar; D is the diameter of the can. If the ratio is different, the calculation error will increase. Number of turns N = 2586/(Fr), where F is the upper tuning frequency (MHz); r - can radius (cm). Spiral winding diameter (in the center of the wire) d = r, winding length I = 1,5r, winding pitch a = I/N, wire diameter b = a/4. It is desirable to keep the distance from the ends of the coil to the lower and upper walls within the limits L = 0,25 ... 0,3D.

When choosing a bank, consider the following. What matters is the cleanliness of the processing of the inner surface, it is good if it is shiny. It is desirable that there are no joints located parallel to the coil, but since they are in most cases, you need to pay attention to their quality, and if necessary, solder. The lower, grounded end of the coil must be brought to the side wall at a right angle.

Based on the above, we can conclude that the jar used by the author is not the best option. The H / D ratio was about 1, because of this, the lower turns were too close to the bottom wall, which means that the quality factor decreased. The calculation error did not exceed 8 ... 10% - the number of turns should be 6,5, and after adjustment it turned out 6.

The antenna was a piece of wire with a diameter of 1 ... 1,5 mm and a quarter wavelength, in this case about 70 cm. The level of the received signal strongly depends on the orientation of the antenna and its location. In the receiver, it is desirable to use high-frequency germanium detector diodes with the smallest possible capacitance.

To get loud headphone reception, you need a large received signal field strength, which is possible in the immediate vicinity of the radio station. In this case, one should strive to increase the quality factor of the resonator by reducing the capacitance of the capacitor C2, i.e., by removing a piece of wire from the spiral.

If the distance to the radio station is significant, reception on telephones is difficult due to the low signal level. Then the signal from the detector must be fed to the ultrasonic frequency converter, while its input resistance must be more than 100 kOhm, and the sensitivity must be 1 ... 3 mV. If there is no such UHF, then it can be made independently, thus making the entire VHF FM receiver. In addition, you can use the existing ultrasonic frequency converter by making a matching stage on a field-effect transistor.

When testing the receiver layout with the author of the article, due to the distance from the transmitting radio stations (the nearest, but not the most powerful, at a distance of 2 km, the rest are further), only one radio station was received on phones with a resistance of several kOhm, and weakly. I had to add UZCH, after which three radio stations (out of seven operating in this range) were received very loudly (about the same) and with good quality. Two of them were received louder when the antenna was oriented horizontally, and one was received vertically. These radio stations are separated by about 2 MHz in frequency, and no mutual interference was observed. The receiver was located on the windowsill, the antenna was about 70 cm long. Measurements showed that the bandwidth of the loaded spiral resonator in this layout was about 800...850 kHz, which corresponds to a quality factor of about 125.

If the signal level is high, it is advisable to increase the quality factor, thereby increasing the selectivity by connecting the input connector closer to the grounded end of the spiral. It should be noted that the receiver does not have an AGC system or a limiter, so the voltage of the AF output signal depends on the level of the received signal. This means that more powerful radio stations are received at a higher volume.

The UZCH scheme is shown in fig. 5, a. Its basis is the K174UN7 chip in a standard simplified inclusion. At the input of the ultrasonic frequency converter, a source follower on the transistor VT1 is installed, which increases the input resistance. The volume is regulated by resistor R3, resistor R4 sets the optimal gain of the microcircuit.

Rice. 5,a

The connection to the receiver should be made with a shielded wire of the shortest possible length. By combining the resonator and UZCH into one design, for example, in a housing from a subscriber loudspeaker, you can make a good VHF FM receiver. If the signal level at the receiving point is so high that the receiver output will have a detected voltage of more than 1 V, the source follower circuit must be modified in accordance with Fig. 5 B.

Rice. 5b

All parts of the UZCH are placed on a printed circuit board made of foil fiberglass, a sketch of which is shown in fig. 6.

Fig. 6

The following parts can be used in the device: field-effect transistor - KP303G, D, KP307A, B; polar capacitors - K50; non-polar - K10-17; variable resistor - SP4, SPO; tuning - SP3-19; fixed resistors - MLT, S2-33.

Literature

1. Polyakov V. Theory: a little about everything. 4.3 AM radio receivers. - Radio, 1999, No. 9, p. 49,50.
2. Polyakov V. Improvement of the detector receiver. - Radio, 2001, No. 1, p. 52, 53.
3. Hanzel G. Handbook of filter design. - M.: Sov. Radio, 1974.

Author: I.Aleksandrov, Kursk

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