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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING VHF detector receivers. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur The concept of "detector receiver" is strongly associated with huge antennas and broadcasting on long and medium waves. In the published article, the author cites experimentally verified schemes of VHF detector receivers designed to listen to VHF FM broadcasts. The very possibility of detector reception on VHF was discovered quite by accident. Once, while walking around the Terletsky park (Moscow, Novogireevo), I decided to listen to the broadcast - fortunately I took with me the simplest loopless detector receiver. The receiver had a telescopic antenna about 1,4 m long. I wonder if reception is possible on such a short antenna? I managed to hear, rather weakly, the simultaneous operation of two stations. But what surprised me: the reception volume periodically increased and fell almost to zero every 5 ... 7 m, and for each station in different ways! It is known that in the Far East, and even in the NE, where the wavelength reaches hundreds of meters, this is impossible. I had to stop at the point of maximum reception volume of one of the stations and listen carefully. It turned out - "Radio Nostalgia", 100,5 FM, broadcasting from nearby Balashikha. There was no direct line of sight of the antennas of the radio center. How could a FM transmission be received by an amplitude detector? Subsequent calculations and experiments show that this is quite possible and completely independent of the receiver itself. The simplest portable VHF detector receiver is made in exactly the same way as a field indicator, only high-impedance headphones must be turned on instead of a measuring device. It makes sense to provide for the adjustment of the detector connection with the circuit in order to select it according to the maximum volume and reception quality. The simplest detector A receiver circuit that meets these requirements is shown in fig. one.
The device contains a whip telescopic antenna WA1, directly connected to the loop L1C1, tuned to the signal frequency. The antenna here is also an element of the circuit, therefore, in order to highlight the maximum signal power, it is necessary to adjust both its length and the tuning frequency of the circuit. In some cases, especially when the antenna length is close to a quarter of the wavelength, it is advisable to connect it to the tap of the contour coil, and select the position of the tap for maximum volume. Communication with the detector is regulated by a tuning capacitor C2. The detector itself is made on two high-frequency germanium diodes VD1 and VD2. The circuit is completely identical to the voltage doubling rectifier circuit, however, the detected voltage would double only if the coupling capacitor C2 was sufficiently large, but the load on the circuit would be excessive and its quality factor low. As a result, the signal voltage in the circuit and the sound volume would decrease. In our case, the capacitance of the coupling capacitor C2 is small and the voltage doubling does not occur. For optimal matching of the detector with the circuit, the capacitance of the coupling capacitor should be equal to the geometric mean between the input resistance of the detector and the resonant resistance of the circuit. Under this condition, the maximum power of the high-frequency signal, corresponding to the maximum loudness, is given to the detector. Capacitor C3 is a blocking capacitor, it closes the high-frequency components of the current at the output of the detector. The load of the latter are telephones with a direct current resistance of at least 4 kOhm. The entire receiver is assembled in a small metal or plastic case. A telescopic antenna with a length of at least 1 m is fixed in the upper part of the case, and a connector or jacks for connecting telephones is fixed at the bottom. Note that the telephone cord serves as the second half of the receiving dipole, or counterweight. Coil L1 is frameless, it contains 5 turns of PEL or PEV wire with a diameter of 0,6 ... 1 mm, wound on a mandrel with a diameter of 7 ... 8 mm. You can select the required inductance by stretching or compressing the turns during tuning. A variable capacitor (KPE) C1 is best used with an air dielectric, for example, type 1KPVM with two or three movable and one or two fixed plates. Its maximum capacitance is small and can be 7 ... 15 pF. If there are more plates (respectively, the capacitance is larger), it is advisable either to remove some of the plates, or to connect a constant or trimmer capacitor in series with the KPI, thus reducing the maximum capacitance. As C1, small-sized "smooth tuning" capacitors from transistor receivers with a KB range are also suitable. Capacitor C2 - ceramic sub-tuning, type KPK-1 or KPK-M with a capacity of 2 ... 7 pF. It is permissible to use other trimmer capacitors, as well as install a KPI similar to C1 by bringing its knob to the receiver panel. This will allow you to adjust the connection "on the go", optimizing reception. Diodes VD1 and VD2, in addition to those indicated in the diagram, can be types GD507B, D18, D20. The blocking capacitor C3 is ceramic, its capacitance is not critical and can vary from 100 to 4700 pF. Setting up the receiver is easy and comes down to tuning the circuit with capacitor C1 to the frequency of the station and adjusting the connection with capacitor C2 until the maximum volume is obtained. In this case, the contour setting will inevitably change, so all operations must be carried out sequentially several times, while choosing the best place for reception. By the way, it does not necessarily have to coincide (and most likely will not) with the place where the field strength is maximum. This should be discussed in more detail and finally explained why this receiver can receive FM signals at all. Interference and FM to AM conversion If the L1C1 circuit of our receiver is adjusted so that the FM signal carrier falls on the slope of the resonant curve, then the FM will be converted to AM. Let's see what the quality factor of the circuit should be for this. Assuming the loop bandwidth to be equal to twice the frequency deviation, we obtain Q = fо/Δ2f = 700 for both the upper and lower VHF bands. The real quality factor of the circuit in the detector receiver will probably be lower due to the low intrinsic quality factor (of the order of 150...200) and shunting of the circuit by both the antenna and the input impedance of the detector. However, a slight FM to AM conversion is possible, and thus the receiver will barely work if its circuit is slightly detuned up or down in frequency. However, there is a much more powerful factor that contributes to the conversion of FM to AM - this is interference. Very rarely, the receiver is in the line of sight of the radio station antenna, more often it is covered by buildings, hills, trees and other reflective objects. Several beams scattered by these objects arrive at the receiver antenna. Even in the line of sight, in addition to the direct beam, several reflected ones come to the antenna. The total signal depends on both the amplitudes and the phases of the summing components. Two signals are added if they are in phase, that is, their path difference is a multiple of an integer number of wavelengths, and subtracted if they are out of phase, when their path difference is the same number of wavelengths plus another half wave. But after all, the wavelength, like the frequency, changes with the FM! Both the path difference of the rays and their relative phase shift will change. If the path difference is large, then even a small change in frequency leads to significant phase shifts. An elementary geometric calculation leads to the relation: Δf/f0 = λ/4ΔC, or ΔС = f0/λ/4Δf, where ΔС is the path difference required for a phase shift of ± π/2, i.e., to obtain the total AM of the total signal; C Δf - frequency deviation. By total AM here we mean the change in the amplitude of the total signal from the sum of the amplitudes of two signals to their difference. The formula can be further simplified if we take into account that the product of frequency and wavelength foλ is equal to the speed of light c: ΔС = с/4 Δf. Now it is easy to calculate that to obtain a full AM two-beam FM signal, a beam path difference of about a kilometer is sufficient. If the travel difference is smaller, then the AM depth will decrease proportionally. Well, what if there are more? Then, in one period of the modulating sound oscillation, the total amplitude of the interfering signal will pass through the maxima and minima several times, and the distortions during the FM to AM conversion will be extremely strong, up to the complete illegibility of the audio signal when received on the AM detector. Interference in FM is an extremely harmful phenomenon. It causes not only an accompanying spurious AM signal, as we have just seen, but also spurious phase modulation, which leads to distortion even when received on a good FM receiver. That is why it is important to move the antenna to that place in space where one signal prevails. It is always better to use a directional antenna as it increases the direct signal and attenuates the reflected signals coming from other directions. Only in our case of the simplest detector receiver did interference play a useful role and made it possible to listen to the transmission, but the transmission can be heard weakly or with great distortion not everywhere, but only in certain places. This explains the periodic changes in the volume of reception in Terletsky Park. Detector with frequency detector A radical way to improve reception is to use a frequency detector instead of an amplitude one. On fig. 2 shows a diagram of a portable detector receiver with a simple frequency detector, made on a single high-frequency germanium transistor VT1. The use of a germanium transistor is due to the fact that its junctions open at a threshold voltage of about 0,15 V, which makes it possible to detect rather weak signals. The junctions of silicon transistors open at a voltage of about 0,5 V, and the sensitivity of the receiver with a silicon transistor is much lower.
As in the previous design, the antenna is connected to the input circuit L1C1, tuned to the signal frequency using KPI C1. The signal from the input circuit is fed to the base of the transistor. Another is inductively connected to the input circuit - L2C2, which is also tuned to the signal frequency. The oscillations in it, due to inductive coupling, are shifted in phase by 90 ° relative to the oscillations in the input circuit. From the tap of the coil L2, the signal is fed to the emitter of the transistor. The blocking capacitor C3 and high-resistance telephones BF1 are included in the collector circuit of the transistor. The transistor opens when positive half-waves of the signal act on its base and emitter, and the instantaneous voltage on the emitter is greater. At the same time, a detected and smoothed current passes through the telephones in its collector circuit. But the positive half-waves overlap only partially when the oscillation phases in the circuits are shifted by 90°, so the detected current does not reach the maximum value determined by the signal level. With FM, depending on the frequency deviation, the phase shift also changes, in accordance with the phase-frequency characteristic (PFC) of the L2C2 circuit. When the frequency deviates to one side, the phase shift decreases and the half-waves of the signals at the base and the emitter overlap more, as a result of which the detected current increases. When the frequency deviates to the other side, the overlap of the half-waves decreases and the current drops. This is how frequency signal detection occurs. The detector transfer coefficient directly depends on the quality factor of the L2C2 circuit, it should be as high as possible (in the limit, as we calculated, up to 700), which is why the connection with the emitter circuit of the transistor is chosen weak. Of course, such a simple detector does not suppress the AM of the received signal; moreover, its detected current is proportional to the signal level at the input, which is an obvious disadvantage. The justification lies only in the exceptional simplicity of the detector. Just like the previous one, the receiver is assembled in a small case, from which a telescopic antenna extends upwards, and telephone jacks are located below. The handles of both KPIs are displayed on the front panel. These capacitors should not be combined into one unit, since, by tuning them separately, it is possible to obtain both greater volume and better reception quality. The receiver coils are frameless, they are wound with PEL 0,7 wire on a mandrel with a diameter of 8 mm. L1 has 5 turns and L2 has 7 turns tapped from the 2nd turn, counting from the ground terminal. If possible, it is advisable to wind the L2 coil with a silver-plated wire to increase its quality factor, while the wire diameter is not critical. The inductance of the coils is selected by squeezing and stretching the turns so that the well-audible VHF stations are in the middle of the tuning range of the corresponding KPI. The distance between the coils within 15 ... 20 mm (the axes of the coils are parallel) is selected by bending their leads soldered to the KPI. With the described receiver, you can conduct a lot of entertaining experiments, exploring the possibility of detector reception on VHF, the features of the passage of waves in urban areas, etc. Experiments to further improve the receiver are not excluded. However, the sound quality when receiving high-impedance headphones with tin membranes leaves much to be desired. In connection with the above, a more advanced receiver has been developed that provides better sound quality and allows the use of various outdoor antennas connected to the receiver by a feed line. Field powered receiver While experimenting with a simple detector receiver, we repeatedly had to make sure that the power of the detected signal was high enough (tens and hundreds of microwatts) and could provide rather loud operation of telephones. But the reception turns out to be unimportant due to the lack of a frequency detector (FR). The second receiver (Fig. 2) solves this problem to some extent, but the signal power is also inefficiently used in it due to the quadrature power supply of the transistor by high-frequency signals. Therefore, it was decided to use two detectors in the receiver: amplitude - to power the transistor; frequency - for better signal detection. The scheme of the developed receiver is shown in fig. 3.
The external antenna (loop dipole) is connected to the receiver by a two-wire line made of a VHF ribbon cable with a wave impedance of 240 ... 300 Ohm. Matching the cable with the antenna is obtained automatically, and matching with the input circuit L1C1 is achieved by selecting the connection point for the tap to the coil. Generally speaking, an unbalanced connection of the feeder to the input circuit reduces the noise immunity of the antenna-feeder system, but, given the low sensitivity of the receiver, this does not really matter here. There are well-known ways to symmetrically connect a feeder using a coupling coil or balancing transformer. Under the conditions of the author, the loop dipole was made of a conventional insulated mounting wire and placed on a balcony, in a place with a maximum field strength. The length of the feeder did not exceed 5 m. With such insignificant lengths, the losses in the feeder are negligible, so a telephone wire can be successfully used. The input circuit L1C1 is tuned to the signal frequency, and the high-frequency voltage released on it is rectified by an amplitude detector made on the high-frequency diode VD1. Since the oscillation amplitude is unchanged during FM, there are practically no requirements for smoothing the rectified DC voltage. However, in order to remove a possible spurious AM signal during multipath propagation (see the interference story above), the capacitance of the smoothing capacitor C4 is chosen to be large. The rectified voltage is used to power the transistor VT1, and to control the current consumption and simultaneously indicate the signal level, a pointer indicator PA1 is used. The quadrature frequency response of the receiver is assembled on a transistor VT1 and a phase-shifting circuit L2C2. A high-frequency signal is fed to the base of the transistor from the coil tap of the input circuit through the coupling capacitor C3, and to the emitter - from the coil tap of the phase-shifting circuit. The detector works exactly the same as in the previous design. To increase the transmission coefficient of the black hole and make better use of the amplifying properties of the transistor, a bias was applied to its base through the resistor R1, which is why it was necessary to install a decoupling capacitor C3. Pay attention to its significant capacitance - it was chosen as such for shorting low-frequency currents to the emitter, i.e. for "grounding" the base at audio frequencies. This increases the gain of the transistor and increases the receive volume. The primary winding of the output transformer T1 is included in the collector circuit of the transistor, which serves to match the high output resistance of the transistor with the low resistance of telephones. The receiver can be used with high-quality stereo phones TDS-1 or TDS-6. Both phones (left and right channels) are connected in parallel. Capacitor C5 is a blocking capacitor, it serves to close high-frequency currents penetrating into the collector circuit. The SB1 button is used to close the collector circuit when setting the input circuit and searching for a signal. At the same time, the sound in the phones disappears, but the sensitivity of the indicator increases significantly. The design of the receiver can be very different, but you need a front panel with KPI C1 and C2 installed on it (they are equipped with separate tuning knobs) and an SB1 button. So that the movements of the hands do not affect the adjustment of the contours, it is desirable to make the panel metal or from foil material. It can also serve as a common wire of the receiver. KPI rotors must have good electrical contact with the panel. Antenna and telephone connectors X1 and X2 can be installed both on the same front panel and on the side or rear walls of the receiver housing. Its dimensions entirely depend on the parts available. Let's say a few words about them. Capacitors C1 and C2 are of the KPV type with a maximum capacitance of 15 ... 25 pF. Capacitors C3 - C5 used ceramic, small. Coils L1 and L2 are frameless, wound on mandrels with a diameter of 8 mm and contain 5 and 7 turns, respectively. Winding length 10 ... 15 mm (adjust when setting). PEL wire 0,6 ... 0,8 mm, but it is better to use silver-plated, especially for the L2 coil. The taps are made from 1 turn to the transistor electrodes and from 1,5 turns to the antenna. Coils can be arranged both coaxially and parallel to each other. The distance between the coils (10 ... 20 mm) is selected during adjustment. The receiver will work even in the absence of inductive coupling between the coils - capacitive coupling through the interelectrode capacitance of the transistor is quite enough. Transformer T1 is taken ready, from the broadcast loudspeaker. As VT1, any germanium transistor with a cutoff frequency of at least 400 MHz is suitable. When using a p-n-p transistor, for example, GT313A, the polarity of switching on the dial indicator and the diode should be reversed. The diode can be any germanium, high-frequency. Any indicator with a total deflection current of 50 - 150 μA is suitable for the receiver, for example, a dial indicator of the recording level from a tape recorder. Setting up the receiver comes down to tuning the circuits to the frequencies of well-audible radio stations, selecting the position of the coil taps for maximum volume and reception quality, as well as the connection between the coils. It is useful to choose the resistor R1, also at maximum volume. With the described antenna on the balcony, the receiver provided high-quality reception of the two stations with the most powerful signal at a distance of at least 4 km from the radio center and in the absence of direct visibility (blocked at home). The collector current of the transistor was 30...50 μA. Of course, the possible designs of detector VHF receivers are not limited to those described. On the contrary, they should be considered only as the first experiments in this interesting direction. If you use an efficient antenna placed on the roof and directed to the radio station of interest, you can get sufficient signal strength even at a considerable distance from the radio station. This opens up very attractive prospects for high-quality headphone reception, and in some cases it may be possible to get loud-speaking reception as well. Improvement of the receivers themselves is possible with the use of more efficient detection circuits and high-quality volumetric, in particular, spiral resonators, as oscillatory circuits. Author: V.Polyakov, Moscow
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