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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Ring stereo decoder in VHF FM receivers. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / radio reception In simple VHF FM direct conversion receivers with phase locked loop, described in [1], there are no stereo reception indicators and Mono-Stereo mode switches, which creates certain inconveniences during their operation. Readers are offered a simple stereo decoder that automatically switches the radio to the "Stereo" mode when a stereo signal appears at its input. Schematic diagram of the stereo decoder is shown in fig. 1. According to the principle of operation, it is similar to the device proposed at the time by S. Novikov [2], but unlike it, it does not have a separate path for extracting and recovering a subcarrier frequency signal (VFC). No measures have been taken to bring the frequency and phase characteristics of the tonal and supratonal parts of the complex stereo signal (CSS) into line in the region of low audio frequencies [3]. For this reason, the low-frequency components of the audio signal (up to approximately 300 Hz) are reproduced in monophonic sound, which is quite acceptable, since, as has been repeatedly indicated, the stereo effect does not appear at these frequencies.
The KSS received at the input of the decoder is amplified by a cascade on the transistor VT1. To avoid distortion, the resistance of its load (resistor R1 and circuit L1C2) must be much less than the output. This requirement is met due to the fact that in linear mode the collector current of the transistor VT1 is determined by the base current. The VLF voltage is allocated by the L1C2 circuit tuned to it and then fed to the "ring" diode mixer, assembled on VD1-VD4 diodes. Under the action of the signal, for example, the left (VD1, VD2) and the right (VD3, VD4) pair of diodes opens and vice versa, and thus the CCC released on the resistor R1 is detected. When monophonic signals are received, the VLF control voltage is absent on the L1C2 circuit, and the signal from the resistor R1 through the corresponding pairs of diodes VD1-VD4 is fed to the output of the stereo decoder. In order to avoid non-linear distortion in this mode, the input impedance of the receiver's stereo amplifier should be more than 10 kOhm. Thus, the stereo decoder switches from the "Mono" mode to the "Stereo" mode automatically if there is a VLF on the L1C2 circuit. On fig. 2 shows a diagram of a device designed to receive transmissions on stereo phones. The input stage is made on one of the transistors of the DA1 microassembly and is a direct conversion device with direct synchronization by the received signal.
Reception is carried out on a whip antenna 1 ... 15 cm long connected to the XW25 jack. Coil L1 serves to tune the input circuit and eliminate side reception channels at harmonics (above the 2nd) of the local oscillator. Diode limiter (VD1, VD2) expands the dynamic range of input signals and reduces receiver overload. From it, the received signal is fed to the L2C2 broadband circuit, tuned to the middle (70 MHz) frequency of the received range. The local oscillator is tuned within 32,9 ... 36,5 MHz by a varicap, which is used as parallel-connected collector junctions of transistors VT1, VT2. The refusal to use the KVS111 varicap assembly is caused by the impossibility of obtaining a sufficiently good quality factor at low (0,3 V) control voltages. Capacitor C7 provides self-excitation of the local oscillator at the second harmonic, C5 blocks it at radio frequency, and C6 creates an optimal phase shift for detecting FM signals. The load functions of a synchronous detector at audio frequencies are performed by resistor R2. The ring stereo decoder (VD3-VD6) receives the signal through a thinly compensated volume control R16C10L4R8. The VLF voltage is allocated by the L5C17 circuit. The AF amplifier is made on transistors VT3-VT6. It has a fairly high input impedance and provides a nominal output power of 2x2 mW for headphones with a resistance of 2x8...100 Ohm. The quiescent current of the transistors of the output stage of the AF amplifier is 7 ... 10 mA. The receiver is powered by a voltage of 1,5 V (one element 316, A332, etc.). Coils L1, L2, L3 contain 12, 7 and 10 turns of wire PEV-2 0,51, respectively. They are wound on rods made of 600NN ferrite, 12 mm long and 2,8 mm in diameter (the rod should be removed from the L2 coil after winding). Coil L4 is placed on a ring of size K10x6x2 made of ferrite 2000NN and contains 1000 turns of wire PEV-2 0,06, coil L5 (260 ... 280 turns of wire PEV-2 0,12) - on a movable frame, put on a piece of rod with a diameter of 8 and 15...20 mm long from 400НН ferrite. Setting up the receiver begins with tuning it to the required frequency range. The easiest way to do this is with an industrial VHF receiver, which is used as an indicator of the radiation of the second harmonic of the local oscillator. The local oscillator is tuned by moving the L3 coil trimmer, which is then fixed with a drop of molten paraffin. Since the local oscillator radiation is very small, the antennas of both receivers must be placed as close to each other as possible. Further, by moving the trimmer of the L1 coil and changing the distance between the turns of the L2 coil, they achieve the maximum holding band of the signals of the received stations, and then tune the L5C8 circuit. coils L3) are guided by the maximum manifestation of the stereo effect. The tuning accuracy can be increased by temporarily shorting resistor R6. After that, by removing the jumper, by selecting its resistance (within 5 ... 9 Ohms), you can slightly increase the crosstalk attenuation between the channels. This completes the setup. The receiver sensitivity is about 50 µV and is the same for both mono and stereo signals, because it is limited by synchronization, and not by atmospheric and self-noise. The noise level during stereo reception increases, as is known [4], by approximately 20 dB, and in order for the sensitivity limited by noise to be the same as that limited by synchronization, it is necessary to introduce an RF amplifier into the radio receiving path. On fig. 3 shows a diagram of a simple stereo tuner designed to connect an AF amplifier with a nominal input voltage of about 30 mV. The "ring" stereo decoder of the tuner uses silicon diodes VD1-VD4, so it was necessary to introduce resistors R14, R15, through which the opening voltage is applied to the diodes.
The winding data of the coils L1-L3 are the same as in the receiver discussed above. The L3 coil can also be wound on a unified four-section frame with a trimmer with a diameter of 2,8 mm made of 600NN ferrite. Its winding in this case should contain approximately 400 turns of wire PEV-2 0,12. The upper limit of the received frequency range (73 MHz) is set by the coil trimmer L2, the lower one (65,8 MHz) - by resistor R6. Resistor R8 regulate crosstalk between the channels of the tuner. Author: A.Zakharov, Krasnodar The receiver described in the article, at the request of the editors of the Radio magazine, was tested by V. T. Polyakov, the author of a number of articles and books about FM receivers with phase auto-tuning of frequency. The tests were carried out in Moscow, in an apartment on the ninth floor of a reinforced concrete building, from the windows of which the television tower of the Ostankino television center located about 20 km away is visible. Here is what V. T. Polyakov said about the work of receiver A Zakharov. "Tests have shown that the sensitivity of A. Zakharov's receiver is comparable to well-known PLL receivers containing a balanced mixer, a DC amplifier at an op-amp and a single-stage RF amplifier. The approximate value of the sensitivity of A. Zakharov's receiver is 100 ... 150 μV. So high sensitivity is achieved due to the small amplitude of oscillations in the heterodyne circuit of the receiver and low supply voltage.With the optimal length (25 ... 30 cm) and the location of the antenna, stable reception of VHF broadcasting stations with very high quality was ensured.The "ring" stereo decoder proposed by the author allows you to get good separation of stereo channels and introduces very little distortion.However, as expected, the real selectivity and noise immunity of such a simple receiver turned out to be low.Even a slight, against the optimal increase in the length of the antenna, led to interference from neighboring stations in frequency, which is explained by the direct timing of their signals. The direct detection of signals from adjacent television channels, which was heard as a strong background with a frame rate, also interfered with reception. Reducing the length of the antenna relative to the optimal one or turning it, leading to a decrease in the level of the signal received at the input of the receiver, largely eliminated interference, but the acquisition and holding band of the useful station narrowed, and the reception became unstable. It seems that in its present form the receiver can provide good reception in cities where there are one, maximum - two VHF stations. VHF PLL receivers built using a balanced mixer, a separate local oscillator and a DC amplifier in the clock loop have a significantly greater dynamic range and better selectivity. So, for example, a VHF tuner from the Start 7104 set, tested under the same conditions, provided stable reception with no interference from other VHF broadcasting and television stations. It seems that the parameters of L. Zakharov's receiver could be significantly improved by introducing an RF amplifier with a small gain and two three circuits, tunable simultaneously with the heterodyne one. With a Q-factor of about 150 easily implemented in the VHF range, the bandwidth of such a preselector would be 300 ... 500 kHz, which would significantly attenuate the signals of neighboring stations in frequency and improve the actual selectivity of the receiver. The complication of the scheme is not so great. Another way to improve the parameters of the receiver (not excluding the first one) is to use a balanced synchronized oscillator on two transistors, in which the signal is fed to the transistors in phase, and the push-pull balanced local oscillator operates at half the frequency of the signal. Such a device should attenuate the direct detection of interfering signals. Of course, these are only assumptions, and experimentation is needed to verify them. Literature
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