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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING A simple tuning system for a VHF FM receiver. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / radio reception The proposed analog tuning system can be built into almost any VHF FM receiver. It does not contain a frequency synthesizer and a microprocessor, which makes it simple and accessible for repetition. The system automatically searches for the next station when you press the "UP" or "DOWN" button, then the AFC system is turned on, supporting fine tuning. Recently, FM radio broadcasting in the VHF band has been developing at a very rapid pace. In our country, broadcasting is carried out in two bands: 65.8 - 73 MHz (OIRT standard) and 88 - 108 MHz (CCIR standard). The first of these ranges is usually called "VHF", and the second - "FM", although this is not entirely true: both ranges lie in the ultrashort wave region, and they both use frequency modulation (FM, or FM - Frequency Modulation). The main difference in broadcasting on these bands is the way the stereo signal is transmitted. The "our" standard uses a polar modulation system, while the "import" standard uses a pilot tone system. In addition, the maximum carrier frequency deviation is different: ±50 kHz and ±75 kHz, respectively. In a polar modulation system, the 31.25 kHz subcarrier is modulated by the amplitude difference signal AB and added to the sum signal A+B. The result is a polar-modulated signal. When modulating the transmitter, the subcarrier is suppressed by 14 dB using a notch circuit with a Q factor of 100±5. To decode such a signal in the receiver, it is sufficient to have a subcarrier recovery stage and two diode detectors, at the output of which signals of the left (A) and right (B) channels are obtained. Thus, this system was initially focused on a simple stereo decoder. However, when trying to create a high-quality stereo decoder, some disadvantages of the system appear. First of all, this is the need for accurate reconstruction of the subcarrier (by exactly 14 dB and a loop with a Q factor of exactly 100). The deviation of these parameters worsens the separation of stereo channels. In addition, the system was not focused on the use of synchronous detection, and a conventional amplitude detector has increased non-linear distortions. The selection of the reference frequency for the synchronous detector from the amplitude-modulated subcarrier is difficult. The system with a pilot tone [1] was initially focused on the use of synchronous detection and sum-difference (matrix) stereo decoders. In this system, the 38 kHz subcarrier is modulated with an amplitude difference signal AB. Matrix stereo decoders use the tonal part of the signal from the frequency detector of the receiver as the sum signal A+B. A special 19 kHz pilot tone is transmitted to obtain the reference frequency of the synchronous detector. When the transmitter is modulated, the pilot tone is suppressed by 20 dB, and the subcarrier is completely suppressed, leaving only the sidebands. Thus, due to the use of synchronous detection, non-linear distortions are drastically reduced. In addition, high-precision subcarrier recovery is not required. The system is generally insensitive to the deviation of the level and even the phase of the subcarrier. The polar modulation system exists only thanks to a large fleet of old radios. Over time, it is increasingly being replaced by a system with a pilot tone. It is known that with stereo reception, the signal-to-noise ratio at the receiver output is much worse (by 20 dB or more) than with mono reception. The main noise is contained in the difference signal AB. Therefore, modern stereo decoders, in order to improve the signal-to-noise ratio, automatically narrow the band and reduce the AB signal level at the matrix input when reception conditions worsen. In this case, instead of increasing the noise level, the separation of stereo channels is somewhat worsened, which is subjectively less noticeable [2]. This principle is used, for example, in the tuners of some models of Pioneer car radios. Let's return to the receiver tuning system. Unlike a system based on a frequency synthesizer, the proposed tuning system can operate on any range. It is not directly tied to any particular reception frequency. Due to the fact that the system does not contain a microprocessor and switching digital circuits, there is no interference from the digital part. This ensures the best signal-to-noise ratio and maximum receiver sensitivity. Some disadvantage of the device is the lack of indication of the number of the received station. A prerequisite for embedding the system in the receiver is the presence of electronic tuning and an AFC signal. Electronic tuning is usually carried out using varicaps, which are supplied with a control voltage of 3 - 24 V, depending on the tuning frequency. Modern high-frequency receiver units often have a narrower tuning voltage range, approximately 1 - 9 V. The proposed system allows you to work with any tuning voltage range, the desired range is provided by the appropriate choice of the supply voltage of the op-amp U4 (Fig. 1). The AFC signal is the DC output of the frequency detector and can be obtained using a low pass filter. It is possible that this signal has a reverse polarity (i.e., with a downward frequency detuning, the AFC signal increases). The desired polarity can be obtained using one op amp, on which an amplifier with a gain of -1 should be assembled.
On fig. 1 shows a complete diagram of a VHF FM receiver. A ready-made VHF-I-2C block was used as an input block. Instead, an input block from a foreign-made car radio or a home-made input block can be successfully used. It should be noted that any input block can be easily converted to the desired range by replacing the coils of the heterodyne and input circuits. From the output of the VHF unit, an intermediate frequency signal of 10.7 MHz is fed to an aperiodic amplifier assembled on transistors VT1 - VT3. From the output of the amplifier, the signal is fed to the piezoceramic band pass filter F1, which forms the bandwidth of the receiver. The signal from the filter output is fed to a specialized U1 microcircuit, which contains an IF limiting amplifier, a frequency detector and an audio frequency preamplifier. The built-in frequency detector is based on a balanced modulator. The signal necessary for its operation, shifted in phase relative to the input, is obtained using the L1C9 oscillatory circuit. The quality factor of this circuit determines the steepness of the conversion. The required quality factor is set by resistor R13. From the output of the pre-amplifier of the audio frequency (pin 8), the signal goes to the amplifying stage on the VT5 transistor, then to the stereo decoder. The R19C14 chain compensates for the uneven frequency response of the path at high frequencies. The pre-distortion correction circuits must be part of the stereo decoder. As
Consider the operation of the tuning system when searching for a radio station up in frequency (Fig. 2a). When the receiver is not tuned to a station, the AFC voltage has some average value (in this case, about 3 V). Approximately the same voltage should be set using the trimmer R51 at point +E. To start the search process, press the "UP" button. In this case, the trigger U5B is reset, and U5A is reset. The analog multiplexer U6 receives address=1. The multiplexer, through the resistor R31, connects a voltage slightly less than + E to the input of the integrator U4. The output voltage of the integrator, and it is the tuning voltage, begins to increase. Along with it, the tuning frequency of the receiver increases (the area indicated by the arrow R in Fig. 2a). When the tuning frequency begins to approach from below the carrier frequency of one of the operating radio stations, the AFC voltage decreases. When it reaches the threshold set by trimmer R28, comparator U3 switches and resets both flip-flops U5A and U5B. In this case, address = 0 is supplied to the multiplexer, the multiplexer connects the AFC voltage to the input of the integrator, which fine-tunes the frequency. The voltage at the output of the integrator (and the tuning frequency of the receiver) change until the voltage of the AFC becomes equal to the voltage +E. And this corresponds to fine tuning (the area indicated by the arrow AFC in Fig. 2a). At this time, the output of the comparator is in a logic high state, which is provided by the hysteresis chain VD3-VD5, R25-R27. This circuit is built in such a way that when the comparator is triggered, the threshold rises just above the +E voltage. On fig. 2, the comparator threshold voltage is denoted by Utrh. To search for a radio station down in frequency, press the "DOWN" button. In this case, the trigger U5B is reset, and U5A is set. The analog multiplexer U6 receives address=2. The multiplexer, through the resistor R34, connects a voltage slightly greater than + E to the input of the integrator U4. The output voltage of the integrator then begins to decrease. Along with it, the tuning frequency decreases (the area indicated by the arrow R in Fig. 2b). When the tuning frequency begins to approach from above the carrier frequency of one of the radio stations, the AFC voltage first increases. If comparator U3 was previously turned on, then it turns off. The AFC voltage reaches a maximum, then begins to decrease, becomes equal to +E at the time of fine tuning, then falls further. When it reaches the set threshold, comparator U3 switches and resets both flip-flops. In this case, the multiplexer connects the AFC voltage to the input of the integrator, which returns the tuning voltage back, providing fine tuning of the frequency (the section indicated by the arrow AFC in Fig. 2b). If the comparator did not have a hysteresis chain, then it would reset already at fine tuning, and an attempt to search down would result in a re-acquisition of the same station. The second channel of the multiplexer U6 is used to drive the LEDs. When searching up, the "UP" LED turns on, when searching down, the "DOWN" LED turns on. When the station is found and the AFC is working, the "LOCK" LED is on. During the search, the receiver output is muted (silent tuning implemented). To do this, the output voltage of the U1 chip is shunted by the transistor VT4. This transistor is controlled by a cascade on VT9, which locks VT4 when the "LOCK" LED lights up. The R48C21VD9 chain provides a signal turn-on delay for the time required by the AFC system to capture the frequency. The tuning system is adjusted in the following sequence. First, set the desired voltage value +E. To do this, the voltage input of the VHF unit is grounded and the voltage of the AFC is measured. The same value is set with a tuning resistor for +E. If the receiver's IF path is implemented differently, then the +E adjustment limits may not be sufficient from below. In this case, an additional divider should be installed, or a suitable stabilizer of a different type should be used instead of U2. Then, with the trimming resistor R28, you should set the comparator threshold so that the system confidently captures the stations. If this threshold is too close to +E, then the tuning system will be stopped by interference. If the threshold is too far from +E, the system will skip stations. When the receiver is tuned to the station and the AFC is working, it is necessary to refine the voltage adjustment + E for the best reception (this adjustment brings the frequency detector to the middle of the linear section). The tuning system is powered by two voltages: +9 V and +30 V. The first can lie within +5..+12 V, the second depends on the tuning voltage range of the applied input block and can vary over a wide range. Instead of LM311, you can use KR554CA3 or one half of LM393 (LM2903). TL061 can replace KR544UD1, KR140UD8. Domestic analogue 4013 - K561TM2 or K176TM2, 4052 - K561KP1. Instead of DTC144E transistors, you can use any low-power npn transistors by adding a divider from identical resistors with a resistance of 10..47 K to the base circuit. The IF path can be made according to a different scheme or taken ready. The main thing is that it provides the AFC voltage. The stereo decoder can be made according to any scheme. A good stereo decoder for a polar modulation system is described in [2].
Figure 3. Schematic diagram of a stereo decoder system with a pilot tone. Specialized stereo decoder chips are also available for the polar modulation system. There is even a chip for a dual-system stereo decoder K174XA51 manufactured by Angstrem JSC. For the pilot tone system, there are many specialized foreign-made microcircuits. As an example, in fig. Figure 3 shows a diagram of a simple stereo decoder based on the AN7421 chip from Matsushita. Literature
Author: Ridiko Leonid Ivanovich, e-mail: wubblick@yahoo.com
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