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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Detectors for PLL receivers. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / radio reception The detectors (Fig. 1 and 2) for direct conversion FM receivers with phase locked loop frequency [1] offered to the attention of readers are made on the basis of a similar device described in [2]. Unlike the prototype, both of them have a high sensitivity, and the detector according to the scheme of Fig. 2, in addition, and higher selectivity. The detecting device, the scheme of which is shown in Fig. 1 is a frequency converter with a combined local oscillator, which simultaneously performs the functions of a synchronous detector. The input circuit L1C2 is tuned to the frequency of the received signal, and the local oscillator circuit L2C5 is tuned to a frequency half that. The conversion takes place at the second harmonic of the local oscillator, so the intermediate frequency lies in the audio frequency range. The local oscillator frequency is controlled by the transistor VT1 itself by changing the capacitance of the collector junction, which depends on the output signal of the detector. The use of inductive coupling (L3) in the local oscillator made it possible to get rid of the resistor in the emitter circuit of the VTI transistor [2] and, thus, to provide greater amplification of the detector at audio frequencies, i.e., to increase its sensitivity (up to 50..,100 μV). The gain of the cascade at these frequencies is Ku=(R3+Rin)Se, where Rin is the input impedance of the 3H amplifier to which the detector is connected, Se is the equivalent slope of the transistor VT1. \ When converting to the second harmonic of the local oscillator, the value of Se reaches its maximum value at a cutoff angle of 60°. It is set by choosing the appropriate coupling coefficient of the coils L2 and L3. In order to provide sufficiently stable tracking of the frequency even when receiving very weak radio signals, the cutoff frequency of the low-pass filter formed by the capacitor C4 and the equivalent input resistance of the transistor VT1 must be below the minimum frequency of the spectrum of the detected audio signal. This condition is met if the capacitance of the capacitor C4 is at least 50 microfarads.
GT313 and GT311 transistors with any letter index can work in the detector (in the latter case, it is necessary to change the polarity of the power supply and oxide capacitors). The coils are wound with PEV-2 wire 0,27 turn to turn on thin-walled paper frames with a diameter of 6 (L1, L2) and 7 mm (L3) and contain, respectively, 5 (tap from the 2nd turn, counting from the output connected to the common wire) , 15 and 5 turns. The local oscillator circuit is rebuilt with a brass screw with an M5 thread, which is inserted into the frame of the L2 coil through a threaded hole in the receiver housing. The antenna is a piece of wire 1 m long. Before setting up the detector, it is necessary to temporarily replace the resistor R1 with a trimmer and maximize the connection between the coils L2. L3 (push one on top of the other). After that, by selecting the resistor R1, a constant voltage is set on the collector of the transistor VT1, equal to 0,8 ... 0,9 V (measured on the capacitor C3). Then, by connecting the detector to an amplifier 34 with a loudspeaker, capacitors C2, C5 and coil trimmer L2, they tune in to some powerful VHF radio station and, by changing the position of the WA1 antenna, achieve its best audibility. Further, the connection between the coils L2, L3 is somewhat reduced and, keeping the voltage on the collector of the transistor VT1 unchanged, they are again tuned to the same radio station. The described manipulations continue until the acquisition bandwidth of the signal of the received radio station becomes the widest. After that, the required tuning interval of the detector is set with the capacitor C5 (it must correspond to the VHF broadcasting range of 65,8 ... 73 MHz), and the input circuit is tuned with the capacitor C2 to the middle of this range. Along with such an advantage as high sensitivity, the described detector also has significant drawbacks: low selectivity and significant gain non-uniformity over the range, due to a change in the operating mode of the transistor when the local oscillator circuit is tuned. The detector, the scheme of which is shown in fig. 2 is designed to receive one radio station. The selectivity and sensitivity of this device has been improved by the introduction of positive feedback (POS). The coil L2 included in its circuit is a coil of wire PEV-2 0.27, which can be moved along the frame of the coil L1 (the winding data of the coils of the heterodyne and input circuits are the same as in the detector according to the circuit in Fig. 1). When setting up this detector, the L2 coil is first moved away from L1 to the maximum distance, and then, having tuned in to a powerful VHF radio station, they are brought closer to it to a distance at which undistorted reception is still preserved. Using the previously described technique, set up the detector with the POS introduced and try again to reduce the distance between the coils L2 and L1. The adjustment procedure is repeated until the minimum possible distance between them is found.
When manufacturing receivers based on the described detectors, it must be remembered that in order to avoid the microphone effect, their design must be as rigid as possible, and the axes of the coils of the input and heterodyne circuits must be mutually perpendicular. It should also be borne in mind that the described detecting devices are very sensitive to supply voltage ripples, therefore, in case of their self-excitation, it is necessary to use a separate voltage regulator. Literature
Author:S. Chekcheev s. Nizhyn, Chernihiv region; Publication: N. Bolshakov, rf.atnn.ru
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