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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Car VHF FM tuner. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / radio reception The creation of specialized microcircuits that provide signal processing in the high-frequency paths of broadcasting receivers has led to the "unification" of their construction. When creating receiving devices, radio amateur designers, in essence, were left with only the sphere of saturating the design with a functional service. Below is a description of the car receiver, in which, in addition to the power switch, there are only three control buttons for switching subbands and tuning to a station operating within the subband. The design provides for automatic scanning when searching for a station, which is very convenient, especially in cases where the driver cannot be distracted from the traffic situation. When developing the described VHF FM tuner, the following tasks were set:
The tasks were achieved using typical, but rarely used modes of operation of the LSI VHF FM receiver K174XA34, electronic switching of subbands by radio waves, non-standard inclusion of the DAC KR572PA1 and an additional input high-line preselector. Main technical characteristics:
The remaining characteristics of the tuner are determined by the parameters of the LSI K174XA34 [1]. The electrical circuit diagram of the receiver is shown in fig. 1. It has three main parts; radio receiving path A1 (input stage on the transistor 1VT1, BIS K174XA34, switching devices for subbands on the multiplexer 1DD1 and pin diodes 1VD1, 1VD2, 1VD4, 1VD5), control unit A2 (control buttons SВ1-SВЗ, elements 2DD1.1, 2DD1.4 , flip-flops 2DD2, 2DD3, comparator 2DA1, multivibrator on elements 2DD1.2, 2DD1.3, reversible counters 2DD4 - 2DD6, DAC on microcircuits 2DA2, 2DAZ) and an AZ display unit (keys on transistors ZVT1-3 \ / T5, LEDs ZVD5 -ЗVD8). The main functions of the control unit are the formation of a signal that controls the tuning frequency of the receiving path, and the switching code of the subbands. The control unit works as follows. When the power is turned on, a pulse is generated at the output of the inverter (element 2DD1.1), which sets the triggers 2DD3.1, 2DD3.2 to zero and writes information from the presetting inputs of counters 2DD4 - 2DD6 to their outputs. as a result, the initial sub-band 65...77 MHz is selected, the minimum voltage and, accordingly, the lower frequency of the sub-band in the receiving path are set at the output of the DAC (microcircuits 2DA2 and 2D10). The lower XNUMX bits of the output code of the counters determine the voltage at the DAC output, and the upper two - one of the four sub-bands, so that when choosing or searching for the necessary radio station, the transition from sub-band to sub-band occurs automatically in a cycle, i.e. pre-selection of the sub-band is not possible.
When you press the increase button SВЗ ("+") or the decrease button SВ2 ("-") of the tuning frequency, the 2DD2.1 RS-trigger generates the corresponding signal to increase or decrease the output code of the reversible counters, and the 2DD3.1 trigger issues a generation enable signal multivibrator on elements 2DD1.2 and 2DD1.3. coming to output 2 of the 2DD1.2 element through the integrating RC circuit 2R14, 2C6 and the inverter on the 2DD1.4 element. Fixing the output state of the counters should occur after the appearance and reliable capture of the input radio signal by the PLL system of the tracking demodulator, which is included in the LSI K174XA34 functional blocks. Such operation of the receiver is provided as follows. The appearance of a radio signal receiver close to the tuning frequency is accompanied by a change in voltage at pin 2 of the LSI 1DA1, the envelope of which is selected by the 2VD1, 2C1, 2R2 circuit and converted by the 1DA2 comparator into a logical unit, supplied through the dividing circuit 3C2, 5R2 to the information input of the trigger 3.1DD2. At its output, this signal appears after the next clock pulse. However, the prohibition of multivibrator generation will occur with a delay determined by the time constant of the 14R2, 6C4 circuit. In addition, the same circuit will average short-term random operation of the comparator from radio interference. As a result, at the capture stage, the natural frequency of the tunable PLL oscillator is "pulled up" to the frequency of the input signal. When you press the button SB1 "Scanning" ("Scan"), the tuning frequency of the receiver will begin to change depending on the previously set state of the RS-trigger 2DD2.1. At the same time, at the output of a single vibrator (RS flip-flop 2DD2.2), a pulse is generated that is applied to the counting input of the flip-flop 2DD3.2, as a result of which a log appears on its direct output. 1. The processes of capturing the radio signal and setting the center frequency in the PLL system in this case are the same as those described earlier, however, the hold mode is interrupted after 5 s if the "Scan" button is not pressed again, and scanning continues further until the next radio signal appears. Log state duration. 1 in time is 5 s and is determined by the elements 2R12, 2C5. At the input of the receiving path, a preselector is turned on, attenuating signals outside the band 55 ... 110 MHz. It is a combination of high-frequency filters: T-shaped on elements 1C1, 1L1 1C2 and L-shaped on elements 1L1, 7CXNUMX. A feature of the amplifying stage on the 1VT1 transistor is an increased dynamic range due to the use of an RF low-noise transistor of the KT368AM type and the presence of current and voltage feedback generated by resistors 1R4 and 1R1, respectively [12]. Taking into account the attenuation in the filters, the transmission coefficient of the entire preselector, measured from the antenna input of the UHF BIS K174XA34 input (pin 12), smoothly increases from 6,5 dB at a frequency of 55 MHz to 12 dB at a frequency of 110 MHz. Such an AFC makes it possible to some extent compensate for the decrease in sensitivity depending on the increase in frequency observed in the LSI K174XA34. A feature of the inclusion of LSI K174XA34 is the use of a generator inductor 1L4 switched by pin diodes 1VD1, 1\/D2, 1VD4,1, 5VD1. Together with them, the capacitors 15C1, 22C1, 24C1, 26C1 for alternating current close the right taps of the inductor according to the scheme to a common wire. In a typical switching circuit, the VCO supply voltage of the microcircuit is supplied through a similar inductor (1), in the considered circuit, the supply voltage of the entire microcircuit is determined by the voltage drop across the quenching resistor 16R1, the internal resistance of the 1DD174 multiplexer keys and the voltage drop across the pin diode. The amount of current flowing through the latter is sufficient for normal switching ranges and provides the necessary quality factor for stable generation. For the above switching circuit, the supply voltage must be stable and exceed the supply voltage of the BIS K34XA2.7, which is in the range of 3.3 ... XNUMX V. The source follower on the 1VT2 field-effect transistor eliminates the influence of the circuits following it and shifts the level of the constant component present at pin 2 of the BIS K174XA34. The display unit displays the number of the operating subrange by the glow of the corresponding LED. The search mode is accompanied by the blinking of this LED due to the switching of the variable component of one of the output signals of the reversible counter by the key on the transistor ZVT1. The design of the tuner can be arbitrary, depending on the conditions of use and the capabilities of the radio amateur himself. The printed circuit board for the entire device has not been developed, special requirements apply only to the 1L4 inductor. It is printed, its configuration is shown in Fig. 2. Lines show cuts made with a cutter with a blade width of 0,25 mm. The points indicate the places where the conclusions of the pin diodes and the conclusion of the 1C21 capacitor connected to point 1 are soldered. The conclusion 25 of the BIS 5DA1 is connected to the same point with an MGTF wire no longer than 1 mm. The entire inductor is surrounded by a screen tip. For the remaining elements of the receiving path, surface mounting was used on the same side of the double-sided board, the second side being the screen. It should be noted that the inductance of the 1L4 coil in research and experimental circuits was chosen with a margin. Frameless inductors 1L1, 1LЗ are wound turn to turn with a silver-plated wire with a diameter of 0,8 mm on a mandrel with a diameter of 3,4 mm and contain 9 and 6 turns, respectively. The 1T1 transformer is wound with two PEV-0,28 wires at once on the K5x1x20 ring made of M3VCh-14 ferrite, each of the windings contains XNUMX turns, the winding is ordinary. In the manufacture of the regulator, resistors of the MLT-0,125 type or the like were used. Resistors 2R6-2R8 can be in the range of 27 ... 68 kOhm. Resistors 1R1, 1R4, 2R16, 2R19 have a tolerance of ±5%. Adjusted resistors - type SPZ-38 or the like, for the automotive version it is better to use closed resistors of the type SPZ-19. Instead of the KT368AM transistor, the KT399AM is suitable. Chip K561IE14 can be replaced by 564IE14 or K561IE11 (564IE11). In the second case, a log is fed to the input of the initial state setting (pin 9). 0. Capacitors 1C23, 2C1, 2C3 - type K73-14 or K73-17; 2C5 - type K53-4 with a tolerance of at least ± 20%; 1C25,1, 30C50 - type K35-1; the rest - ceramic, of any type; capacitors 1C1, 2C1, 7C5 must have a tolerance of ± 75% and TKE M90, blocking can be groups TKE H750, the rest - normalized TKE no worse than M1. High-frequency inductor 2L1.2 - type DM-XNUMX with the value of inductance indicated on the diagram. After checking the operating modes of the tuner elements and the performance of the blocks that do not require adjustment, the receiver tuning is as follows. 1. With a tuned resistor 2R17 for the 1VD3 varicap, set the initial bias voltage of 2,2 V at the output of the DAC (pin 6 of the op-amp 2DAZ) in the initial state of the reversible counter 2DD4 - 2DD6, at the output of which, after applying the supply voltage, code 0111 1111 1111 (high level - junior category). Then, increasing the generation frequency of the multivibrator by decreasing the capacitance of the 2C7 capacitor, use an oscilloscope to verify the linearity of the change in the output voltage of the entire DAC. With voltage restrictions, you should select the values \u2b\u16bof the resistors 2R19, XNUMXRXNUMX. 2. Adjusted resistor 2R2 to set the threshold of the comparator 2DA1 to ensure reliable capture and retention of the radio signal. To do this, apply a test FM signal to the receiver input with a level corresponding to the nominal sensitivity, turn on the search mode using the SВ2 or SВЗ buttons. If necessary, reset to the initial state - by turning off the supply voltage of the control unit. Another way is to receive obviously the weakest radio signal. 3. Specify the connection positions of the pin diodes on the printed inductor 1L4 in accordance with the specified technical characteristics. To do this, submit a log to the information input of the trigger 2DD3.1. 1, this disables generation. Then, apply an FM signal with the frequencies of the lower boundaries of the subbands to the receiver input from the reference generator, set zeros on the control inputs of the 2DAZ DAC using the SВ2 or SВЗ buttons, and set the code of the corresponding subband at the control inputs of the 1DD1 multiplexer. Between pin 10 of the 2DD3.1 chip and the common wire, a resistor with a resistance of about 10 MΩ is required. Literature
Author: Yu.Ezhkov, Omsk
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