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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Radiotelephone from handset. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Telephony The radiotelephone is designed to be connected to a conventional telephone line and to the mains. It consists of a base unit, which is connected to the line, and a portable handset with a dialer. The connection between the base unit and the handset is wireless using two radio channels in the range of 64-73 and 88-108 MHz. The radiotelephone is not intended for use in the field or automobile conditions, it has a short range, and its purpose is to “untie” the subscriber from the telephone socket and allow him to move with the handset within an apartment, a small company, or a private house with a not very large area.
The portable unit is based on the simplest Chinese-made handset. All details connecting it to the line have been removed, including the combined talk-pulse amplifier. The lever switch, the electrodynamic receiving capsule, and the dialer circuit with a microcircuit, buttons and power circuits are left. The printed circuit board of the handset remains, only unnecessary elements are removed from it, The schematic diagram of the portable unit is shown in Figure P.01. It consists of three functional units (not counting the dialer, which is already in the phone): a VHF FM radio receiver on the A1 chip, a VHF FM transmitter on transistors VT4, VT5, and a control unit on the D1 chip and VT6 transistor.
The radio receiver is made on the K174XA34 chip. The choice is due to the fact that this microcircuit has already begun to appear on sale, and besides, it has a high sensitivity at low supply voltage and low current consumption. The second version of the handset was made according to the same scheme, but using a radio receiving path from a cheap pocket VHF FM radio receiver made in China or India (it was not possible to establish exactly, but it was clear that it was not Japan or South Korea). Apparently, this path was made on a chip similar to K174XA34, because there was only one circuit and a minimum of attachments. In order to transfer the path to the handset, the wiring of its board had to be copied to a new handset board, and then the microcircuit and related elements were carefully soldered. In other matters, it is possible to use almost any FM path with sufficient sensitivity and minimum dimensions, even on the basis of UPCHZ-2, but in this case the current consumption increases greatly. And it is not at all necessary that the base unit has the same path as the portable one. The signal from the twisted antenna WA 1 enters through the filter plug L3 C16, which blocks the path of the signal from the transmitter, to the input circuit at L2 C14. Then the signal is processed by the A1 microcircuit and the low-frequency signal from its output is fed through the volume control R2 to the 3h amplifier on transistors VT1-VT3. UZCH is made according to the well-known two-stage circuit with a push-pull output on germanium transistors At the UZCH output, an electrodynamic capsule from a handset is connected. The transmitter itself is made on a VT5 transistor in a single-stage circuit. The carrier frequency is determined by the parameters of the circuit 16 C22 C23 C21 VD3. Modulation is performed by changing the capacitance of the VD3 varicap, which is part of this circuit. To transmit speech, an electret microphone from tape recorders is used. The fact is that in the handset the same capsule was used as a microphone as for reception. In this case, it provided low quality and required additional amplifying stages. The signal from the microphone is amplified by an emitter follower on VT4. At the varicap, 3h enters through a divider from R7 and R8. Power is supplied to the transmitter through a key stage on a VT6 transistor, which opens when the tube is removed. At this time, switch S1 is set to the position shown in the diagram. Thanks to the capacitor C30, after the handset is hung up, power is supplied to the transmitter for a few more seconds, allowing it to transmit the clear signal. To transmit information about the state of the hook switch of the handset (positions - "off-hook" and "hung up") and dialing signals, practically for controlling the base unit's pulse key, a conventional single-channel frequency-coded radio control system is used, such as the one used in systems radio control models. The code frequency signal 2925 Hz is formed by a multivibrator on the elements D1.1 D1.2. It starts by applying a zero level to pin 2 of the D1.1 element. When dialing signals need to be transmitted, this pin receives negative pulses from the dialer chip of the handset. For the transmission of off-hook and on-hook signals. longer pulses are fed to this pin. Formed by element D1.3. When removing the tube, switch S2 is set to the position shown in the diagram. In this case, the capacitor C28 begins to charge through the resistor R14. during the charging time of this capacitor, a low logic level is present at the output of D1.3, and as a result, the multivibrator works all this time. In order for the base unit to switch to the off-hook state, a pulse duration of about 1 second is required. As soon as the tube is lowered, the switch S1 is moved to the opposite position, and now the same pulse at the output D1.3 is formed by the charging time of C29 through R13. C28 at this time is discharged through R15 As a result, the circuit works like this: as soon as you pick up the phone, a signal modulated by a burst of pulses with a frequency of 2925 Hz is emitted into the air, which lasts about 1 second. Then you dial the number and the signal is modulated in a sequence of shorter bursts. When you are talking, the signal is modulated 3H from the microphone, and when the handset is hung up, the signal is again modulated in a burst, lasting about 1 second. There is no special ringing device in the handset, the base unit generates a signal modulated with a frequency of 1000 Hz, which is reproduced by capsule B1. At the same time, the base unit generates an acoustic signal with a built-in piezoelectric buzzer. The details of the circuit according to Figure P.01 are mounted on one printed circuit board made of one-sided foil fiberglass. . The board is located in the upper part of the tube, in the space between the receiving capsule (B1) and the rear wall. The RF path of the receiver is shielded with a brass "P" shaped plate, marked in fig. P.02 dotted line, holes for trimmer capacitors are drilled in it
Two twisted antennas are installed on the upper end of the tube. As frames for them, cases of disposable felt-tip pens with a diameter of 5-6 mm and a length of 100 mm are used. They are wound with a PEV wire of 0,5, 15 turns in increments of 6 mm. Then soft black polyvinyl chloride tubes are pulled over them. Distance between antennas - 40 mm. The following details are used in the proposed design. All fixed resistors are MLT 0,125, a variable volume control resistor of the SDR-3 type with a switch and an end handle (used as volume controls for pocket receivers). Trimmer resistor -SP4a. Trimmer capacitors ceramic PDA, constant KG, KD, KM, K10-7, electrolytic K53-14 Zener diode KS147 can be replaced by KS'133, instead of a varicap, you can also use a zener diode for a voltage of more than 9 8 (for example, D814D-1). Chip K176LE5 can be replaced by K561LE5. Transistors with any letter indices, MKE-3 microphone, or another with a built-in amplifier. Coils L1, L2, L3. L5, L6 frameless. They are wound on mandrels with a diameter of 3 mm, which are then removed, L1 contains 13 turns, L2 - L3 turns with a tap from the 3rd, L3 contains 7 turns. For winding, a PEV wire 0,35 L6 - 5 turns is used. L5 - 2 turns of wire PEV 0,35. Inductors L7 and L4 are wound on fixed resistors MLT 0.125, and contain 60 turns of PEV wire 0,12 each. When mounting the tube, output 1 of the dialer microcircuit must be connected through a KD503 diode and a 10 kΩ resistor connected in series to the connection point R15 and R17, and output 17 through a 47 kΩ resistor to the connection point R16 C29. With a common wire, you need to connect the pins 2, 6, 10, 11 connected together. Take the pulse signal from the 18th pin (data for the KS5805A chip). Setting up the radio receiving path must begin with setting the voltage at the emitters VT2 VT3, equal to half the supply voltage, by selecting R3. Then you need to unsolder C7 and solder a piece of wire 10-15 cm long to terminal 14 A1. and try to tune in to one of the radio stations in the 64-73 MHz range by rotating the C1 rotor. Now you need to debug the wire and solder C7, and then adjust the L2 C14 circuit in such a way as to ensure reliable reception of this radio station Setting up the transmitter comes down to setting the carrier frequency to that part of the 88-108 MHz range in which there are no radio stations in the area, this can be determined by a model receiver. And setting the maximum transmission range (also on an exemplary receiver). Perform this setting by sequentially adjusting the capacitors C22 and C23. The R7 trimmer is set for the best sound quality of speech delivered in front of a microphone, when listening to it through an exemplary receiver. Now you need to adjust the L3 C16 plug filter for the minimum signal penetration from the transmitter to the receiver input, Setting the control unit comes down to setting the frequency at the output of the multivibrator to 2925 Hz (not necessarily so accurately) by selecting R11. And the selection of R13 and R14 in such a way that when switching S1, the multivibrator forms a pack with a duration of 1 second. The schematic diagram of the base unit is shown in Figure R.03-The radio receiving path is made according to the same scheme as in the handset, with the difference that the path is tuned to a frequency in the range of 88-108 MHz, and its output ultrasonic frequency converter is replaced by a resonant comparator on chip A2 and circuit L4 C1b. As already noted, the scheme is built according to the radio control scheme with frequency-time division of channels. This means that when dialing a number or when picking up or dropping the handset, the handset transmitter emits a frequency-modulated pulse on the air, the modulation frequency of which is 2925 Hz, and the duration depends on the transmitted command (dialing or "handset position"). The L4 C16 circuit is tuned to this frequency. At a time when the input signal is absent, the comparator inputs receive the constant component of the detector output through resistors R2, R3, R4. At the same time, an additional bias voltage is supplied to the inverting input through resistor R6. As a result, the voltage at the inverting input is slightly higher and zero is set at the output of the comparator. When a signal is received with the tuning frequency of the L4 C16 circuit, a positive voltage appears on the cathode of the VD2 diode. As a result, the voltage at the direct input is higher than at the inverse one, and the comparator output goes into a single state. The duration of this state depends on the duration of the frequency-modulated pulse emitted by the tube transmitter. From the output of the comparator, a positive pulse is fed to two timing circuits R18 C28 and R17 C27. If a long pulse arrives, the capacitor C27 has time to charge to a single level and the pulse enters the input "C" of the trigger D2. As a result, the trigger takes the opposite position and transfers (using the electromagnetic relay P1, which replaces the lever switch) the line to the busy or cleared state. The initial state when the power is turned on is the end state, the contacts K1 are in the open state. When switched on to the busy state (the handset is off), the contacts K1 close and a conversation-pulse cascade on transistors VT2.VT3 is connected to the line. When dialing a number, the duration of the pulses is much shorter and C27 does not have time to charge, so the trigger state remains unchanged. Short pulses are fed to input D1, which, using diode VD9, reduces the bias voltage at the base of transistor VT2, putting the entire cascade into a state with a low collector current. Collector voltage pulses are perceived by the line as dialer operation. After the number is dialed, the D1 output is set to one, and the talk-pulse stage goes into a high current state. Now its collector current changes in accordance with the speech signals coming from the output of the detector A1 to its input through the capacitor C25. To listen to the ringing signal, conversational signals coming from the line, a transmitter on the transistor VT1 is used, which is made in the same way as in the handset, but operates in the range of 64-73 MHz. Its RF signal is modulated using a VD3 varicap. 3H signals are fed to it through capacitor C24, VD4 serves to limit the amplitude of these signals. The ringing signal emitted by the handset is not loud, therefore, an additional "ringer" on the VT5 transistor and the BQ1 piezoelectric element is used. To power the circuit, a 9V source is used, consisting of a transformer T1 and a rectifier on VD13-VD16 with a stabilizer on VT6 The details of the circuit according to Figure P 03 are mounted on one board made of one-sided foil fiberglass. The board pattern and the wiring diagram are shown in Figures P.04 and P.05. The board is placed in a plastic case with dimensions of 160x200x80 mm, a power transformer is also located there. The RF path of the receiver is shielded with a brass "P" shaped plate, holes for trimmer capacitors are drilled in it. On two opposite sides of the case, two folding telescopic antennas with swivel hinges (from small-sized radio tape recorders) are installed. And the design uses fixed resistors MPT 0,125. Trimmer capacitors KPK, permanent KD, KT, K10-7, electrolytic K50-14 or K50-35. The VD1 zener diode can be replaced with KS133, VD12 with D816D-1, VD4 with any voltage of 12-31V or a zener diode circuit, for example, two in series - D814D. Chips K4 can be replaced by similar K561. Electromagnetic relay for 176V - RES 12, such relays are equipped with kits for self-assembly of the remote control of the TV (for the power switch) You can use another relay for 22-10 V. and a current of not more than 12 mA Piezo emitter from a handset. Coils L1, L2, L3, L5. L6 frameless, have the same design as in the "tube". L1 contains 7 turns, L2 - 7 with a tap from the 2nd, L3 - 13 turns, L6 -10 turns, L5 - 3 turns. The L7 coil is wound on a frame from the VEF-0,125 receiver IF circuit with a tuning core and armored cups (you can use SB-10A with a trimmer). The coil contains 60 turns of PEV-0,12. For the transformer, the Sh-15x23 core was used. The primary winding contains 4400 turns of PEV 0,09, the secondary - 230 turns of PEV 0,23. You can use any ready-made transformer with an output AC voltage of 8-12V and a power of 10W. The setting of the base unit must be carried out together with the “handset” that has undergone preliminary configuration. When transmitting a speech signal from the tube, you need to tune it to the frequency of the transmitter of the tube, the circuit with coils L1 and L2. You can control the signal using a control ultrasonic frequency converter, to the input of which you apply an AF signal from pin 16 A1 (through a capacitor). Then you need to unsolder the cathode of the VD5 diode (Fig. P.01) from the dialer chip and connect it to a common wire. Now you need to control the level at pin 9 A2 (Fig. P.03) with an oscilloscope and adjust L4 (P.03) and select the resistor R11 (P.01) to set a position in which, when connecting the VD5 cathode (Fig. P.01) with a common wire, at pin 9 A2 (Fig. P.03) a unit is set. Now you need to restore the connection VD5 (P.01) and set the sensitivity of the comparator A2 (P.03) by selecting R5 so that it works confidently when dialing pulses arrive and does not work during a conversation. Now you need to choose the value of R17 in such a way that the level at pin 1 D2 (P.03) changes when you press the button on the lever switch of the handset and does not change when you dial a number. Setting the talk-key node to VT2 and VT3 (P.03) is reduced to setting the voltage on the collectors of these transistors 7-10V in the absence of a signal from the tube and closed contacts K1, by selecting R13. The transmitter is set up in the same way as the handset transmitter. Using an exemplary receiver, they choose a place on the scale of the 64-73 MHz range, in which there are no radio stations, and tune the transmitter with capacitors C22 and C23 to this place in the range. For ease of setup, it makes sense to send a signal to capacitor C24 from the GZCH, after disconnecting the device from the telephone line. Then you need to tune the handset receiver to the frequency of the base unit transmitter by tuning C1 and C14 (P.01). Now it remains to configure the L3 circuit C10 (P-03) for the maximum suppression of the signal from the transmitter (P.03) Author: Pavlov S.I.; Publication: N. Bolshakov, rf.atnn.ru
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