Frequency synthesizer for a portable radio station. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Frequency synthesizers

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The device generates a frequency grid in the range 27150 - 27262,5 kHz during transmission and in the range 27615 - 27727,5 kHz during reception for 1 channels with a step of 12,5 kHz. It is designed to be built into a 27 MHz VHF portable radio station powered by a 9 volt current source. When developing a frequency synthesizer, the requirements of simplicity, minimum power consumption and availability of components were taken into account. The synthesizer is built on the basis of a phase locked loop (PLL), which includes a variable division ratio divider (PVCD), a pulsed frequency-phase detector (PFD), a low-pass filter (LPF) and a transceiver generator. In addition, the synthesizer includes a reference frequency driver, a diode encoder of the channel number and a +5V voltage regulator.

Schematic diagram of the synthesizer

From the transceiver generator on the transistor VT2, through the buffer amplifier VT3, the signal enters the DPKD (chips DD1, DD2, DD3, DD5, transistors VT4 ... VT6). Its division factor is determined by the formula: K=K1xK4xK5+(KZ-K4xK5)xK2, where K1, K2 - division ratio of the DD1 counter at different levels of the control signal supplied to its preset input from the DD3 chip (K1 = 13, K2 = 12); K3 - the division factor of the counter DD5, which varies depending on the mode of operation of the radio station ("Reception - Transmission") by applying a voltage of + 9V to the contact "Upit.prm." from the corresponding switch when receiving and removing it when transmitting (when receiving K3 = 1841, when transmitting - 1810); K4 - division ratio of the counter DD2 (K4-10); K5 - the division factor of the counter DD3, which varies from the choice of the channel number from 0 (Nk=1) to 9 (Nk=10), set by the switch SA1. Transistor VT4 is used as a signal level converter, VT5, VT6 - inverters.

The specific values ​​​​of the division coefficients adopted in the circuit are due to the range of frequencies generated by the synthesizer, the frequency shift between two adjacent channels (12,5 kHz) and between the frequencies generated during reception and transmission (465 kHz), as well as the comparison frequency with which the signal voltage comes from the output DPKD to IFFD (1,25 kHz). For example, if Nk=5 is set, then K5=4, and in the transmission mode, Kprd.=13x10x4+(1810-40)x12=21760. With a comparison frequency fср=1,25 kHz, we obtain a generation frequency of 21760x1,25=27200 kHz. In the receive mode, Kprm.=13x10x4+(1841-40)x12=22132 and the generation frequency is 22132x1,25=27665 kHz. DPKD works as follows.

The next signal with a voltage of one level from the output G of the DD5 microcircuit produces a preset channel number at the inputs of the counter DD3. In this case, the signals from the output P of the DD3 microcircuit and the collector of the transistor VT6 set the division factor of the counter DD1 K1-13 and allow the counter DD 2 to count. R DD13 and single (from the collector VT1) set K10=3 and stop the counter DD6 until the next signal voltage log. "1" at the output G of the chip DD12. the reference frequency signal generator consists of a master oscillator on a VT2 transistor and a frequency divider on a DD1 chip. The frequency of the master oscillator is stabilized by a quartz resonator BQ5. At the output G of the DD7 chip, a signal with a frequency of 4 kHz is generated. With the resonator frequency of 1 kHz adopted on the circuit, the division factor of the counter DD4 is 1,25 (set by jumpers at the Ci inputs). It is possible to use resonators with other frequencies from 500 kHz to 4 MHz, multiples of 400 kHz. The required division factor is set by soldering the jumpers at the inputs DD125. Setting the reference oscillator to the received frequency is the selection of capacitors C1,5, C125. The ICFD includes two D-flip-flops DD4, transistors VT13, VT14. On the diodes VD6, VD 8, a logic "AND" circuit of the signals coming from the inverse outputs of the triggers DD9, DD18 is assembled.

The circuit sets flip-flops to a single state. If the phases of the signals entering the inputs C of the triggers coincide, or there is a small difference in the phases, the transistors VT8, VT9 are closed. With an increase in the phase difference, depending on their ratio, either the VT8 transistor or the UT9 transistor opens and the low-pass filter capacitors are charged or discharged, which includes C15, R33, C19, R35, C20. Passing further through the double T-shaped bridge (R29, R30, R34, C16 ... C18), which suppresses the residual background of 1,25 kHz, the signal voltage is supplied to the varicaps VD3, VD4 of the transceiver generator, tuning its frequency so that a frequency of 1,25 kHz is set at the output of the DPCD. resistor R1 is set to + 5V at the output of the stabilizer, assembled on a transistor VT1, a diode VD 1, a zener diode VD2. Diode VD1 is used for thermal compensation of the stabilization voltage. The channel number encoder is assembled on diodes VD6 ... VD17. An inverse binary code is set at the output of the encoder. In transmit mode, the +9V supply voltage from the Receive-Transmit switch, which is part of the radio station, is supplied to the microphone amplifier and transmitter power amplifier. such a signal swing at the output of the microphone amplifier, in which the frequency deviation of the transceiver generator would not exceed 3 kHz. In the receive mode, the supply voltage is supplied by the same switch to the receiving path of the radio station.

The second group of switch contacts switches the antenna jack from the receiver input to the transmitter power amplifier output and vice versa. The frequency synthesizer, together with the transceiver generator, is assembled on one double-sided printed circuit board 60x114 mm in size. In the frequency synthesizer, resistors of the MLT, S2-23, S2-33 type, electrolytic capacitors of the K53-18 type, capacitors C7, C10 - of the KD26 type, and the rest - KM-56 are used. The inductor L1 is wound on a plastic frame with a diameter of 5 mm with a core of 100 HF wire PEV-2 0,5 mm. The number of turns is 4 with a tap from the middle. Channel switch SA1 - type PR2-10P1NVR.

Synthesizer setup starts with a stabilizer + 5V. Then a reference frequency of 1,25 kHz is set at the output G of the DD4 chip by setting the oscillator on the VT7 transistor and setting the required division ratio of the counter DD4. Then the PLL ring breaks - the output of the T-bridge is disconnected from the transceiver oscillator. The output of the microphone amplifier is also turned off. To the generator, at the connection point of the VD3, VD4 varicaps, a 10-20 kΩ variable resistor engine is connected, one output of which is connected to the + 9V circuit, and the other to the common wire. By changing the voltage on the resistor engine within 2 - 4 V and rotating the core of the inductor L1, the generation frequency of 27150 kHz is achieved. Further, by setting Nk=1 and the "Transmission" mode, the voltage of the signal with a frequency of 5 kHz is controlled at the output G of the counter DD1,25. Then, by removing the variable resistor and closing the PLL ring again, the generation frequency of 27150 kHz is controlled in the same mode. If necessary, the value of the resistor R35 is selected. When changing the operating mode from transmission to reception, the generation frequency should be set to 27615 kHz. Further, by switching the channel number to 10, the generation frequency is controlled in both modes.

When switching from a channel to an adjacent channel, the generation frequency should change by 12,5 kHz. Finally, by connecting the output of the microphone amplifier to the transceiver generator and setting the "Transmission" mode, they ensure that the generator frequency deviation does not exceed 3 kHz. The easiest way to do this is to listen to a tuned radio station through a receiver and achieve audibly undistorted reception. This completes the setup of the synthesizer. The total current consumed by the synthesizer and the transceiver generator for +9V power supply does not exceed 8 - 10 mA.

When choosing a frequency range, the requirements for the practical sufficiency of 10 channels and the possibility of operating at a frequency adopted for many single-channel radio stations in the 27 MHz band were taken into account. With some complication of the circuit, by adding another one to the DD3 chip - of the K561IE11 type, as a high-order counter, and changing the channel number encoder circuit with the division factor of the DD5 counter - you can increase the number of channels to 256. It is also easy to change the frequency interval between adjacent channels, for example, set to 10 kHz. To do this, you need to set the reference frequency to 1 kHz, change the division factor of the DD5 counter and rebuild the T-bridge to a frequency of 1 kHz.

Based on this scheme, it is possible to build frequency synthesizers for other frequency ranges. It seems that it can also be used as a base for the development of an industrial single-chip synthesizer circuit.

Author: S.Shevchenko, Simferopol; Publication: N. Bolshakov, rf.atnn.ru

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