Frequency synthesizer based on modern element base. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Frequency synthesizers

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At present, the industry has mastered the production of integrated circuits, which make it possible to build compact frequency synthesizers with high operating parameters. Synthesizers can be used in radio receivers, radio transmitters and other similar equipment. It is especially important to have a compact and economical frequency synthesizer (MF) for the introduced communication equipment for the 27 MHz and 144 ... 146 MHz bands. One of these microcircuits is a microcircuit of the KR1015XK2 type [1]. The frequency synthesizer with its use is built according to the block diagram described in "RL" N 1/92. an intermediate frequency of 2 MHz is shown in Fig. 4.

Schematic diagram of the frequency synthesizer. Fig.1

Let's take a look at his work. The DD2 microcircuit KR1015XK2 has a 20-bit receiving register with serial input of information, into which data is written that sets the division coefficients of a variable division ratio divider (DPKD), an absorbing counter (PS) and a frequency divider with a fixed division ratio (DFKD). The format of the receiving register and the timing diagram of the process of writing information to it are shown in Fig.2.

Fig.2 (click to enlarge)

The rewriting of information from the receiving register of the microcircuit to its buffer register is carried out by applying a pulse of positive polarity to pin 7 after the twentieth information bit arrives at pin 5. At pin 6 of the microcircuit, clocks are received to write information to the receiving register. The first digit of the register determines the division factor of the DPCD. If it is set to 0, then its division factor is 1024; if it is 1, then 2560. Bits 2 to 8 determine the counting factor of the absorbing counter, and bits 9 to 20, the division factor of the DPCD. The 20th bit is entered into the microcircuit first, the last - 1.

The voltage-controlled generator C4 (VCO) is made according to the capacitive three-point circuit with a common base on a VT2 transistor of the KT316D type. The tuning of its frequency is carried out by the VD2 varicap of the KV109B type. From GU The signal is fed to amplifiers based on transistors VT3 and VT4 of the KT399A type. From the output of the transistor VT4, the signal through the capacitances C24 and C25 is fed to the receiver and transmitter, respectively. From the output of the amplifier on the transistor VT3, the signal comes to a frequency divider 64/65, made on a DD1 microcircuit of the KF193PTs8 type. From pin 2 of this microcircuit, the pre-divided signal of the operating frequency enters the DD2 microcircuit of the KR1015XK2 type, which plays the role of DPKD, DFKD, IChFD and has elements of a low-pass filter in its composition. The reference oscillator is also built on this chip. Its frequency is determined by a ZQ1 quartz resonator at a frequency of 12,800 MHz. Thus, when writing "1" to the first bit of the control register of the DD2 microcircuit, this frequency will be divided up to a frequency of 12,5 kHz, at which the comparison takes place. The signal from the IFFD through the output 16DD2 is fed to the low-pass filter on the elements C7, C8, R4, which determines the capture band and the retention band of the PLL.

With the elements indicated in the diagram, the PLL capture bandwidth will be about 15 MHz, and the settling time will be about 250 ms. Further, the control voltage through the resistor R6 and the coil L1 is supplied to the varicap VD2 and rebuilds the VCO. From pin 1 of the DD2 microcircuit, the signal that fixes the PLL tracking failure is fed to the detector on the VD1 diode of the KD522B type and then to the key on the VT1 transistor of the KT315B type, which controls the LED. When the PLL ring is captured, the HL1 LED will be extinguished. Information is written to the DD2 chip through pin 1 of the board, and the clock frequency and strobe signal are fed through pins 3 and 2 of the board, respectively. The modulating voltage is supplied to pin 4 of the board and then through the low-pass filter on the elements C12, R8, C14 - to the VD2 varicap, where frequency modulation is carried out. The required frequency deviation can be set by resistor R7.

To write the frequency code into the register of the microcircuit DD2 MF and to control the "reception-transmission" switching, the control unit shown in Fig. 3 is used.

Schematic diagram of the control unit. Fig.3

The operation of the control unit is based on the fact that the required frequency code is set at the inputs of the multiplexers DD5, DD7, DD10 of the type K561KP2 and then, in serial form, is issued through output 1 of the board. Since the DD10 multiplexer is used to output eight zeros, it can be successfully replaced with a source follower on the KP303V transistor, but when building such a synthesizer for the 430 ... 470 MHz range, it will be needed. The number of the required channel is set on the switches SA1 and SA2, and the SA1 switch has 16 positions, and the SA2 switch has 10 positions. The SA2 switch sets the channel group number, and the SA1 switch sets the channel number in the group. In this way, 160 channels can be set at 12,5 kHz. An adder is built on DD3, DD8, DD9T type K561IM1 microcircuits, which subtracts the intermediate frequency in the receive mode. The adder works in two's complement. The clock generator of the control unit is assembled on the elements DD1.2 and DD1.4, the "reception-transmission" switching unit has an SB1 button, a VD1 diode - of the KD522B type and a single vibrator - on the elements of DD1.1 and DD1.3 of the K561LA7 type. On the DD4 counter of the K561IE10 type and the D6.1 element, a counting unit up to 20 is built. The midrange is powered by two stabilized sources with a voltage of +5 V and +9 V.

The frequency synthesizer together with the control unit is assembled on a printed circuit board made of double-sided foil fiberglass with a thickness of 1,5 mm and a size of 190x55 mm. When manufacturing the board, it is necessary to preserve the maximum area of ​​the common wire both on the side of the installation of the elements and on the opposite side. Coil L2 is frameless, has 4 turns, held RPS 0,8, wound on a mandrel with a diameter of 5 mm. Coil winding length - 6 mm. Coils L1, L3 and L4 are DM-01 type chokes with an inductance of 10 μH. They can also be wound on rings made of ferrite grade 600 NN ... 2000NN, size K7x4x2, laying 15 turns of PEV-2 wire 0,25 mm thick evenly around the entire circumference. Setting up the frequency synthesizer comes down to setting the required frequency generated by the VCO using capacitor C15. However, with the help of capacitor C6, it is necessary to set the frequency of the reference oscillator as close as possible to 12,8 MHz. You may need to select low-pass filter elements: R4, C7, C8. This is necessary to expand the PLL capture bandwidth in the absence of it at the time of the transition from receive to transmit, although the latter is unlikely. This completes the setup of the synthesizer.

Literature

1. Yakubaitis S.V. Digital and analog integrated circuits., M.: Radio and communication, 1989, p.496.

Author: V. Stasenko, Rossosh; Publication: N. Bolshakov, rf.atnn.ru

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