Frequency synthesizer for a portable radio station. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Frequency synthesizer for a portable radio station. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Frequency synthesizers

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The publication in "RL" N 8/91 of the scheme and description of the "Portable radio station for personal use" aroused great reader interest. Analyzing incoming mail, I came to the conclusion that the main obstacle in repeating this design is the acquisition of quartz resonators with a frequency spacing of 465 kHz. Another disadvantage is the single-channel radio station. Therefore, to improve it, a frequency synthesizer was developed that uses only one quartz resonator for frequencies from 500 kHz to 2 MHz.

The frequency synthesizer allows you to work on all 11 channels allowed to work with frequency modulation in the 27 MHz band. It can also be performed in a single-channel version (in this case, the circuit will be simplified) and also rebuilt to frequencies allowed for operation with amplitude modulation.

The block diagram of the frequency synthesizer is shown in Fig.1. The synthesizer is based on the principle of a phase-locked loop (PLL) ring and a frequency divider with a variable division factor (CVD).

Frequency synthesizer for portable radio station
Ris.1

The controlled oscillator G1 operates at the transmit or local oscillator frequency, depending on the state of the "receive - transmit" PTT. From its output, the signal goes to the receiver, transmitter and DPKD, which consists of a counter with a switchable PD division factor. The latter divides the input frequency by 10 and 11, depending on the selected channel and the state of the absorption counter LN. Then the signal goes to the DPCD itself, where the required channel is set and the frequency shift is taken into account when switching from reception to transmission. The total division factor of the frequency divider from the input of the PD to the output of the DPKD is determined as follows:

N=a+10*b,

where a, b are the coefficients set by the CPS frequency setting unit.

From the output of the DPCD, a signal with a frequency of about 1,25 kHz is fed to a pulse-frequency phase detector (PFD). The reference frequency generated by the generator G2 and reduced by the divider D to 1,25 kHz also comes here. The output voltage from the IFPD is filtered by a low-pass filter, which determines the lock and hold bands of the PLL. Then it goes to the varicaps of the controlled oscillator G1 and adjusts it until the reference frequency and the frequency of the oscillator G1 match, taking into account the division coefficients. The comparison is carried out at a frequency of 1,25 kHz.

Schematic diagram of the frequency synthesizer is shown in Fig.2. The reference oscillator is made on the D2.1 element of the K564LN2 microcircuit. The quartz resonator Z1 is applied at a frequency of 500 kHz. A frequency divider with a fixed division factor divides this frequency by 400, i.e. up to 1,25 kHz. It is made on a D4 K564IE15 chip. A signal with this frequency is fed as a reference to the IChFD, assembled on the elements D1, D2.2, D3.1 and transistors VT1, VT2.

(click to enlarge)

The voltage-controlled generator is made on a VT4 transistor of the KT316D type according to the inductive three-point circuit. Its frequency is tuned using the varicap matrix KVS111 A by the voltage supplied from the IChFD through a low-pass filter on the elements C3, R6, C4. The modulating voltage also comes from the microphone amplifier through the resistor R8.

The signal from the VCO comes to the receiver and transmitter of the radio station through the capacitances C10, C11. Then it enters a buffer amplifier based on a VT5 transistor of the KT315V type. Reinforced, it is fed to the 10/11 divider, made on the D5 K153IE10 chip. A high-speed signal inverter is built on the VT3 transistor.

From pin 11 of the D5 chip, the signal is fed to the D6.1 trigger, the K564TM2 chip, which divides the frequency by a factor of two. This is done because the counter DPKD D9 type K564IE15 with a supply voltage of 5 V can only operate stably at a frequency not higher than 1,5 MHz. The divider switching control unit 10/11 is built on elements D2.4, D3.2, D3.3 and an absorbing counter D7 of the K564IE11 type.

DPKD is assembled on a D9 chip. Its division factor is controlled by codes from ROM D8. The K573RF4 chip was used as a ROM, but it is better to use 2764C to reduce the current consumption.

The channel number is set using switch SA1. On elements D2.5, D3.4, a circuit for suppressing the bounce of the contacts of the "reception-transmission" switch is built. From it, the control signal is fed to the DPKD to organize a frequency shift of 465 kHz during the transition from reception to transmission. The synthesizer is powered by a voltage regulator built on a VT6 transistor and a VD2 zener diode.

Structurally, the frequency synthesizer is made on a printed circuit board made of double-sided foil fiberglass 65 x 60 in size, which is located in the radio station housing in place of the batteries (see "RL" No. 8). Batteries are located behind the printed circuit board in a special container. At the same time, the thickness of the radio station body increases from 20 to 33 mm.

The supply voltage of the radio station in this design should be increased to 9 V, which will contribute to the stable operation of the frequency synthesizer, as well as increase the output power of the transmitter.

Coil L1 is wound on a frame with a diameter of 5 mm and has 15 turns of PEV-2 wire with a diameter of 0,25 mm with a tap from 5 turns, counting from the grounded end. Coil L2 is wound on a ferrite ring of size K7x4x2 made of F600NN ferrite and contains 20 turns of the same wire. The codes recorded in the ROM are shown in the table.

Channel number	Broadcast. Address	Broadcast. The code	Reception. Address	Reception. The code
1	001	06	801	47
2	002	66	802	A7
3	004	07	804	48
4	008	67	808	A8
5	010	08	910	49
6	020	68	820	A9
7	040	09	840	50
8	080	69	880	V0
9	100	10	900	51
10	200	70	A00	V1
11	400	11	С00	52

Any information can be written to the remaining ROM addresses. The SA1 channel switch is displayed on the panel next to the noise reduction knob.

When connecting the synthesizer to the radio station, you should use the diagrams in Fig. 3 and Fig. 4. The radio stage, which previously served as a local oscillator, will now be a buffer amplifier. The heterodyne part of the DA1 KD74PS1 chip also acts as a buffer. Coils L1 and L2 in fig. 4 are wound on a ring of M50VCh2 ferrite with a size of K7x4x2 and contain 10 turns of PEV-2 wire with a diameter of 0,25 mm. The windings are wound with two wires at the same time, twisted with a small pitch.

Frequency synthesizer for portable radio station

Synthesizer setup comes down to setting the frequency of the VCO with the core of the L1 coil so that when switching channels and switching from reception to transmission, a confident frequency capture occurs using the PLL ring. The capture can be judged by the shape of the voltage at pin 12 DA1.2. The "picture" on the oscilloscope screen must be stable. Resistor R8 is selected for the absence of PLL tracking failure at the loudest sounds pronounced into the microphone.

The synthesizer can use chips of types K564, K561, K176, D5 - type K555. Transistors can be used such as KT312, KT315, KT316, etc. Varicap matrix KVS111 \A can be replaced by two varicaps of types KV109, KV110, KV124, D901. The K573RF6 chip is also applicable in the ROM. When using the K573RF2 and K573RF5 microcircuits, the number of channels will be reduced to 10. The ROM matrix can also be assembled on diodes of the KD522B type, although this will take up much more space.

Instead of resistors R18 - R28, it is desirable to use resistor blocks of types B19-1 or B19-2 of the corresponding rating. When using quartz resonators in the synthesizer to a frequency different from that chosen by the author, it is necessary to rebuild the division factor of the D4 microcircuit using the appropriate wiring of the jumpers so that at pin 23 the pulses follow at a frequency of 1,25 kHz.

A properly configured frequency synthesizer draws no more than 9 - 15 mA of current from a 20 V power supply.

When repeating a radio station with a frequency synthesizer, it is better to change the printed circuit board by redesigning it. This will reduce the size of the entire device. In the author's version, this radio has dimensions of 150 x 70 x 25 mm when powered by batteries.

Author: V. Stasenko (RA3QEJ), Voronezh; Publication: N. Bolshakov, rf.atnn.ru

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