Synthesizer frequency range 144 MHz. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Synthesizer frequency range 144 MHz. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Civil radio communications

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The master oscillators and "smooth range" local oscillators, which smoothly "creep" in frequency with temperature changes and sharply "jump" with power surges, go further and further into the past. Attempts to stabilize the frequency with a quartz resonator made it impossible to tune it. And only with the advent of simple and cheap frequency synthesizers it became possible to have a signal with "quartz" stability at almost any desired frequency. True, the frequency tuning is now not smooth, but abrupt, which in some cases (for example, when working on VHF with FM) is even useful. We offer the reader a description of a simple frequency synthesizer in the 2-meter range.

This frequency synthesizer is intended for use in VHF radio stations with FM operating in the 144 MHz band and having a receiver with an intermediate frequency of 10,7 MHz.

The synthesizer has four functional modes of operation:

"Channel". In this mode, the frequency band of the output signal of the synthesizer during transmission is 144500 ... 146000 kHz, and during reception it is 10700 kHz lower than the frequency of the signal during transmission, i.e. 133800 ... 135300 kHz. The mode is designed for simplex communication in one channel, when both correspondents operate at the same frequency.
Step. It is designed to select and set the frequency tuning step: 2,5, 12,5,25 kHz.
"Repeater". Here, the frequency band of the output signal of the synthesizer during transmission is 145000 ... 145200 kHz, and during reception it is 10700 - 600 kHz lower than the frequency of the signal during transmission, i.e. 134900 ... 135100 kHz. This mode allows you to receive signals from repeaters that respond at a frequency shifted 600 kHz up, i.e., in the frequency band 145600 ... 145800 kHz.
Scan. This mode searches for stations operating in the frequency band 144500 MHz ... 146000 kHz, with the step selected in the "Step" mode.

The minimum number of frequency synthesizer control buttons provide quick access to all functional modes of operation. The disadvantages of the device include only a relatively long frequency setting time, within one second.

The frequency synthesizer is based on a Motorola MC145170-2 chip [1]. The "service" was implemented on an Atmel AT90S8515 microcontroller [2]. The user interface is the buttons SB1 - SB3 and a nine-digit seven-segment indicator HG1.

The synthesizer circuit is shown in fig. 1. The signal from the output of the voltage controlled generator (VCO) is fed through the "Input Fgun" to pin 4 of the DD1 microcircuit. Here it is compared with the reference frequency received from the built-in oscillator on the ZQ1 quartz resonator. Error signals from the phase detectors of the microcircuit are fed to a low-pass filter (LPF) assembled on the elements R1-R6, C1, C4, C11, and then through the "Uynp Output" to the VCO frequency control elements. Thus, the phase locked loop (PLL) is closed.

(click to enlarge)

The DD1 microcircuit is controlled by the DD2 microcontroller via three wires from the outputs PB4, PB5 and PB7. From the output PB7 (pin 8) comes the synchronization signal "sck", from the output PB5 {pin 6) - the data signal "data", from the output PB4 (pin 5) - the signal to allow the replacement of old data with newly arrived "enbl".

The signals from the outputs of ports A and C of the microcontroller are decoded by microcircuits DD3, DD4 and displayed on the alphanumeric display HG1. The execution of commands is confirmed by sound signals reproduced by the piezoelectric sound emitter BF1.

In the author's version of the device, a VCO from the commercially available radio station "Estakada" was used, its diagram is shown in fig. 2. The VCO output for the receiver mixer should output a signal at a frequency of 10,7 or 11,3 MHz (depending on the mode) lower than for the transmitter. Therefore, it is necessary to provide a VCO tuning range of 133800 ... 135300 kHz for the receiver and 144500 ... 146000 kHz for the transmitter. With such a large separation of transmit and receive frequencies, it turned out to be advisable to use two separate generators assembled on transistors VT1 VT3. Depending on the mode of operation of the radio station, key stages on transistors VT2 and VT4 supply power to one or the other generator.

(click to enlarge)

The output signals of the generators are fed to two buffer stages, assembled on transistors VT5 and VT6. They are constantly on and, in addition to decoupling the generators from the rest of the stages of the radio station, provide a bias voltage to the bases of the generator transistors. It is removed from the divider R13R14 and through the transistor VT5, which is an emitter follower for the bias voltage, it enters the bases of the generator transistors.

The connection diagram of the VCO with the synthesizer, including the power circuits, is shown in fig. 3. Since the required supply voltages for the synthesizer and VCO are different, 5 and 8 V, respectively, two separate voltage regulators are used: the first microcircuit DA1 type 78L05, and the second, assembled on discrete elements VT1, VD1, R1 and C1.

144 MHz Frequency Synthesizer

When you first turn on the manufactured synthesizer, power can only be supplied to it after the microcontroller is programmed, the synthesizer is assembled and carefully checked for the absence of installation errors. After turning on the power, the indicator should show "Ch 145500", which means that the synthesizer is operating in the "Channel" mode at a frequency of 145500 kHz. In this mode, you can change the frequency using the SB2 - "Down" (down), SB3 - "Up" (up) buttons in steps of 2,5 kHz, 12,5 kHz, 25 kHz (default 2,5 kHz). Scanning the range with the same selected step is carried out by simultaneously pressing the buttons SB2, SB3, the frequency capture occurs when a positive pulse is applied to the "Capture" input.

After a single press of the SB1 button, the device switches to the "Repeater" mode. The display will show "R0 145600", which means tuning to the frequency of the zero repeater channel with a frequency separation of reception and transmission of 600 kHz. In this mode, it is possible to change the frequency using the SB2 "Down" and SB3 "Up" buttons in 25 kHz steps.

The next pressing of the SB1 button puts the synthesizer into the "Step" mode - setting the frequency change step, it is selected with the SB2, SB3 buttons. The display will show "Step 2,5". Then another press of the button puts the synthesizer back into "Channel" mode.

Let's give two examples.

First:

"Ch 144550" - you worked at a frequency of 144,550 MHz;
"Step 2,5" - frequency grid step before changing;
"Step 12,5" - the same, after the change;
"Ch 144550m - return to the operating frequency.

Second:

"R2 145650" - you worked at a frequency of 145,650 MHz;
"Step 12,5" - frequency grid step is set to 12,5 kHz;
"Ch 144550" - switching to the "Channel" mode at a frequency of 144,550 MHz;
"R0 145600" - return to the zero repeater channel.

When you have verified that the controller program is working properly, you can proceed to test the DD1 chip. Connect the oscilloscope to pin 3 of the DDI chip, where a 10 MHz RF signal should be present. If it is, it means that the microcircuit is working normally and correctly perceives the controller data. Using a frequency meter, set the frequency of the ZQ1 crystal oscillator to 10000 kHz using capacitor C4.

After that, connect the VCO according to the diagram in fig. 3, and to "output 1" - a frequency counter or a control receiver. To turn on the receiving frequencies, supply voltage + 5 V to the input of the VCO "ON Rx." To turn on the transmission frequencies, apply + 5 V to the input of the VCO "ON. Tx." and connect the "in TX" input to the common wire. If the VCO frequency is different from the desired one, check the VCO frequency overlap again, on the control receiver in this case (in the absence of capture) the background will be heard in a fairly wide frequency spectrum.

In the synthesizer, instead of the ALC318 indicator, it is permissible to use any similar one, the inductance of the inductor L1 is not critical, it can be more than 10 μH. In the VCO, the chokes L1, L3, L5 are wound on ferrite rings with a diameter of 7 mm and have 10 turns of wire with a diameter of 0,2 mm, the L6 coil contains 2 turns, L2, L4, L7 - 6 turns of silver-plated wire with a diameter of 0,5 mm. These coils are frameless - they are wound on a mandrel with a diameter of 4 mm.

microcontroller firmware file

Literature

MC145170: PLL Frequency Sintesizer with Serial Interface - e-motorola.com/ files/rf if/doc/data sheet/MC145170-2.pdf.
AT90S8515: 8-bit RISC microcontroller - atmel.com/dyn/resources/prod_documents/DOC0841.pdf.

Author: A. Chetovich (EU6AI), Republic of Belarus, Glubokoe, Vitebsk region.

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