VHF frequency synthesizer. Scheme, description

Free technical library

ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
Free library / Schemes of radio-electronic and electrical devices

VHF frequency synthesizer. Encyclopedia of radio electronics and electrical engineering

Free technical library

Encyclopedia of radio electronics and electrical engineering / Civil radio communications

Comments on the article

Recently, quite a few synthesizers for 144 MHz band equipment have appeared in print. The version of the synthesizer proposed in this article is interesting in that it uses an inexpensive LM7001J synthesizer chip used in household radio receivers.

The synthesizer is designed to work in FM transceivers with an intermediate frequency of 10,7 MHz. It provides the formation of a signal with a frequency of 133,3 ... 135,3 MHz in the receive mode and 144 ... 146 MHz in the transmit mode with a frequency grid step of 25 kHz. It provides the ability to scan in receive mode over the entire operating frequency range. The synthesizer has a non-volatile memory for three user frequencies. It also contains 9 repeater channels (R0-R8). In the transmit mode, the synthesizer performs frequency modulation of the RF signal. The synthesizer is fed with a voltage of 8 ... 15 V. The consumption current is not more than 50 mA. The level of the high-frequency signal at its output at a load of 50 ohms is at least 0,1 V.

Synthesizer operation

When the supply voltage is applied, the synthesizer immediately starts working at the frequency recorded in the 1st memory cell. While the supply voltage is below 4,2 V, pin 1 (RES) of the DD1 microcontroller will have a logic zero level generated by the DA1 supervisor chip, which generates a reset signal. When this value is reached, the level will jump to "1". This eliminates the distortion of RAM information that occurs with a smooth increase in the supply voltage. The HG1 indicator shows the frequency at which the synthesizer will operate in transmit mode. To switch to the frequency recorded in one of the memory cells, you must press the corresponding button "1" - "3" (S1 - S3). Each press of the "UP" or "DN" button (S6 and S7) shifts the operating frequency up or down by 25 kHz, respectively. Pressing the "SCAN" button (S5) activates the scanning mode in the entire operating frequency range. When a carrier appears on the channel, scanning is suspended and resumes a few seconds after it disappears. The signal to stop scanning is the logic zero level applied to the "SCAN" output of the synthesizer. To exit the scanning mode, just press one of the "UP", "DN", "SCAN" buttons.

When you press the "REP" (S4) button, the synthesizer switches to the mode of working with repeater channels. The transition through the channels is carried out with the "UP" and "DN" buttons. In this case, the indicator directly displays the channel number (R0 - R8). Scanning in repeater mode is not provided. This mode is exited by pressing the "REP" button again.

To write the frequency to a memory cell, you must dial the frequency value on the indicator, press the button with the cell number and, without releasing it, press the "REP" button. When the power is turned off, the information recorded in the memory cells is retained.

Operating principle

The internal structure of the LM7001 chip, according to the documentation, allows you to build a frequency synthesizer for frequencies of 45 ... 130 MHz with a step of 25, 50 or 100 kHz. However, several copies of this microcircuit available to the author worked without problems at the amateur band frequencies of 2 meters. You can learn more about this chip from [3] or on the Internet at sites with technical information (for example, at [4]).

The electrical circuit diagram of the synthesizer is shown in fig. one.

(click to enlarge)

The synthesizer chip is controlled by the DD1 AT90S1200 microcontroller. This type of controller is chosen by the author as one of the cheapest on the market. Frequency indication is made with the help of LCD indicator used in imported telephones and caller IDs. The DD1 microcontroller, when the keys are pressed, processes commands, outputs data to the indicator and controls the operation of the DA2 synthesizer via a three-wire bus (pins 6, 7, 8 DD1). It is clocked from the internal DA2 oscillator operating at a frequency of 7,2 MHz. To transfer the synthesizer to the transmission mode, it is necessary to apply a zero logic level to the "TX" output of the synthesizer.

The control voltage generator (VCO) is assembled on a VT3 transistor according to the "capacitive three-point" scheme. A VD5 varicap is used as a frequency tuning element. The VCO inductor consists of two parts. In receive mode, both parts of the coil "work", while transmitting - only one (large] part. Open drains of three keys (BO1 - WHO) on field-effect transistors that are part of the LM7001 microcircuit are connected to its outputs 7-9 The state of these keys changes when the corresponding control bits are changed. The microcircuit is programmed so that during reception, the key BO2 is closed, and the WHO is open. At the same time, the diode VD4 is closed and the coil L1 is fully turned on. When switching to the transmission mode, the key B02 opens, WHO closes, the diode opens VD4 and capacitance C7 grounds the alternating current of the smaller part of the coil.The buffer stage of the VCO signal is assembled on the transistor VT4.

The composite cascade, assembled on transistors VT1 and VT2, acts as an inverting amplifier for the PLL error signal and an active filter.

In transmit mode, the synthesizer signal is modulated in frequency by the speech signal applied to the "MOD" input of the synthesizer. The level of frequency deviation of the output signal depends on the amplitude of the speech signal. The amplitude of the speech signal must be such as to ensure the deviation of the output signal within the required limits. It is advisable to select its value already in the assembled radio station. The quality of the transmitted signal can be assessed using a closely spaced monitoring receiver.

The supply voltage of the indicator HG1 (1,5 V) is removed from the divider R1VD1 - VD3. To match the levels of logical signals applied to the indicator, use resistive dividers R2 - R5.

Construction and details

The entire structure is assembled on a single printed circuit board with dimensions of 148x50 mm, made of one-sided textolite (Fig. 2).

VHF frequency synthesizer

The drawing of its trace is shown in fig. 3, and the arrangement of elements - in fig. 4

VHF frequency synthesizer

The design used constant capacitors type K10-17 or KM. Trimmer capacitor C3 - type KT4-23. Electrolytic capacitors C14 and C15 - type K50-35. Fixed resistors - type C2-23, C1-4. To rebuild the VCO, the author used the KV134AT-9 varicaps available to him. Instead, you can successfully use any high-frequency low-voltage varicaps with an initial capacitance of 18-22 pF. The DA1 supervisor chip can be replaced with an imported analog PST529D. A ten-digit LCD module with an HT1611 controller from Holtek was used as an indicator. The inductor L1 has 0,5 and 2,5 turns (counting from the "cold" end) with a 0,45 mm wire on a 4 mm mandrel. Choke L2 is wound on resistor R24 and contains 15 turns of wire with a diameter of 0,15 mm.

Setting

After assembling the synthesizer, it is necessary to unsolder the upper (according to the diagram) output of the resistor R17 and apply a voltage of +2,5 V to it from an external source. Turning on the synthesizer, it is transferred to the transmission mode and the frequency of the VCO is measured at the "OUT" output using a frequency meter. By shifting and pushing the turns of most of the inductor L1, they ensure that the frequency of the generated signal is as close as possible to the value of 145,5 MHz. After that, the synthesizer is switched to receive mode and the frequency value is again controlled. By changing the shape of the smaller part of the coil, the frequency generated by the VCO is set to be close to 134,8 MHz. At the end of the VCO frequency adjustment, the coil turns are fixed with paraffin or wax, the output of the resistor R17 is soldered to the board. Next, a frequency meter is connected to the output of the synthesizer. Adjusting C3 ensures that the frequency of the generated signal on any channel differs from the required one by no more than a few hundred hertz. The final stage is to check the operation of the synthesizer in all modes. The control voltage on the varicap in the operating frequency range should be within 1,5 ... 4,5 V.

Microcontroller programming

To program the AT90S1200, the author used the RopuRgod2000 programmer developed by Claudio Lanconelli. The latest software releases, programmer diagrams for various types of microcontrollers and detailed instructions for use can be found in [5], and useful information on using the programmer can be found in [1]. The programmer contains a base unit connected to the COM or LPT port of the computer, and replaceable adapters for each family of microcontrollers.

However, if you plan to program only a certain type of microcircuit, for example, AT90S1200 and AT90S2313, then you can use a simplified adapter for the COM port (Fig. 5).

VHF frequency synthesizer

Data for programming the microcontroller and RAM

Using the synthesizer

During the operation of the synthesizer, in order to reduce parasitic pickups that reduce the signal quality, it must be placed in a shielded compartment. The design proposed by the author (the location of the microcontroller, synthesizer chip and VCO on the same board) is not always convenient. If necessary, you can place the synthesizer chip and VCO on a separate board, and also use a different VCO circuit. It is not necessary to change the microcontroller firmware program.

Literature

Dolgiy A. Development and debugging of devices on the MK. - Radio, 2001, No. 6, pp. 24-26; 2001, no. 7. p. 19-21.
Matantsev A. Controller and synthesizer. - Radio, 2001, No. 3, p. 62, 63.
Reference sheet. - Radio, 2003, No. 4, p. 49, 50.
Website in the description section.
Website .

Author: A. Temerev (UR5VUL), Svetlovodsk, Ukraine

See other articles Section Civil radio communications.

Read and write useful comments on this article.

<< Back

Latest news of science and technology, new electronics:

Artificial leather for touch emulation 15.04.2024

In a modern technology world where distance is becoming increasingly commonplace, maintaining connection and a sense of closeness is important. Recent developments in artificial skin by German scientists from Saarland University represent a new era in virtual interactions. German researchers from Saarland University have developed ultra-thin films that can transmit the sensation of touch over a distance. This cutting-edge technology provides new opportunities for virtual communication, especially for those who find themselves far from their loved ones. The ultra-thin films developed by the researchers, just 50 micrometers thick, can be integrated into textiles and worn like a second skin. These films act as sensors that recognize tactile signals from mom or dad, and as actuators that transmit these movements to the baby. Parents' touch to the fabric activates sensors that react to pressure and deform the ultra-thin film. This ... >>

Petgugu Global cat litter 15.04.2024

Taking care of pets can often be a challenge, especially when it comes to keeping your home clean. A new interesting solution from the Petgugu Global startup has been presented, which will make life easier for cat owners and help them keep their home perfectly clean and tidy. Startup Petgugu Global has unveiled a unique cat toilet that can automatically flush feces, keeping your home clean and fresh. This innovative device is equipped with various smart sensors that monitor your pet's toilet activity and activate to automatically clean after use. The device connects to the sewer system and ensures efficient waste removal without the need for intervention from the owner. Additionally, the toilet has a large flushable storage capacity, making it ideal for multi-cat households. The Petgugu cat litter bowl is designed for use with water-soluble litters and offers a range of additional ... >>

The attractiveness of caring men 14.04.2024

The stereotype that women prefer "bad boys" has long been widespread. However, recent research conducted by British scientists from Monash University offers a new perspective on this issue. They looked at how women responded to men's emotional responsibility and willingness to help others. The study's findings could change our understanding of what makes men attractive to women. A study conducted by scientists from Monash University leads to new findings about men's attractiveness to women. In the experiment, women were shown photographs of men with brief stories about their behavior in various situations, including their reaction to an encounter with a homeless person. Some of the men ignored the homeless man, while others helped him, such as buying him food. A study found that men who showed empathy and kindness were more attractive to women compared to men who showed empathy and kindness. ... >>

Random news from the Archive

TG700 test modules for video test generation 22.01.2003

TEKTRONIX Corporation has released a family of test modules TG700 to generate video tests.

It includes analog (AVG7) and wideband analog (AWVG7). AVG7 is a multi-format generator of test signals, drawings, texts. AWVG7 has a higher resolution.

Other interesting news:

▪ Smart surveillance camera with tracking function

▪ Non-melting ice cream

▪ OLED panel 0,97 mm thick

▪ Acnodes PCM8019 Rugged Embedded Computer

▪ Kamchatka deer were microchipped

News feed of science and technology, new electronics

Interesting materials of the Free Technical Library:

▪ Telephony site section. Article selection

▪ article Hidden loop. Tips for the home master

▪ article What is dodecaphony in music? Detailed answer

▪ article General information about the human body and its interaction with the environment