VHF FM tuner. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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VHF FM tuner. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / radio reception

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VHF broadcasting stations are one of the main sources of business, music, entertainment and other information. The number of radio stations is constantly growing, in Moscow, for example, there are several dozen of them. The quality of the signal transmitted in this range is comparable to the sound quality of a CD player. The advent of specialized frequency synthesizers has greatly simplified the design and manufacture of digital tuners. Microcontroller control allows you to significantly expand the consumer qualities of the tuner at relatively low hardware costs. Changing functions comes down to writing an appropriate control program for the microcontroller.

When developing the devices, a block design principle was applied, which makes it possible to manufacture and adjust individual units independently of each other. The presence of connectors makes it possible to combine various nodes into a single system with the desired parameters and greatly facilitates the debugging of the end device. The tuner consists of a control unit, a radio receiver and a timbre unit. Let's consider the operation of each block in more detail.

Rice. 1 (click to enlarge)

The control block diagram is shown in fig. 1. Its basis is the DD2 microcontroller, thanks to its use, it is possible to control four devices simultaneously via the I2C bus. Operating modes are displayed on the two-line alphanumeric LCD HG1. Push-button and remote control via IR channel using the RC5 standard remote control, as well as via a serial asynchronous interface is possible.

All port D lines are dedicated to tuner control. Lines PD0-PD6 are connected to the buttons SB1 - SB7, and PD7 - to the output of the IR receiver for remote control of the tuner. By software, all lines of this port are configured for input, in addition, the internal resistors of the microcontroller are connected to the lines PD0-PD6. LCD control is carried out along the lines РВ2-РВ7. To save the resources of the microcontroller, a four-bit indicator control scheme was used, which made it possible to use the remaining bits of the port for other purposes. The PB0 and PB1 lines control the I2C bus, with the help of which the frequency synthesizer, the tone block and the DS1 chip are controlled - a non-volatile memory that stores the tuner settings. The choice of control mode is a short press on the SB3 button. The SB1 button decreases, and SB2 increases the value of the adjustable parameter, such as volume.

Rice. 2 (click to enlarge)

The diagram of the radio receiving part is shown in fig. 2. It is assembled on a DD1 frequency synthesizer chip and a VHF FM receiver chip DA1. Both microcircuits are included according to standard schemes. The SAA1057 chip is one of the first digital synthesizers developed by Philips for consumer radio equipment. Its distinguishing feature is control over a three-wire bus. This is one of the early variants of the I2C bus. Therefore, to control the synthesizer, it was necessary to use an additional signal CS0, which is generated on the PC0 line of the microcontroller in the control unit. The synthesizer includes a programmable counter (prescaler), elements of the PLL system and an oscillator with an external quartz resonator, the installation accuracy and frequency stability of which determine the accuracy and stability of the synthesizer frequency. The receiver is assembled according to the superheterodyne circuit and operates at a local oscillator frequency below the signal frequency. Program the synthesizer with two 16-bit words. The first contains data on the tuning frequency, the second - service information. The transfer of information is no different from the typical I2C protocol, except for the presence of an additional signal CS0 (DLEN).

All modes of operation and settings of the tuner are stored in the DS1 chip (see Fig. 1) with the same I2C serial interface. Recording of parameter values occurs automatically at any access to the menu (just like it is done in TVs and music centers). At initial power-up, the microcontroller reads the contents of the memory. The first channel is selected by default. In the absence of data corresponding to the parameters of the connected timbre block, the average values of all the main parameters are automatically set: volume, timbre, balance. In receiver mode, buttons SB4 and SB6 switch memory cells (for 100 stations). If you press the SB5 button, the frequency tuning mode is turned on (it is displayed on the indicator), and the SB4, SB6 buttons tune from 88 to 108 MHz and back. When you press the SB5 button again, the selected frequency is memorized for the current station. Each memory cell is set to 88 MHz by default. In station tuning mode, only volume control is available.

VHF FM tuner
Fig. 3

The tone block (Fig. 3) is assembled on a TDA8425 (DA1) microcircuit, included according to a typical circuit. It includes a switch for two stereo inputs and controls for volume, balance, bass and treble. All software control, via the I2C bus. The main functions of the tone block:

- the ability to select for each channel as a signal source one of the two inputs;
- Pseudo-stereo, spatial stereo, linear stereo and forced mono modes;
- volume control in each channel and balance;
- tone control;
- mute mode (MUTE) and turn it on (for this, the SB7 button is used).

Controlling a microcircuit via the I2C bus consists in writing certain information to its internal registers. The control word format is

S_SLAVE ADDRESS_A_SUBADDRESS_ A_DATA_A_P, where S - starting combination, SLAVE ADDRESS - device address (code) (for TDA8425 processor - 1000010);

A - control word field separator (high level issued by the device as a response to a correctly received data byte); SUBADDRESS - address of the parameter control register; DATA - parameter value setting data;

P - stop combination, signaling the end of the transfer of the control word.

The control word is transmitted by the control unit every time a parameter needs to be changed. But first you need to address the chip itself. To do this, the first byte with the device address is sent to the microprocessor.

For remote control of the tuner, you can use any television remote control with the RC5 protocol (Philips, LG, etc., working on this protocol). The remote control uses only five buttons: TV, MUTE, SLEEP, VOL, CH. The purpose of the buttons is as follows:

MUTE - turn off the sound (corresponds to the SB7 button);

SLEEP - mode selection (corresponding to the SB3 button);

VOL - adjustment of the "more-less" parameter (corresponds to the buttons SB1 and SB2);

CH - station selection (corresponds to buttons SB4 and SB6).

VHF FM tuner
Fig. 4

The elements of the control unit are mounted on a printed circuit board made of fiberglass foiled on one side with a thickness of 1,5 ... 2 mm, the drawing of which is shown in fig. 4. The tone block is assembled on a single-sided board with a thickness of 1.1,5 mm, the drawing of which is shown in fig. 5. For the radio-receiving part, a printed circuit board (Fig. 6) made of fiberglass 2 mm thick, foil-coated on both sides, was used. All parts are mounted on one side, and the second is left metallized and used as a common wire. Some of the leads of the parts are soldered directly to the printed conductors. The conclusions that are installed in the holes are soldered on both sides.

VHF FM tuner
Fig. 5

VHF FM tuner
Fig. 6

Fixed resistors MLT, S2-23, tuning resistors - SP5-2, oxide capacitors - imported, the rest - K10-17. Microcircuits are installed in the panel. Quartz resonator - any suitable one, for example, HC-49U or HC-49S, L2 - EC24 series choke with an inductance of 10.100 μH, the remaining coils are wound with PEV-2 wire 0,5 mm on a mandrel with a diameter of 2,5 mm and contain L1 - 12 (with tap in the middle), L3 - 12, L4 - 10 turns. Connectors - series pLd double row with a pitch of 2,54 mm. To connect the nodes, multi-wire flat cables are used. The power supply must provide a stabilized voltage of 12 V at a current of up to 100 mA. When using a powerful final UZCH, the maximum current of the power supply should be increased in accordance with the UZCh current consumed. We can recommend UZCH on the TDA1552 chip, it is easy to use with a minimum of external attachments and the necessary built-in overvoltage and short circuit protection functions. Buttons - self-returning angular series TC-02xx. The configuration of the microcontroller during its programming is shown in fig. 7.

VHF FM tuner
Fig. 7

To configure the receiving part, you must first "run" it without a synthesizer. To do this, the SAA1057 chip is removed from the panel (or not temporarily mounted on the board). A constant voltage in the range of 9 V is applied to the connection point of resistors R11 and R0.12. To do this, you can use a variable resistor by connecting it to a common wire and a 12 V power line, and the middle output to resistors R9, R11. It is advisable to temporarily install a capacitor with a capacity of several microfarads between the middle terminal of the resistor and the common wire. The receiver must tune in to stations on the entire band. If necessary, by changing the inductance of the coil L3 (shifting and pushing the coils) set the lower limit of the range, and coil L4 - the sensitivity of the receiver.

The program for the microcontroller can be downloaded from ftp://ftp.radio.ru/pub/2013/06/Tuner.zip.

Author: S. Bashirov

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