Design of simplex wireless calling and data transmission system

This paper implements a wireless call system with simplex voice and data transmission functions based on ATMEGA16L single-chip microcomputer. The voice and data transmission of the master station is realized through the encoding circuit, single-chip FM transmission circuit and high-frequency power amplifier circuit; the voice and data reception of the slave station is realized through the decoding circuit, FM demodulation and voice power amplifier circuit. The encoding and decoding are realized by the MSK modulation mode modulation and demodulation chip MSM6882; the display circuit of the master and slave uses LCD liquid crystal display, and the input circuit uses PS2 keyboard scanning.

1. Overview

At present, the application fields of wireless voice and data communication are constantly expanding, and the application forms are also becoming more diversified. Such as mobile communication systems, intelligent transportation systems, remote control networks, etc. This design completes a simplex wireless call system to realize simplex voice and data transmission services from the master station to multiple slave stations. The master station transmits a voice signal or text message. Its transmission frequency is 36MHz, and the peak power of the transmission is adjustable to achieve low-power long-distance transmission. The call or broadcast function to any slave station can be selected. The sending and receiving of text messages uses a black and white LCD display, supports Chinese character display, supports graphic display, supports high-speed automatic screen refresh, has a cold backlight function, can view the screen in the dark, and has a friendly interface.

It can be conveniently applied to short-distance communication, such as community communication, office communication or broadcasting system. With a little improvement, it can complete half-duplex and full-duplex communication, expanding its application range.

2. System Design

This design is divided into data encoding and decoding module, wireless transmission module, wireless receiving module and control module. The data encoding and decoding module adopts minimum shift keying (MSK) modulation. The data modem chip MSM6882 communicates with the control part through the serial port to realize the conversion of data between digital signals and analog signals. The wireless transmission module adopts the MC2833 single-chip frequency modulation circuit. The wireless receiving module is completed by the MC3362 secondary frequency conversion demodulation circuit. The control part is completed by the single-chip microcomputer ATmega16L, which performs data channel selection control, call address and mode control, keyboard input control and data display and storage functions. The system block diagram is shown in Figure 1:

Figure 1 System overall block diagram

3. System Hardware Design

1. Design of wireless transmission module

The transmitting part of this design is based on the MC2833 single-chip FM transmitting integrated circuit produced by MOTOROLA. It consists of a microphone amplifier, a variable reactor, a radio frequency oscillator, an output buffer, and two auxiliary transistors. The specific circuit is shown in Figure 2:

Since this system works at a frequency of 36MHz, we use a 12MHz crystal as the fundamental crystal. L1 is used to compensate for the reactance of the modulator and fine-tune the output frequency. The value is 4.7uH. The modulation signal is added from pin 3 for frequency modulation. The oscillator output frequency is amplified by the buffer and multiplied three times by the external frequency selection network, and then output from pin 9 through the internal Q1 and Q2 amplifiers. That is, the output RF is 36MHz. The parameters of the LC frequency selection network can be obtained by

If you want to increase the transmission power, you can choose the variable gain amplifier AD603 developed by AD company with low noise, high bandwidth and good stability.

2. Design of wireless receiving module

The function of a wireless receiver is to successfully demodulate the desired signal.

At the same time, the design of the receiver must also consider reducing costs and power consumption to a minimum. The single-chip integrated FM receiver chip MC3362 launched by Motorola has the characteristics of low noise, low power consumption, good image selectivity and wide dynamic range. The MC3362 chip contains all the circuits for secondary frequency conversion, and its operating voltage can be in a wide range of 2.0V-6.0V. The working principle is that the RF signal received by the antenna is amplified and output by the input matching circuit, and then mixed with the first local oscillator signal in the first mixer on the chip, and the frequency is converted to 10.7 MHz, and filtered by the ceramic filter connected to the 19th terminal, and sent to the input terminal (18th terminal) of the second mixer. The 10.7MHz output of the first mixer and the 10.245 MHz signal of the second local oscillator are mixed for the second time on the chip to obtain a second intermediate frequency signal of 455 kHz, and the 455 kHz ceramic filter connected to the 5th terminal is selected and connected to the limiter. The audio signal obtained after limiting and phase shift product frequency discrimination is output on the 13th terminal. There is a comparator between terminals 14 and 15 to detect the zero crossing point of FSK modulation. The demodulated data signal is output at terminal l5.

The first local oscillator frequency is 36M-10.7M=25.3M, which is composed of an LC oscillation circuit. When high precision is required, a phase-locked loop circuit is used; the second local oscillator frequency is 10.7M-0.455M=10.245M, which is composed of an external crystal and an oscillation capacitor. The second local oscillator frequency can be adjusted by changing the oscillation capacitor.

Pin 12 is connected to the middle frequency for tuning. The specific circuit is shown in Figure 3.

3. Design of encoding and decoding modules

The modulation modes of modems mainly include frequency shift keying (FSK), relative phase shift keying (DPSK), etc. Among them, minimum shift keying (MSK) modulation is a better one among FSK modes. We use the MSK (minimum shift keying) coding modulation and demodulation chip MSM6882 produced by OKI Company of Japan. The device contains receiving, transmitting and clock generating circuits, and the data transmission baud rate can be selected between 1200bps and 2400bps. The chip integrates two functions of modulation and demodulation, and adopts the data transmission mode of analog serial port, with its own clock inside, less MCU resource occupation, easy interface design and high system stability.

In this design, a 3.6864MHz crystal oscillator is used. Considering the small amount of data transmission and the susceptibility of the wireless channel to interference, the data transmission baud rate is selected as 1200bps to ensure correct transmission. In this mode, the ST pin of MSM6882 generates a 1.2kHz square wave, latches the data of the SD pin on the rising edge, and modulates it. 1 is modulated into a 1.2kHz sine wave, and 0 is modulated into a 1.8kHz sine wave. In the receiving slave station, we use the mode corresponding to the master station to enable the data to be received correctly, input the modulation signal from the Ai pin, and the data is output from the RD and pin RT.

The 1.2kHz square wave is used as the receiving synchronization clock, and the data is output on the falling edge. The application wiring diagram and schematic diagram of MSM6882 are shown in Figures 4 and 5.

4. Software Design

This design uses Atmel's mega16L single-chip microprocessor, which integrates a 10-bit A/D converter and 1KB of on-chip RAM. It works in a pipeline mode and has a fast instruction execution rate. It has rich internal resources and is convenient for input and output of data. It has strong arithmetic operation functions, flexible software programming, and a large degree of freedom. Various algorithms and logic controls can be implemented by software programming. The use of 8MHz clock frequency can meet the requirements of 1.2kbs data transmission and processing. The software development platform uses CodeVisionAVR, uses C language, and uses AVR Studio 4 for software debugging. The program mainly includes keyboard and LCD drive, EEPROM storage, sending and receiving, verification and anti-interference.

The flowchart is as follows:

Figure 6 Transmitter flow chart

Figure 7 Receiver flow chart

After the transmitter starts the analog channel, it resets the LCD and initializes the MSM6882. For voice services, the digital modulator is turned off. If it is a data SMS, digital modulation and keyboard decoding are started. The edited information and sending mode can be displayed through the LCD. You can select any slave station or broadcast. After confirmation, send it through the send key. For the convenience of application, the SMS content will be written into the EEPROM of ATMEGA16L for storage. The data in the EEPROM can be called out after each power-on reset.

The receiving end receives data in the same mode as the transmitting end. After initialization, the address is recognized and the service is determined. If it is a voice, the speaker is turned on. If it is a text message, the receiving interrupt service program is entered: data is received at the falling edge of the receiving synchronization clock, and the data is received and saved starting from the bit after the synchronization code. After receiving a frame of data, it is displayed after verification. If the verification is wrong, the subroutine is jumped out and other programs are continued.

V. Results Analysis

After testing, voice and data can be accurately transmitted over 10 meters when the transmission power is 3mW. Any slave station or broadcast can be paged. This design still has memory storage function after power failure, and the power can be adjusted within the range of 20mW. The innovation of this design is that the data transmission is completed with a single-chip digital modulation and demodulation chip, and low-power long-distance transmission of voice is realized.