Design of digital FM radio using single chip microcomputer + CXA1019S + phase-locked loop BU2614

Abstract: This article uses digital phase-locked frequency synthesis technology to form the radio's electrical tuning part, completing the radio's tuning, channel selection, search and storage functions. This article focuses on the FM frequency modulation circuit composed of the radio integrated chip CXA1019S produced by SONY, the phase-locked loop circuit composed of the frequency synthesizer chip BU2614, and the keyboard, display and storage circuit composed of the MCS-51 series microcontroller and its peripheral circuits.

Chapter 1 Introduction

The development of radio has gone through a process from discrete components to integration, but the tuning circuit and local oscillator circuit, as important components of the radio, have always adopted the traditional capacitor and inductor manual tuning method. In recent years, with the rapid development of radio communication technology, phase-locked loop and frequency synthesis technology have been widely used in various fields. Because the phase-locked loop has tracking characteristics, narrow-band filtering characteristics and no residual frequency difference in the locked state. Therefore, the use of a phase-locked loop in frequency synthesis technology can generate an oscillation signal source with high frequency and accuracy. Now, using the frequency generated by this oscillation signal source as the tuning frequency and local oscillator frequency of the radio circuit, digital radio can be realized. Using a single-chip microcomputer to control the frequency division number in the phase-locked loop can change the output frequency of the oscillation signal source to achieve the purpose of tuning. The requirements of this design mainly include the following aspects:

1. Receive FM signal frequency range 88MHZ~108MHZ.

2 The modulation signal frequency range is 100HZ~15000HZ, and the maximum frequency deviation is 75KHZ.

3 Maximum undistorted output power ≥ 100mW (load impedance 8Ω).

4 Receiver sensitivity ≤1mW.

5 Image rejection performance is better than 20Db.

6. It can realize digital automatic channel search, manual channel adjustment, channel storage and frequency display functions.

Chapter 2 Scheme Design and Demonstration

FM radio generally includes: antenna, front-end input circuit, mixer, local oscillator, intermediate amplifier, demodulation, amplification and input, etc. In this design, the high-frequency amplifier, mixer, intermediate amplifier, demodulation and other circuits use the FM/AM radio integrated chip CXA1019S produced by SONY; automatic tuning, program-controlled search, radio carrier frequency display and other functions are completed by the phase-locked frequency synthesizer chip BU2614, MCS-51 series single-chip microcomputer and corresponding peripheral circuits. The use of dedicated chips can make the entire system small in size, light in weight, reliable, sensitive and low in power consumption.

2.1 Design of the whole circuit solution

This design uses frequency synthesis technology to complete the electronic tuning of the radio. The frequency division ratio is changed by communicating with the microcontroller through the serial port of BU2614. The frequency divider and frequency detector inside BU2614 are used together with the local oscillator VCO of CXA1019S to form a digitally controlled phase-locked loop. The frequency division ratio is changed to change the receiving frequency. Channel selection, frequency display, and channel storage are completed by the microcontroller AT89C52 and MAX7219, 93C46 chips. The system block diagram is shown in Figure-1:

2.2 Design of each circuit scheme

1) Radio unit There are many integrated chips for radios on the market, but in order to meet the performance requirements of this design, we use the integrated chip CXA1019S produced by Sony. This chip is used in portable radios and helmet radios. It has the advantages of high receiving sensitivity, good image suppression performance, few peripheral components, and high output power. When FM, VCC=5V, the working current is 5.3mA; when VCC=6V and RL=8ohms, the output power is 500mW.

2) Phase-locked frequency synthesis unit The receiving frequency range of FM radio is 88M~108MHZ, so the highest frequency of the selected frequency synthesizer chip must be able to reach 110MHZ to meet the requirements. Common frequency synthesizer chips now include MC145151, MC145157, MC145158, etc., but their highest working frequency can only reach 30MHZ. If these chips are used to form a phase-locked frequency synthesizer of about 100MHZ, they must be used in conjunction with MC12009 and MC12013 dividers. The more high-frequency lines pass through, the more susceptible they are to interference. Therefore, one chip should be used as much as possible to complete the phase-locked frequency synthesis function. The phase-locked frequency synthesis tuning integrated chip BU2614 produced by ROHM can reach a maximum frequency of 130MHZ, which fully meets the requirements. In addition, the chip has a high-sensitivity RF amplifier and supports IF counting function.

3) Display unit Most of the commonly used display interface circuits are composed of chips such as 8155 and 8279. These chips need to occupy P0 and P2 ports when connected to the microcontroller. In addition, the dynamic scanning method occupies a large amount of internal system resources of the microcontroller. In order to simplify the peripheral circuit of the microcontroller, we use MAXIM's serial 8-bit digital static display chip MAX7219 to form a 6-bit static display module, which only occupies three port lines of AT89C52 to complete the display function.

4) Keyboard circuit Since this design uses many keys, we adopt the method of separately identifying function keys and number keys, that is, the function keys use the query method, and the number keys use the coded dynamic scanning method. This can not only reduce the time occupied by scanning, but also simplify the program.

5) Power-off data storage unit According to the requirements of this design, the machine has the function of saving the stored radio stations after power failure. There are many types of EEPROMs on the market, such as 2764A, 2864A, etc., which are parallel EEPROMs, large in size and do not have the function of saving data after power failure. Compared with parallel EEPROM, serial EEPROM is small in size, cheap, and has simple circuit connection. For example, serial EEPROM 93C46 is an electrically erasable programmable read-only memory with online data erasing and rewriting functions, which can meet the requirements of saving data after power failure. In addition, the serial form can save the port resources of the microcontroller.

6) Program operation monitoring unit In order to enhance the reliability of program operation, it is necessary to monitor the program operation status to prevent the program from jumping into a temporary dead loop, causing the entire system to be completely paralyzed. Therefore, it is necessary to set a watchdog circuit (WATCHDOG circuit) in the circuit to monitor the operation of the system. Currently, the commonly used watchdog circuits include both hardware-composed WATCHDOG and pure software-composed WATCHDOG. However, the pure software WATCHDOG system requires the setting of advanced interrupt subroutines, which occupy more internal resources of the microcontroller and will inevitably affect the operation speed of the entire machine. If a hardware-composed WATCHDOG system is used, its hardware part is completely independent of the CPU, which will greatly improve its reliability.

7) Power supply unit Since BU2614, CXA1019S and the microcontroller system require a 5V power supply voltage, and the varactor diode requires a voltage above 9V, if a single +5V power supply is used, a DC-DC module must be used to boost the voltage to +12V.

Chapter 3 Hardware System

3.1 Receiver Circuit

   The CXA1019S chip and its peripheral typical application circuit provided by this project are used. The signal input from the antenna is filtered by an 88MHZ-108MHZ bandpass filter and sent to the CXA1019S for high-frequency amplification, mixing, intermediate frequency amplification, and frequency discrimination processing to demodulate the audio signal. This circuit is based on the CXA1019S typical application circuit without the AM part, as shown in Figure 2:

                        figure 2

The high-frequency signal is sent from the antenna to the 13th pin (FM high-frequency input) of the CXA1019S chip through the BPF filter, and high-frequency amplification is performed inside the chip. The amplified signal is selected by L1, C6, C5 and VD1 connected to the 10th pin. The capacitance of the varactor diode is changed by changing the reverse bias voltage of the varactor diode VD1 to achieve the purpose of frequency tuning. L2, C7, C8 and VD2 connected to the 8th pin form the FM local oscillator frequency selection network, and the local oscillator frequency is also changed by adjusting the reverse bias voltage of the varactor diode VD2. The selected FM radio signal is mixed inside the chip, and the mixed 10.7MHZ tuning signal is output at the 15th pin and sent to CF (10.7MHZ ceramic filter) through the R1 (330) resistor. After frequency selection, it is sent to the 18th pin of the chip for FM intermediate frequency amplification. The amplified FM signal is demodulated internally, and the demodulation network is connected to the ceramic bandpass filter (10.7MHZ) at both ends of DICF at pin 3. The demodulated audio signal is output from pin 24 and directly coupled to pin 25 via capacitor E4. It is then sent to the speaker from pin 28 after internal audio power amplification. The volume is controlled by sliding the volume potentiometer. When the sliding end of the potentiometer changes, the DC voltage changes accordingly, thereby achieving the purpose of controlling the volume. The relevant theoretical calculations are as follows:

(1) Calculation of band coverage coefficient