函数信号发生器外文文献翻译(2)

1.Introduction

Signal generator can be used as a signal source, which provides a known test signal to the circuit. It can be used to measure parameters of interest. In a variety of experimental test applications and processing, the signal source as the excitation source can simulate a variety of test signals, providing the practical needs to the circuit [1].

Signal generator play a very important role in communications, electronic measuring instruments, electronic circuits, automatic control, radar and other electronic systems. With the rapid development of digital technology, there has been high-precision digital-analog converter,digital control method by using a standard reference frequency generated by multiple frequency technology, which is the DDS technology [2]. Arbitrary waveform signal generator can meet the complex fields, user-defined signal, and the combination of computer technology and make it more accurate and stable. With the current development

of electronic technology, the signal generator has a great application.

2. System

We designed a signal generator, which can produce several arbitrary waveforms. We can control the size of amplitude and frequency, providing a variety of signals to the test circuit.

Using AT89S52 micro controller, AD9850 (DDS) and digital-analog conversion technology, we complete hardware and software design. Through the keyboard input module, we can adjust the amplitude and frequency of the waveform. Waveform generation process is achieved by the micro controller. So theoretically we can write a program to generate any desired waveform. SCM generates digital signal. In order to obtain the desired waveform, we should have a single chip changing digital signals into analog signals.

AD9850 of the DDS chip is used to generate the waveform. The frequency data of external input can be transformed to frequency phase control words of chip by simple parallel or serial communication interface with the single chip microcomputer. Amplitude modulation is realized by the AD7520 chip. The system design is shown in Fig. 1.

The system uses SCM to control DDS chip AD9850 which produce the required signal for test instrument. SCM controls the multiplexer. The square wave or sine is DA converter AD7520's voltage reference signal. AD7520's output signal, after conditioning and amplification, is a 15V peak frequency signal. After complementary push-pull amplifier circuit, we finally get the necessary frequency signals. To ensure the signal stability peak and frequency when the load changes, the amplifier circuit in the amplifier outputs are added with the appropriate negative feedback.

3.Hardware

The circuit design is mainly two aspects including hardware and software design. The hardware part is the signal generation circuit, MCU control circuit, amplitude control circuit and display circuit. Software is mainly composed of main program and interrupt routines.

The core controller is AT89S52. ATMEL AT89S52 produced by the U.S. is a low-voltage, high performance CMOS 8 bit micro controller chip with repeated contains 8k byte erasable read-only Flash program memory and 256 bytes of random access data

memory. AT89S52 can operate down to 0Hz static logic to support two kinds of software-selectable power-saving mode [3].

In this design, SCM is the most important core component. Its work time is 12MHz. The internal flash stores the working procedures. Through the keyboard input, the circuit can know what is required to generate the waveform, the amplitude and frequency. We can also adjust any waveform of amplitude and frequency.

Fig. 1. The system scheme

In this system, we use an independent-type keyboard. The design uses a matrix keyboard, using the software scan button access to key information as necessary and achieving with software programming. This can save hardware resources, simplifying circuit design. By the matrix keyboard we can easily set on a variety of micro controllers.

In this design, P0 port is connected with the LCD display, AD9850 and the AD7520 IC chip is for data transmission. Because the original I/O ports are not enough, we need micro controller chip on the expansion of other peripheral interfaces. In this circuit, we use the 8255 chip, which is a programmable parallel I/O interface chip.

The highly integrated frequency synthesizer AD9850 is a typical use of DDS technology products. AD9850 uses advanced CMOS process. Its power supply is only 155mw at 3.3V. The extended industrial temperature range is from -40 to +80 centigrade degree with 28-pin Shrink Small Outline surface mount form.

AD9850 DDS system includes programmable and high-speed comparators, all-digital programming to achieve the control of frequency synthesis. AD9850 can produce a frequency and phase is programmable control of the analog sine wave output when connected to the precision clock and the frequency of write. The sine wave frequency signal can be directly used as the source or the internal conversion of high-speed

comparator for the square wave output. In the 125MHz clock, the 32-bit frequency control word can AD9850 output frequency resolution of 0.0291Hz [4].

In this circuit design, the control signal input from the D0-D7 into the register. In rising edge of W_CLK (P3.0) the first byte is loaded and the pointer moves to the next input register. A continuous 5 rising edge, it stops working. Then when the rising edge FQ_UD (P3.1), the data is loaded into the frequency/phase register. At this time the DDS output frequency and phase updates. Then the pointer reset and the circuit waits for the next frequency/phase control word input.

Display Module. Considering the actual situation, we adopt dot matrix LED to realize the display. LCD1602 can display the output waveform frequency, amplitude, and the specific type. LCD1602 and the PA port of 8255 chip are connected. Three terminals RS, RW, E connected the P2.7-P2.5 of micro controller respectively. The software can control the type of waveform display and waveform frequency.

RS is a register choice is to select the data register high, low, the instruction register. RW is the read write signal line, height is read, and write operation is low. When the RS and RW are low, the LCD can be written instruction or display the address. When RS is low and RW is high, the LCD can be read. When RS is low and high-RW, the LCD can be written data. E pin is the energy side. When the E pin is from high to low, the LCD execute commands.

AD9805 outputs constant wave amplitude. We use a D/A converter AD7520 to achieve amplitude adjustment.

The internal resistor network of AD7520 chip is composed of a programmable variable gain amplifier. AD7520 integrates the resistance network of 10 analog switches which are programmable. Internal resistor network with the AD7520 programmable amplifier are used as gain of the circuit [5].

Magnification can be adjusted between 1 to 1024, as long as the change BIT1-BIT10 the corresponding bit changes logic state can control the magnification. When BIT1-BIT10 is 3FFH, the magnification is 1 time, which is 0dB. When 98H, the magnification is10 times, which is 20dB. To realize Programmable gain, we connect PC port of 74LS373 and P2 port of SCM with AD7520 pin BIT1-BIT10. Through program, the micro controller control I/O port output state to complete the programmable gain [5].

The system requires a 5V DC power supply to all circuits. DC power supply is generally composed of power transformer, rectifier, filter circuit and the voltage regulator circuit.

The role of power transformer is to transform 220V AC voltage power into AC voltage required by rectifier filter circuit. The role of rectifier is to transform AC voltage into DC voltage single pulse. Filter circuit is to filter out the rectified output voltage ripple. After the rectification circuit output, the waveform still contains a large AC component which will affect the normal operation of the load circuit. We need to filter through the filter capacitor ripple. Filter circuit is with capacitor filter circuit.