The Design Of Low-Frequency Digital Function Generator

Signal generator is a teaching and research and engineering practice, one of the most important instruments. In the past it made by hardware,system structure is complicated, have a high failure rate, poor maintainability and operability. With the development of computer technology, the signal generator is designed by more computer technology, and it's very different in variety, price and performance, very different. This paper presents a signal generator design and implementation methods, and calls the function signal generator can output sine wave, square wave, triangle wave, and with amplitude modulation, keyboard settings, the frequency display.The main design elements are as follows:(1) Hardware design: 1. VCO circuit design unit; 2. DAC frequency control circuit; 3. DAC phase Control Circuit; 4. Programmable gain amplifier cell design; 5. MCU display and keyboard interface circuit 6. DC power supply circuit design;(2) Software design: SCM software driver design.VCO is the main design elements of this unit, the signal generator produces waveforms with this unit. It can produce different frequency sine wave, triangle wave and square wave. VCO input voltage changes the output frequency of a structure, where we use an integrated voltage-controlled oscillator MAX038, and using D / A for frequency control. MAX038 is a highly integrated precision voltage-controlled oscillator, which can produce frequency-controlled smooth, triangle wave, sine wave and TTL square-wave and other signals.MAX038 can also be controlled by controlling the voltage output waveform of duty cycle, the VCO control voltage and duty ratio control voltage we use TLC5618D / A converter to control, in order to accurately achieve a given output frequency, we use 74HC393 and the internal counter AT89C51 combined output frequencies on a precise measurement and closed-loop feedback control.As a general-purpose signal source, the waveform of the output amplitude should be allowed to fluctuate within a certain range to meet the different experiments. We designed a multi-stage amplifier to meet the needs.In this design in order to change the output amplitude AD811 broadband for the current feedback op amp, the gain bandwidth product of 140MHz,±15V power supply, the case of a gain of +10, -3 dB bandwidth of up to 100MHz, is very suitable for system requirements for broadband amplification, a±12V output swing and output current up to 100mA, fully meet the peak to peak demands, the external circuit is also very simple to avoid the use of transistor amplifier prone to debug difficult circumstances, to prevent self-excitation in the RP-3 times the actual voltage, and does not affect the index, and reliability are increased.To make my door signal source to achieve digital input and output easier to use simple, we use 51 as the system MCU control center here, including 51 single-chip peripheral circuits design and human-computer interfaces. In order to achieve human-machine interface circuit to set the main output frequency, output amplitude, waveform selection function and display the output state, amplitude and waveform information and other functions, for which we design matrix keyboard and LCD display unit.Design of power supply in the machine, we use the most basic series of linear regulated power supply. Using±5V and±12V power supply, so use the device regulator LM7805, LM7905, LM7812, LM7912 voltage regulator as the output transformer used double 12V AC transformer.Software for the MCU system programming language we use C51 in WAVE development environment for editing the design.AT89C51 as the control machine control, to achieve all-digital input and output waveform frequency, amplitude and other information on the visual display in the LCD.Tested, fully able to meet the following specifications: 1. Output frequency range 1Hz ~ 5Mhz; 2. VPP output voltage range of 0 ~ 10V adjustable and have TTL-level output; 3. Output waveform: sine, square, triangle; 4. Output impedance of 50?; 5. Frequency settings and choose the keyboard input waveform; 6. Output state with liquid crystal display ;The overall signal generator design using common components. Simple structure, low cost and high practical value. Practice has proved that the system theory and hardware design is successful.