Design of Multi-channel Signal Generation System Based on Virtual Instrument

0 Introduction

Arbitrary waveform generator is a commonly used signal source, which can be widely used in scientific research, production practice, teaching practice and other fields. Traditional waveform generators mostly use analog circuit technology, generally composed of discrete components, with complex circuit structure, large size, poor stability and accuracy, and can only generate a few simple waveforms such as sine wave, square wave, triangle wave, etc., and it is difficult to generate more complex waveform signals.

With the improvement of microprocessor performance, most new waveform generators have adopted software simulation instead of simply using discrete components to build circuits, thus effectively solving the shortcomings of traditional signal generators. The emergence of virtual instruments is a revolution in the history of instrument development, representing the latest direction and trend of instrument development, and is an important field of information technology.

This paper will combine virtual instrument technology to research and design a multi-channel arbitrary waveform generator, as well as signal acquisition and analysis. Its hardware development platform uses ADLINK's DAQ2206 data acquisition card, and the software development platform is NI's LabVIEW software.

LabVIEW (Laboratory Virtual Instrument Engineering Workbench) is an innovative software from National Instruments, known as the language of "scientists and engineers". LabVIEW provides a convenient and easy graphical design and development integrated environment for designers who are not familiar with text language programming to build computer instrument systems in the field of measurement and control.

LabVIEW integrates all functions that meet the communication requirements of GPIB, VXI, RS-232 and RS-485 protocol hardware and data acquisition cards. It also has built-in library functions that facilitate the application of software standards such as TCP/IP and ActiveX. It can be used to easily build your own virtual instrument, and its graphical interface makes the programming and use process lively and interesting.

The graphical programming language in LabVIEW is also called "G" language. When programming with this language, program code is basically not written, but flow charts are used instead. This method makes use of terms, icons and concepts familiar to technicians as much as possible, so LabVIEW is a tool for end users.

In the design of waveform generator, the use of LabVIEW simulation software can not only realize the convenient way of instrument programming and data acquisition, but also greatly improve the work efficiency in principle research, design, testing and realization of instrument functions.

1 System Overall Plan

The system is mainly composed of two parts: hardware and software. The hardware part includes data acquisition card and PC, and the software part is mainly a virtual instrument test system developed based on LabVIEW software. The overall scheme of the system is shown in Figure 1.

Insert the system data acquisition card DAQ2206 into the computer and PCI expansion slot, and the computer can generate several commonly used communication modulation signals (such as 2ASK, 2FSK, 2PSK) through LabVIEW software, and then convert these signals into analog signal output through DAC. The data acquisition card can transmit the analog signal to the oscilloscope, and the oscilloscope will display the waveform of the corresponding signal. In addition, the acquisition card can also receive signals from the waveform generator, and then transmit them to the computer through ADC, and analyze and display the signals through LabVIEW software.

The system data acquisition mainly supports NI's DAQ2206 card through LabVIEW simulation software. Insert the DAQ2206 acquisition card into the computer slot and set the relevant parameters. Then use LabVIEW for programming and development to realize the data acquisition function.

This data acquisition card is a hardware module of the virtual instrument. Because it is related to the interface with the PC bus, the hardware module should have a relatively standardized standard; at the same time, in order to facilitate the compilation of the underlying driver software, it should also have a standard interface to be transparent to the driver. This interface includes reading the hardware status information and setting the hardware. The computer interface can provide a standard hardware connection for the hardware module and the operating system.

The instrument driver module in the system can use the hardware module to provide a standard interface to the system to realize the control of the hardware and complete the management of the hardware; at the same time, it can provide a standard calling interface to the upper-level module (i.e. the virtual instrument software development platform module) to make the high-level software design independent of the hardware.

The virtual instrument software development platform is a user platform for users to design and generate a single virtual instrument or a virtual instrument test system. It should provide a graphical programming design environment to facilitate user development and shorten the development cycle. The data analysis module library and interface control library are the basis of LabVIEW and LabWindow/CVI. You can also develop your own virtual instrument platform in accordance with the idea of ​​standardization, drawing on the modules of the data analysis module library and interface control library, and using high-level programming languages ​​such as VC++, VB, and C++Builder. However, this module must provide an interface to the upper layer, which is also the development interface provided to the virtual instrument or virtual instrument test system.

For users, the design of the software panel directly determines the instrument's ease of use as well as its functionality and expandability.

2 Signal Generation

2.1 Generation of baseband signals

The baseband signal is a random unipolar rectangular pulse sequence. The baseband signal can be designed by the following G language program.

First, we write an array that can be arbitrarily encoded (0 or 1), and then form a pulse of a certain pulse width by looping each codeword, and then form an arbitrary pulse signal, which is the baseband signal. Figure 2 shows the block diagram of baseband signal generation. [page]

2.2 Generation of 2ASK Signal

2ASK is a binary amplitude keying signal. Usually, the analog amplitude modulation method can be used to form a 2ASK signal. A binary amplitude keying signal can be expressed as the product of a unipolar rectangular pulse sequence and a sinusoidal carrier. Figure 3 shows the overall block diagram of the 2ASK signal generation. It can be seen from the figure that it is obtained by multiplying the baseband signal and the sinusoidal carrier signal.

2.3 Generation of 2FSK Signal

2FSK is a binary frequency shift keying signal. Here, the keying method is used to realize the 2FSK signal, that is, the switch circuit controlled by the rectangular pulse sequence is used to select two different independent frequencies. Its generation block diagram is shown in Figure 4.

First, define two carrier signals f1 and f2, and then select the two carriers through a selector. If f1=1 Hz, f2=4Hz, and the number of cycles of the rectangular pulse is 5, then the frequency of the rectangular pulse is 0.2 Hz. Generally, when the pulse is 0, the carrier signal with a frequency of 1 Hz is selected; when the pulse signal is 1, the carrier signal with a frequency of 4 Hz is selected.

2.4 Generation of 2PSK Signal

When programming a 2PSK waveform generator, you only need to make some modifications based on 2FSK. However, the reset signal at the signal input terminal usually needs to be assigned a value. This requires adding a Boolean control variable. Because only by resetting the signal after each cycle can the required 2PSK signal be obtained. If this Boolean control variable is not added, phase shift keying cannot be achieved. [page]

3-signal analog input

3.1 Analog input related parameters

Analog input is the most basic function of acquisition. It generally converts an analog signal into a digital signal through a multiplexer (MUX), amplifier, sample-and-hold circuit, and A/D. The performance and parameters of A/D directly affect the quality of analog input. When designing, you should choose the appropriate A/D according to the actual required accuracy.

3.2 Analog input related modules

Figure 5 is the initialization module of the data acquisition card, and Figure 6 is the A/D value conversion module. They are both related modules of analog input.

4 analog outputs

4.1 Analog output related modules

Analog output usually uses an intermediate analog output module. The data acquisition card can be initialized first to specify the signal output channel, then the buffer is established, and finally the multi-channel signal information is written out. Figure 7 shows the initialization block diagram of the analog output signal.

4.2 Writing multiple analog output signals into buffer

This paper uses the circular writing method to write two signals into the buffer of the data acquisition card. Figure 8 shows the circular sampling block diagram.

5 Conclusion

Arbitrary waveform generator is a commonly used signal source, which can be widely used in scientific research, production practice and teaching practice. The emergence of virtual instruments is a revolution in the history of instrument development, representing the latest direction and trend of instrument development, and is an important field of information technology. Among the new waveform generators, waveform generators based on virtual instruments have developed rapidly in recent years and will gradually replace traditional signal generators and become the mainstream in the research field. Virtual instrument technology is not only the direction of instrument development in the 21st century, but will also gradually replace traditional hardware electronic instruments, so that thousands of traditional instruments can be integrated into the computer system.