Research On Virtual Digital Storage Oscilloscope Based On USB Interface In Photoelectric Test System

This paper develops a high speed digital storage oscilloscope based on USB interface, which is used to construct photoelectric test system. As a peripheral of computer, this oscilloscope can also be used as an independent virtual test instrument. The virtual digital storage oscilloscope has two independent channels, and each channel has good adjustability to analog signal and triggering mechanism. It reaches the high speed of 100MS/s in real-time sampling mode, and a design even makes the equivalent sampling rate high as 100GS/s possible in under-sampling mode. The hardware can be divided into four modules, that is, signal conditioner, data sampling, control and interface.The analog signal conditioner consists of several sub circuits, including attenuators, amplifiers, filters, level shifters, converters of single end signal to differential signal, and in addition there are comparators to provide trigger signal. The combination of them ensures this instrument has both good adjustability to input signal and high measuring accuracy. About data sampling module, the usage of a dual 10-bit, 120MS/s ADC makes this instrument have both high sampling accuracy and fast sampling rate; By connecting two 8-bit programmable delay chips sequently, and setting their time resolutions 2.5ns and 10ps respectively, the design achieves the adjustable equivalent sampling rate from 3.2GS/S to 100GS/s, which is suitable to measure the repeated high speed signals. About control module, a high performance processor plays the key role of control and communication. About the module of interface to computer, USB is adopted because of its characteristics of popularly used, easily installed, conveniently extended and high transfer rate. By implementing all the digital logic function and data storage function with a FPGA, the instrument is greatly enhanced in integration, reliability and flexibility. An user interface programmed with VB is designed to control the instrument and display waveform. The stored data of time domain can be analyzed by a MATLAB program, so that its frequency domain information can be got.By studying the theory of real-time sampling and under sampling, especially by simulating the error factors in detail, the system design is well guided. Furthermore the system and subsystem is tested and analyzed, which shows that the virtual digital storage oscilloscope achieves good performance, and its accuracy in real-timesampling mode even exceeds that of TDS3052B, a slap-up digital storage oscilloscope made by Tektronix Corporation, USA.This digital storage oscilloscope based on USB interface has the advantage of convenience, flexibility and easily using, and its cost is low but performance is good. It can treat up to 10MHz signal in real-time sampling mode, and 50MHz signal in under sampling mode, and can display, analyze, store the sampled data.This paper has the characteristics as follows: First, combining with USB interface, the virtual instruments are easily connected and used. Second, the design of signal conditioner makes this system have good adjustability to input signal. Third, the low noise of signal conditioner together with the high accuracy of ADC ensures this instrument's high accuracy. Fourth, the method of connecting two programmable delay chips in serials makes a serial of questions solved easily. Finally, the usage of a high performance FPGA, which acts as digital logic function and data storage RAM, greatly enhances the integration of this instrument, and ensures its performance as well.