Digital Oscilloscope Automatic Calibration System

With the development of electronic technology, digital oscilloscopes have greatly expanded their working capabilities with digital technology and software. The shortcomings of early products such as low sampling rate, large dead time, and low screen refresh rate have been greatly improved. The previously difficult to observe complex signals such as modulation signals, communication eye diagrams, and video signals are becoming easier to observe. Digital oscilloscopes can calculate and analyze data, and are particularly suitable for capturing all the details and abnormal phenomena generated in complex dynamic signals. Therefore, they have been widely used in scientific research and industrial production. In order to make the oscilloscope work in a qualified state, it is an urgent task for test engineers to regularly, quickly, and comprehensively calibrate the oscilloscope to ensure its value traceability.

Manual calibration is inefficient and prone to errors. The calibration of each oscilloscope requires test engineers to read a lot of information; the automatic test system has the characteristics of accurately and quickly measuring parameters, intuitively displaying test results, and automatically storing test data, which are not achievable by traditional manual testing. Using an automatic test system to realize program-controlled calibration of oscilloscopes will be the trend of instrument calibration.

GPIB, VXI, and PXI are standard buses for automatic test systems. GPIB has won the recognition of users for its stable performance, convenient operation, and low price. GPIB is selected as the bus of the test system here.

1 Hardware Design

The hardware of the GPIB-based digital oscilloscope automatic calibration system consists of a GPIB controller, FLUKE5500A, a digital oscilloscope to be calibrated, a PC, and peripherals such as a printer. The system composition is shown in Figure 1.



1.1 GPIB interface controller

1.1.1 GPIB bus

GPIB is a practical instrument interface system developed by Hewlett-Packard in the late 1960s and early 1970s. Because it is very convenient to control test instruments and has a high transmission speed (1Mbps), GPIB was defined as the IEEE488 standard in 1975 and revised to IEEE488.1-1987 in 1987. The GPIB bus is a digital 24-pin parallel bus, with 8 ground and shield wires, and the other 16 TTL level signal transmission lines, including 8 data lines, 5 interface management lines and 3 data transmission control lines. GPIB uses 8-bit parallel, byte serial, asynchronous communication mode, and all bytes are transmitted sequentially through the bus.

GPIB system devices have three attributes: controller, speaker and listener. The actual device has one, two or three of them. As a controller, it can address and specify the device with speaker attribute connected to the bus as a speaker and the device with listener attribute as a listener, including specifying itself. The speaker can send data to other devices through the bus. The listener can receive data sent by the speaker from the bus. Generally speaking, in the GPIB system, the computer is the controller, with the three attributes of speaking, listening and controlling. To avoid bus conflicts, IEEE488 stipulates that there can only be one speaker at a time, but there can be several listeners at the same time. Since the operating speeds of various devices in the GPIB system may vary greatly, in order to ensure that multi-line messages can be transmitted bidirectionally, asynchronously and reliably, three handshake lines are set in the GPIB bus, namely the data valid line, the unready to receive line and the unreceived data line.
1.1.2 BC-1402-2 interface controller

The GPIB controller used in this system is the BC-1401-2 USB-GPIB interface controller developed by Beka Technology. It has a USB interface, which converts the USB bus into a GPIB bus to operate GPIB instruments. Its features are: fully compliant with IEEE488.1 and IEEE488.2 international standards, supporting PCI, USB, and Ethernet industrial standards; the data transmission rate is 900kbps, which is suitable for high-speed data transmission between PC and instruments; it provides a set of I/O GPIB operation function libraries, whose functions are the same as those of the ES1400 series interface controllers of the ISA bus; it provides a set of virtual instrument software architecture VISA (Virtual Instrument Software Architecture) function libraries that comply with the VPP specification, which enables all applications developed using VISA functions to be replaced with different models of GPIB interface controllers from different manufacturers without any modification of the application; the interface controller can be used to compile test programs in multiple languages ​​such as C/C++, VC++, VB, LabView, LabWindows/CVI, HP-VEE, Delphi, etc., which is convenient and flexible.

1.2FLUKE5500A

FLUKE5500A is a high-performance multi-function calibrator from Fluke Corporation of the United States. It can calibrate a variety of instruments such as handheld and desktop multimeters, oscilloscopes, oscilloscopes, power meters, electronic temperature meters, data loggers, power harmonic analyzers, process calibrators, etc. FLUKE5500A provides three standard interfaces: GPIB (IEEE-488), RS-232, and 5725A; in terms of safety, it meets the standards of IEC1010-1 (1992-1), ANSI/ISA-S82.01-1994, and CAN/CSA-C22.2NO.1010.1-92; FLUKE5500A output voltage can reach 1100V, current output can reach 11A, and it can provide multiple waveforms and harmonics of DC voltage and current, AC voltage and current, and output two voltages at the same time, or one voltage and one current, to simulate power, resistance, capacitance, thermocouples, and RTD. The oscilloscope calibration kit also provides a stable sine wave, fast edge, time mark and amplitude signal.

1.3 Master PC

PC, as the "master controller" of the system, controls FLUKE5500A and the oscilloscope to be tested by issuing commands to the GPIB interface controller, mainly including the following aspects: instrument initialization, reset, instrument parameter setting; command FLUKE5500A to generate standard signals, while the oscilloscope to be tested displays; read/save instrument data and transmit it to PC, etc.

2 Software Design

2.1 Selection of Software Platform

Software is the core of this digital oscilloscope automatic calibration system. Whether the software and hardware can work stably and harmoniously is the basis for the system to quickly and reliably calibrate the digital oscilloscope. This system uses the stable Windows 2003 Server operating system, SQL Server 2005 (development version) database and Visual.NET 2005 as the development platform, and C/C++ as the programming language. At the same time, NI's LabWindows/CVI7.0 is used as the driver for some programs. At the same time, MAX (Measurement & Automation) is used as the IVI driver configuration program.

2.2 Key technologies VISA and IVI

VISA is the I/O interface software standard established by the VXIplug&play Alliance. The purpose of establishing VISA is to ensure that instruments from different manufacturers and with different interface standards are compatible with each other, can communicate and exchange data. Its notable features are: VISA is implemented using advanced object-oriented programming ideas; it is a super-integration of all current instrument interface type function functions, and it is very concise, with only more than 90 functions; VISA, as a standard function, has nothing to do with the instrument's I/O interface type, which facilitates program porting. For driver and application developers, VISA library functions are a set of functions that can be easily called to control various devices such as GPIB, VXI, PXI, etc.

IVI (Interchangeable Virtual Instrument) is an instrument driver programming interface launched by the IVI Foundation to further improve the executable performance of instrument drivers, achieve true instrument interchangeability, and realize that applications are completely independent of hardware. The IVI system consists of five parts: IVI class driver, specific driver, IVI engine, IVI configuration utility, and IVI configuration information file. The class driver implements the encapsulation of upper-level unified functions and faces the operator, while the specific driver completes the communication with the specific instrument. The test program calls the class driver, and uses the class driver to call the specific driver to achieve the independence of the test program and hardware. The IVI engine completes the state cache, instrument attribute tracking, and mapping functions from the classification driver to the specific driver. The IVI configuration utility uses the software MAX to create and configure IVI logical names. In the test program, the operation is mapped to the specific instrument and instrument driver by transmitting the logical name to a classification driver initialization function. The IVI configuration information file records all logical names and mapping information from the class driver to the specific instrument driver. Its structure is shown in Figure 2.

2.3 Test software architecture

2.3.1 Test software module

The test software is divided into three parts: test data management module, test parameter management module, and test program module. The test data management module manages the instrument calibration date, calibration personnel, calibrated items for specific instruments, calibration data, etc. Test parameter management manages the calibration items and standard values ​​of specific instruments in the database. The test program module calls the corresponding test instrument for testing based on the test parameters selected by the user on the soft panel, compares the test data with the standards in the database, and determines whether it is qualified.

2.3.2 Test software structured process

After the system self-test is started, the calibration operator selects/enters the instrument model to be calibrated on the software interface. The program calls the corresponding calibration items, standard values ​​of the items to be tested, and the connection diagram between the instrument to be tested and the FLUKE5500A and GPIB controller in the database according to the instrument model. The inspector connects the instrument according to the connection diagram (FLASH animation). After confirming that the connection is correct, check whether there is an IVI driver. After installing the driver, run the MAX configuration tool. After completing the configuration, you can run the corresponding test program, save the test results to the database, and print the corresponding pass/fail report. The flow chart is shown in Figure 3.



2.4 Developing IVI Drivers

For IVI instruments, the manufacturer will provide IVI drivers. Only a small amount of code needs to be written to implement the instrument calibration. The main program is simple and easy to manage. The goal of the IVI Foundation is to support 95% of the instruments. The calibration of digital instruments based on IVI technology will be the inevitable path for instrument calibration.

However, not all instruments support IVI. For non-IVI instruments, use the IVI driver development wizard in LabWindows/CVI to encapsulate all the underlying commands in the instrument program control command tree into a series of high-level functions with image panels to complete the development of the IVI driver and make it an IVI instrument. Its characteristics are that the workload of developing IVI drivers in the early stage is large, but the workload of developing and maintaining test programs in the later stage is small.

2.5 Database management

Database management mainly includes six modules: user management, model management of the instrument to be tested, verification project management, verification report management, verification project index management, and data query.

3 Application examples

The test system constructed by this method was used to calibrate IVI instruments such as HP54815, and IVI drivers were developed for non-IVI instruments such as XJ4321. The five items of vertical sensitivity, transient response, steady-state response, scan time factor error, and scan time factor linear error were calibrated, and the verification results were saved and the verification certificate was printed. Practice has proved that the verification process has become fast and simple; the results of automatic verification and manual verification are consistent.

The digital oscilloscope verification system introduced in this paper uses GPIB as the bus, and comprehensively uses IVI technology and database technology to realize the automatic verification of digital oscilloscopes. It has the characteristics of easy operation, strong scalability, and good working stability. It provides a reference for the establishment of a comprehensive digital instrument automatic verification system for power meters, spectrum analyzers, arbitrary waveform/function generators, and digital multimeters.