Download this article in .PDF format
This file type includes high resolution graphics and schematics.

In contrast to manual calibration methods, an automated calibration approach developed by ATE Systems provides the means for calibrating a VNA-based measurement system at two or more ports and across 2000 test points at an intermediate-frequency (IF) bandwidth of 3 kHz. This automated calibration approach (Fig. 1) makes possible an in situ calibration of a VNA-based ATE system, without removing a DUT or the calibration standards. It eliminates the errors that can result from manual calibration methods, and can provide the levels of accuracy possible with through-reflect-line (TRL) calibration approaches.

1. This block diagram shows how the automatic calibration solution is connected to a VNA-based test system and enables a reference plane to be established at the DUT without constantly changing calibration standards.

This in situ calibration process also removes any inaccuracies resulting from test cables shifting position due to flexure. The automated calibration process requires very little time, allowing it to be performed more frequently (on a daily basis or as needed). The speed of the calibration approach increases the availability of the test system for production-line measurements and reduces the effective cost per DUT test.

The automated calibration solution supports measurements on devices and components with two or more ports—it can be configured for four or more ports depending upon a DUT, and the measurement reference planes can be established right at the DUT. It works with systems based on commercial VNAs and includes an instrument correction module (ICM) that mounts directly onto the front panel of the analyzer and one or more fixture characterization modules (FCMs) for connection to a DUT. FCMs are connected to an FCM controller which coordinates switching and multiplexing for multiport calibrations (Fig. 2). Once this solution is attached to the test system, there is no need to connect a calibration kit, so the chances for measurement errors due to using the wrong calibration standard or applying too much torque during the connection are eliminated.

2. This is an example of a measurement system using the automatic calibration solution.

One of the benefits of using an automated calibration approach is the convenience of programmable software. For ATE Systems’ automated calibration solutions, the control operating software, which can be loaded into the measurement analyzer (VNA) or into the system controller, is based on Standard Commands for Programmable Instruments (SCPI) commands. It is also compatible with Virtual Instrument Software Architecture (VISA) code and drivers and includes instrument wrappers for use with LabVIEW/CVI test software from National Instruments.

The software can run TCP/IP over Ethernet and incorporates an Internet browser interface to configure the controller module. The programmable software supports automated calibration routines using Tru Cal calibration software from Tru-Cal Metrology. It also features an Application Programming Interface (API) for use with all ATE calibration algorithms based on standard Windows 32 (WIN32) dynamic-link-library (DLL) code. The software, which is built upon rudimentary C data types for maximum compatibility, can be called from any programming language/IDE that supports DLL access, such as C, C++, LabVIEW, CVI, or Virtual Basic.

Of course, while proper calibration techniques can minimize systematic errors in a measurement setup, they do not address random errors. It is difficult to eliminate these errors, which originate from such sources as noise and environmental effects, but they can be minimized. Noise issues such as electromagnetic interference (EMI) can be reduced by means of shielding materials, and maintaining a narrow operating temperature range can reduce temperature effects on the measurement environment. In addition, by employing averaging and narrow IF bandwidths during calibrations and measurements, high measurement accuracy can be achieved and maintained. The use of automated calibration systems and techniques can certainly simplify the process of maintaining that high measurement accuracy.

Roy Zohrabian, Business Development Manager

ATE Systems, 85 Rangeway Rd., Building 1, North Billerica, MA 01862; (978) 362-1850.

Download this article in .PDF format
This file type includes high resolution graphics and schematics.