HP Modular Instrumentation User Manual

Calibration and Specification Considerations

When Using Modular Instrumentation

Speaker/Author: Michael Dobbert

Agilent Technologies

1400 Fountaingrove Parkway, Mail Stop: 2USA

Santa Rosa, CA 95403

E-mail: [e-mail]

Author: Mark Catelani

Agilent Technologies

1400 Fountaingrove Parkway, Mail Stop: 3LSW

Santa Rosa, CA 95403

E-mail: [e-mail]

Abstract

Modular instrumentation, such as PXI or AXIe modular instruments, offers significant configuration
flexibility, plus interchangeability, speed, and size advantages when it comes to deploying measurement
systems. However, the architecture that enables these advantages also presents unique challenges when
calibrating modular instruments. Calibration often occurs outside of the use environment. For modular
instrumentation, this may mean performing calibration on a module with a different chassis and its
related electronics. Additionally, the module’s ambient environmental conditions depend upon chassis
fan speed, the use of slot blockers and EMC filler panels and the presence of other modules. The
operating software and CPU for modular instruments are contained outside the module in an external
computer, which may not travel with the module for calibration. Modular instrumentation may require
multiple modules configured together to provide measurement capability. This may require calibration
on the set of modules as a system or, a method to relate system level performance to the calibrated
performance of individual modules. These issues affect both the calibration and the calibration report
and influence how manufacturers may define specifications for modular instrumentation. This paper
examines these issues in detail and considers both in situ calibration and calibration performed outside
the use environment. Recommended is information to be included on the measurement report that is
unique to calibration of modular instrumentation. Addressed are the requirements for assuring the
ability to make traceable measurements using calibrated modular instrumentation.

1. Introduction

Modular instrumentation refers to instruments designed to conform to one of several modular platform
definitions. This includes instruments that conform to the PXI or AXIe specifications. Modular
instruments plug into a compatible system chassis and the calibration of individual modular instruments
is performed either in situ, or at a calibration laboratory. A goal of the calibration is to enable the
modular instruments to make accurate and traceable measurements. Figure 1 shows an example of a
PXI modular chassis with several modules inserted into it and filler panels covering the unused chassis
slots.

2013 NCSL International Workshop and Symposium

1

Figure 1. PXI Modular Instrument.

Modular platforms provide distinct advantages for testing. The modular system chassis supplies
mechanical support, power, cooling, and communication/control services to the modular instruments.
Since these functions and services are common to, but separate from the individual modules, the
modular instruments are themselves compact. This allows for small floor space requirements, plus the
ability to quickly add, remove or replace modular instruments.

Taking full advantage of the modular platform, however, does present unique challenges when
calibrating individual modular instruments. When removing a modular instrument from the chassis and
sending it to the calibration laboratory, not all of the functional components that affect measurement
accuracy travel with the instrument. Additionally, the operating environment for an individual
instrument in the use location may be significantly different from the operating environment in the
chassis at the calibration laboratory. Finally, specifications may cover the accuracy of multiple modules
working together as a system.

The remainder of this paper examines the calibration issues associated with modular instruments and
provides recommendations for dealing with them.

2. Environmental Conditions

Almost without exception, instrument specifications include the environmental conditions over which
the instrument shall meet its accuracy specifications. This is true for all instruments, not simply modular
instruments. ISO/IEC 17025 [1] places a requirement on calibration laboratories to ensure the
environmental conditions of the laboratory meet the requirements of the instrument calibration and do
not adversely affect the results1. What does this mean for modular instruments? Even while holding the
ambient conditions outside the modular chassis constant, the ambient conditions for individual modular

ISO/IEC 17025:2005 section 5.3.1.

2013 NCSL International Workshop and Symposium