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Digital Object Identifier 10.1109/MMM.2008.919925

86 June 2008

Andrej Rumiantsev

and Nick Ridler

t was during the late 1950s that the

need for reliable measurement, and

therefore reliable measurement

standards, at RF and microwave

frequencies began to emerge. This led

to the introduction of precision coaxial air

lines as primary reference standards of

impedance [1], [2]; see Figure 1. These lines

use conductors made from very-high-con-

ductivity metals and air as the dielectric,

due to the simple and predictable electro-

magnetic properties (i.e., permeability and

permittivity) of air at RF and microwave

frequencies [3]. This ensured that the prop-

erties of these lines were very close to those

of ideal lines [4].

Also during the late 1950s and through-

out the 1960s, much work was undertaken

to develop precision coaxial connectors to

ensure that very repeatable and repro-

ducible measurements could be made at

microwave frequencies [5], [6]. To help

focus this effort, committees were estab-

lished (including an IEEE subcommittee on

precision coaxial connectors [7]) tasked

with producing standards for these preci-

sion connectors. Finally, by the late 1960s,

the first fully automated vector network

analyzers (VNAs) providing high-precision

measurement capabilities were introduced

(e.g., [8], [9]). The stage was now set for

work to begin on introducing reliable mea-

surement assurance techniques for mea-

surements made using VNAs (Figure 2).

However, there were several other key

developments that took place during the 1970s,

1980s, and 1990s that greatly improved the state

of the art of measurements made using VNAs.

These included the introduction of:

• smaller precision coaxial connectors

(beginning with the 3.5-mm connector

[10] and ending with the 1-mm connec-

tor [11]), enabling measurements to be

made over wider bandwidths

• VNA calibration and verification kits

containing high-precision devices suit-

able for calibrating and/or verifying the

performance of the VNAs

• reliable VNA calibration techniques

[including thru-reflect-line (TRL) [12],

line-reflect-line (LRL) [13], etc.)

Andrej Rumiantsev is with SUSS MicroTec Test

Systems GmbH, Germany. Nick Ridler is with the

National Physical Laboratory, United Kingdom.

June 2008 87

• six-port VNAs [14] used by national measurement

standards laboratories [such as the National

Institute of Standards and Technology (NIST) in

the United States and the National Physical

Laboratory (NPL) in the United Kingdom, etc.] to

provide an independent measurement method to

verify the performance of the commercially avail-

able VNAs.

Finally, also by the late 1980s and early 1990s, national

measurement standards laboratories (i.e., NIST, NPL, etc.)

began turning their attention to demonstrating the reliabil-

ity of VNA measurements made on planar circuits (such as

on-wafer measurements) to support the rapidly develop-

ing microelectronics industry. Both NIST and NPL pro-

duced standard wafers [15], [16] that contained the planar

circuit equivalent to the coaxial air line—i.e., precision sec-

tions of coplanar waveguide and/or microstrip transmis-

sion line. These lines provided the reference standards for

calibrating VNAs for on-wafer measurements.

All of the above activities greatly improved the state

of the art for practitioners and users of VNA measure-

ments. Also, in addition to all these activities, much was

done by measurement experts working in industrial,

academic, and government laboratories to establish

traceability and other quality assurance mechanisms for

these VNA measurements. These topics are discussed in

”What is Traceability?” and “Measurement Assurance.”

Systematic Measurement Errors

What Is Calibration and Error Correction?

Calibration is defined as the “set of operations that estab-

lish, under specified conditions, the relationship between

values of quantities indicated by a measuring instrument

or measuring system, or values represented by a material

measure or a reference material, and the corresponding

values realized by standards” [17]. As such, calibration

traditionally involves having an instrument or component

sent away periodically to a standards and/or calibration

laboratory, who then undertake the calibration process.

This often results in a certificate of calibration being issued

that demonstrates the current condition of the instrument

or component.

However, in the context of a VNA, the term calibra-

tion can have at least two different meanings. First, the

traditional concept of calibration can still be applied,

with the VNA being sent away for calibration, typically

every year or so. (Alternatively, some companies offer

periodic on-site calibration, performed by a visiting cal-

ibration specialist.) However, of more relevance to this

article is another form of calibration that is performed

locally, usually each time the instrument is set up and

configured for a given series of measurements. This sec-

ond form of calibration is intended to remove systemat-

ic errors from the instrument hardware (and to take into

account the presence of any accessories that may have

been added to enable specific measurements to be per-

formed) at the required frequencies for the measure-

ments. For example, measurements may be required to

be made in an on-wafer environment. In which case,

first cables need to be connected to the VNA front-panel

connectors, followed by coaxial adaptors, and finally

on-wafer probes (Figure 3). This second form of calibra-

tion will correct for the effects of these added compo-

nents as well as correct the systematic errors in the

VNA. This is why this type of calibration is often

referred to as error correction, and it is this type of cali-

bration that will be discussed in this article.

The demand for increased measurement accuracy

from the VNA can be achieved by improving the hard-

ware, the models used for characterizing measurement

errors, the calibration methods used for calculating

these errors, and the definitions of calibration stan-

dards. For

-parameters, the systematic errors are often

represented using so-called error models of the mea-

surement system (i.e., VNA). The number of error coef-

ficients included in the error model, as well as the type

of error model, depends on

Figure 1. An example of precision reference coaxial air

lines of different length.

Figure 2. A coaxial mm-wave measurement bench based

on the Agilent 8510 VNA. This analyzer was the industry

reference for microwave measurements for many years.

(a) (b)

88 June 2008

Traceability, in the context of a measurement, is

defined as the “property of the result of a measure-

ment or the value of a standard whereby it can be

related to stated references, usually national or inter-

national standards, through an unbroken chain of

comparisons all having stated uncertainties” [17].

Applying this concept to a VNA measurement, the

stated references could be precision air lines (or their

equivalent), the VNA is the transfer device used as

part of the unbroken chain of comparisons, and the

precision connectors enabling these comparisons to

be made within acceptable limits to the uncertainty of

measurement.

The benefit of having a measurement that is trace-

able stems from the fact that it can be used to

demonstrate the equivalence of measurements made

independently of one another. This is of paramount

importance in a customer/supplier relationship where

a common understanding is needed of the parame-

ters that define (or specify) the performance of the

device being bought or sold. Therefore, if two mea-

surements of a quantity are made independently, and

these measurements are both traceable, then their

values will agree to within the stated uncertainties of

the measurements. This is, therefore, an extremely

valuable process that can provide the necessary

underpinning assurance that is needed when operat-

ing within a truly global marketplace where the cus-

tomer and supplier may be located in different parts

of the world.

The vital role that traceability can play was recog-

nized long ago and led to the introduction of national

measurement accreditation schemes so that cus-

tomers and suppliers could fully demonstrate the

quality of their measurements to an independent third

party (i.e., the accreditation body). These days, such

accreditation processes are controlled by international

standards (e.g., [72]), thus ensuring that the accredita-

tion process is itself applied uniformly across all types

of measurements and at all locations around the

world. Most countries maintain a national accreditation

body for this purpose, and these bodies are them-

selves linked through international accreditation orga-

nizations such as the International Laboratory

Accreditation Cooperation (ILAC, www.ilac.org).

When traceability is harmonized within an agreed

system of units (e.g., the international system of units,

SI), then not only is it possible to demonstrate equiva-

lence between measurements of the same quantity,

but it also becomes possible to demonstrate the

equivalence of measurements of different quantities.

This is achieved through the relationship of these

quantities to the so-called base quantities within the

system of units. (In SI, the seven base quantities are

length, mass, time, electric current, thermodynamic

temperature, amount of substance, and luminous

intensity.)

By following the traceability path of a measure-

ment back to its fundamental base quantities, it is

possible to demonstrate the harmonization of the

measurement within the system of units. For example,

a reflection measurement made along a transmission

line can usually be traced back to dimensional mea-

surements, since it is the dimensions of the transmis-

sion line that determine the impedance and therefore

the amount of signal that is reflected by the line. The

base quantity for dimensional measurements is

length. Similarly, for power and noise measurements,

these can usually be related back to heating effects.

Therefore, the base quantity is thermodynamic tem-

perature. In just about all microwave measurements,

the frequency of the measurement needs to be

known. Since frequency is the reciprocal of periodic

time, the base quantity is time.

A key role of a national measurement standards

laboratory (such as NIST, NPL, etc.) is to maintain pri-

mary reference standards of measurement. For exam-

ple, at microwave frequencies, these are usually stan-

dards of power, impedance, attenuation, noise, etc. In

addition, the national measurement standards labora-

tory is tasked with realizing the seven SI base quanti-

ties. By linking these two roles, the national measure-

ment standards laboratory is able to deliver a wide

range of traceable measurements to industry that are

also harmonized within the SI.

The subsequent 'linking' of the capabilities of one

national measurement standards laboratory to others

is achieved through participation in international mea-

surement comparison programs conducted under the

auspices of organizations such as the International

Bureau of Weights and Measures (BIPM,

www.bipm.org) and their consultative committees.

The results from these comparison exercises are ana-

lyzed and placed on a database maintained by the

BIPM that demonstrates the capability of each nation-

al laboratory.

Finally, it is worth mentioning that these days, with

the global accessibility of the Internet, measurement

services that make extensive use of the Internet are

beginning to be developed. These services are starting

to play a role in providing traceable measurements in

a highly efficient manner. For example, a system has

recently been put in place by NPL that uses the

Internet to provide traceability for high-precision mea-

surements using VNAs at any location around the

world [73].

What Is Traceability?

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