Data processing: measuring – calibrating – or testing – Calibration or correction system – Circuit tuning
Reexamination Certificate
2002-10-15
2004-12-28
Barlow, John (Department: 2863)
Data processing: measuring, calibrating, or testing
Calibration or correction system
Circuit tuning
C324S601000
Reexamination Certificate
active
06836743
ABSTRACT:
TECHNICAL FIELD
The invention relates to electronic test equipment. In particular, the present invention relates to calibration of electronic test equipment systems such as vector network analyzers.
BACKGROUND ART
A network analyzer is a test system that characterizes the performance of radio frequency (RF) and microwave/millimeter wave DUTs in terms of network scattering parameters. Network scattering parameters, more commonly known as ‘S-parameters’, are transmission and reflection (T/R) coefficients for the DUT computed from measurements of voltage waves traveling toward and away from a port or ports of the DUT. In general, S-parameters are expressed either in terms of a magnitude and phase or in an equivalent form as a complex number having a real part and an imaginary part. For example, a set of four S-parameters, namely S
11
, S
12
, S
21
, and S
22
each represented by a complex number, provide a complete characterization of a linear RF performance of a given two-port DUT at a single frequency. A network analyzer capable of measuring both the magnitude and phase of the S-parameters of the DUT is called a vector network analyzer (VNA).
As with all test equipment, VNAs can and do introduce errors into measured S-parameter data. The presence of these errors distorts or corrupts the measurement of actual S-parameters of the DUT by the test system. The characterization and subsequent removal of the effects of systematic errors is often called error correction or calibration. Generally, a VNA calibration involves determining values for error coefficients associated with an error model of a measurement system. For calibration purposes, the ‘measurement system’ generally includes the VNA along with any cables, adapters, test fixtures that are to be employed while testing a DUT. Thus a VNA error model attempts to account for all, or at least the most significant, sources of the systematic errors in terms of constituent error coefficients of the error model. Once determined through VNA calibration, the error coefficients are used in conjunction with the error model to mathematically correct for the effects of the systematic errors in the measured S-parameter data for the DUT produced by the VNA. The data after calibration-related correction is typically called ‘calibrated data’ and represents a more accurate indication of actual performance of the DUT than uncalibrated or raw data.
All of the major systematic errors associated with using a VNA to measure S-parameters can be accounted for by six types of errors: directivity and crosstalk related to signal leakage, source and load impedance mismatches related to reflections, and frequency response errors related to reflection and transmission tracking within test receivers of the network analyzer. Thus, for a VNA measuring S-parameters of a general two-port DUT, there are six forward-error terms and six reverse-error terms for a total of twelve error coefficients or terms (including two terms that combine the various transmission crosstalk terms into a forward crosstalk or a reverse crosstalk term). Such a full measurement calibration for a general two-port DUT is often referred to as a ‘twelve-term’ error correction or calibration using a twelve-term error model. An extension of the twelve-term error model for a full measurement calibration of a multiport network analyzer (i.e., a network analyzer having more than two ports) often is referred to as a twelve-term error model also, even though such an error model necessarily has more than twelve terms.
In addition to the twelve-term error models, simpler and in some cases, more accurate error models known as ‘eight-term’ error models have been developed and are routinely used in situations and under constraining circumstance that allow their use. The eight-term error models actually include two additional terms, for a total of ten, when crosstalk is considered. Thus, the eight-term error model includes two fewer error terms than the twelve-term model when considering a two-port network analyzer or a two-port DUT. A principal difference between the eight-term and twelve-term models is that the twelve-term model has a separate error term for a source match and a load match at each test port of the VNA. Eight-term models, and extensions of such models to multiport VNAs, have only a single match term for each test port of the VNA.
Unfortunately, due to the presence of a single port match error term, eight-error terms are unable to explicitly account for actual differences in the source match and the load match at a test port of the VNA for many VNA configurations. In particular, a VNA may be completely and correctly calibrated using an eight-term error model only if the equivalent test port source match and load match are equal. As such, there are often severe limitations to the applicability of eight-term error models for VNA calibration.
Since VNA calibration using an eight-term model often offers significant practical advantages relative to calibration using a twelve-term model, approaches have been developed to circumvent or overcome the inadequacy of the eight-term model to account for source/load differences. In some cases, an eight-term calibration may be employed when the source/load differences are small enough to simply ignore. In other cases, the difference is accounted for by a mathematical compensation using additional measurements of the test system. Conventionally, accounting and compensating for source/load match differences through the use of additional measurements require supplementary test hardware, such as the use of precision, broadband dual reflectometers at each test port. Thus, the use of an eight-term error calibration for a VNA with a source/load match difference at one or more test ports generally requires a more expensive test system.
Accordingly, it would be advantageous to compensate a calibration of a VNA for the effects of differences in test port source and load match, especially when employing a calibration that does not inherently account for such differences. Moreover, it is desirable that such a compensated calibration not require the use of dual reflectometers at each of the test ports. Such a compensated calibration would solve a long-standing need in the area of VNA calibration and, in particular, in the area of VNA calibration using eight-term error models.
SUMMARY OF THE INVENTION
The present invention compensates a calibration of a vector network analyzer (VNA) using an error model calibration. In particular, the present invention compensates or corrects for differences in a source match and a load match of a test port of the VNA, where the differences are otherwise unaccounted for by the error model being employed. In addition, the present invention also may compensate for differences in a directivity transmission tracking term and a reflection tracking term of the error model associated with a test port of the VNA due to switching of receivers at the test port. The present invention may be used to compensate any eight-term model-based VNA calibration or multi-port extension thereof such that accurate, ‘calibrated’ measurements of a device under test (DUT) are produced by the VNA employing the calibration.
In an aspect of the present invention, a method of compensating a calibration of a VNA is provided. The method comprises characterizing a source match and a load match of a test port of the VNA. The method of compensating further comprises computing a test port delta-match factor from the characterized source match and load match. The method applies to a multiport VNA having two or more test ports.
In some embodiments, the method of compensating a calibration further comprises correcting or compensating measured S-parameters data for a device under test (DUT) using the delta-match factor. Raw or uncalibrated S-parameter data for the DUT is first compensated for the source/load match difference using the delta-match factor to generate compensated raw S-parameter data. Then the compensated raw S-parameter data is corrected for systematic err
Blackham David V.
Rytting Douglas K.
Agilent Technologie,s Inc.
Barlow John
Le Toan M.
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