Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Calibration
Reexamination Certificate
1997-10-22
2001-06-19
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Calibration
C324S638000, C324S646000
Reexamination Certificate
active
06249128
ABSTRACT:
This invention relates generally to test systems for microwave components and more particularly to calibration methods for such systems to improve accuracy.
Microwave devices are more and more being made using semiconductor manufacturing techniques. Such techniques allow the devices to be made in large quantities at lower cost. Such devices must be tested at a low cost, too. However, test accuracy should not be sacrificed
An important way that test costs are kept low is through the use of automated test equipment. Devices are inserted mechanically into the test equipment and a series of tests are quickly run on the device. Modem automated test equipment can test the microwave circuitry on a semiconductor device. In addition, it can generate and measure digital signals, too. Thus, microwave devices can be fully tested in rapid succession.
To ensure accuracy, the test system is calibrated. Traditional calibration of microwave instrumentation is done by connecting a series of calibration references to the test ports of the test instrument. The test system then measures these calibration references and, because the actual values of the calibration references are known, the measurement error made by the test system can be identified. A series of parameters, often called s-parameters, is computed which form a mathematical model of the measurement circuitry leading up to the point where the calibration references are connected. The model can be used to predict distortion in a signal passing through the measurement instrument. Thus, the effect of the signal distortion, or error, can be eliminated mathematically.
A significant advance in calibration for automatic test equipment is described in U.S. Pat. No. 5,572,160 to Wadell, which is hereby incorporated by reference. That patent describes automatic test equipment in which calibration references are mounted inside the automatic test system. Such a mounting arrangement is contrary to traditional calibration thinking which dictates that the calibration references are mounted in the place where a device under test would normally be mounted. However, that patent describes a unique calibration process used to allow accurate calibration with internal calibration references.
The above described calibration processes provide what is sometimes called VNA calibration. VNA measurements are used to determine what are essentially transmission and reflection coefficients of a device. These coefficients are based on the ratios of measured incident, transmitted and reflected power. The actual power used does not matter for VNA measurements because it becomes irrelevant when the ratios are computed.
To determine the actual power applied to the device under test, a power measuring device is used to measure the source power. The above mentioned patent to Wadell describes such a system used to measure source power.
However, the above described calibration measurements do not adjust for errors which change the power provided to the load. One such error is called “source match”. Source match introduces an error when an incident signal is partially reflected by a device under test. The reflected portion of the wave travels back into the measurement instrument. A portion of the reflected wave will be reflected by the measurement instrument, to make a second order reflected signal traveling back to the load. This reflected portion can be predicted using the VNA calibration measurements. One of the s-parameters describes this reflection. Because the reflected portion can be computed, an adjustment can be made to avoid having the reflected portion cause an error.
However, not all of the signal reflected from the load will be reflected back to the load when it reaches the VNA circuitry. Part of that signal will travel through the circuitry until it reaches the source. At the source, some part of the reflected signal will again be reflected, creating another second level reflection. Because the VNA calibration process can not account for variations in signal level from the source, the second level reflection from the source can not be accounted for using traditional VNA calibration.
Typically, the second level reflection from the source is ignored. Often, it is assumed that the measurement instrument is perfectly matched to the device under test. and that there is no reflection from the device under test back toward the source. An effort is often made in designing a measurement system to ensure that the impedance of the source matches the impedance of the VNA circuitry. If the source impedance matches the impedance of the circuitry, there will be virtually no reflection from the source. However, it is often costly to design test equipment to ensure that the impedance of the source matches the impedance of the circuitry to which it is connected.
In many instances, the amount of reflected signal that reaches the source is very small. Thus, finding and correcting for the source match error has traditionally been ignored without significant errors. However, there are some instances where it would be desirable to make a lower cost test system by not precisely matching the source to the other circuitry while still maintaining accuracy through calibration. There will also be instances where high accuracy will be required and errors caused by source match need to be removed through calibration even though they are small. For example, if the test instrument is being used to measure the gain of a device or the 3 dB compression point, it is important that the true power into the device be accurately measured.
Unless some mechanism is used to adjust for source match errors, power sensitive measurements will vary with the reflection coefficient from the device under test. Thus, the test process will exhibit device to device variation, which is very undesirable. Likewise, power sensitive measurements will vary with the reflection coefficient from the source. Thus, the test process will exhibit tester to tester variations. Any form of variation in the test process of an automated factory is highly undesirable. Therefore, there is a great need to have a simple and accurate way to adjust for source match errors.
SUMMARY OF THE INVENTION
With the foregoing background in mind, it is an object of the invention to provide a simple and accurate way to adjust for source match errors.
The foregoing and other objects are achieved in an automatic test system containing calibration references that can be switchably connected into a signal path. A calibration routine is performed in which first one calibration reference is connected to the device and then a second calibration reference is connected such that the phase relationship of the measurements made with the first and second calibration references is preserved. These measurements are used in computing an adjustment for source match error, which is then subsequently used to reduce the effects of the error.
REFERENCES:
patent: 4808912 (1989-02-01), Potter et al.
patent: 4839578 (1989-06-01), Roos
patent: 4853613 (1989-08-01), Sequiera
patent: 4982164 (1991-01-01), Schiek et al.
patent: 5047725 (1991-09-01), Strid et al.
patent: 5191294 (1993-03-01), Grace et al.
patent: 5241277 (1993-08-01), Kefalas
patent: 5276411 (1994-01-01), Woodin, Jr. et al.
patent: 5434511 (1995-07-01), Adamian et al.
patent: 5537046 (1996-07-01), Adamian
patent: 5572160 (1996-11-01), Wadell
patent: 5587934 (1996-12-01), Oldfield et al.
patent: 0 333 521 A1 (1989-09-01), None
patent: 0 715 177 A2 (1996-06-01), None
patent: WO 89/04001 (1989-05-01), None
Deb Anjan K.
Metjahic Safet
Teradyne Legal Department
Teradyne, Inc.
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