Data processing: measuring – calibrating – or testing – Calibration or correction system – Circuit tuning
Reexamination Certificate
2000-12-22
2003-05-20
Barlow, John (Department: 2863)
Data processing: measuring, calibrating, or testing
Calibration or correction system
Circuit tuning
Reexamination Certificate
active
06567762
ABSTRACT:
TECHNICAL FIELD
The invention relates to measurement systems. In particular, the invention relates to extending the dynamic range of lower power signal measurements in the presence of higher power signals.
BACKGROUND ART
A critical facet of the design and manufacture of modem communications and related signal transmission systems is the measurement and characterization of signal distortion introduced by the elements that make up the system. All system elements, most notably active devices, such as amplifiers, have non-ideal operational characteristics. These non-ideal operational characteristics can and do distort the signals that pass through or are processed by the elements of the system. The signal distortion introduced by the non-ideal characteristics of the system elements often interferes with the operation of the system. Measurement, characterization and control of system element-related distortion are of paramount importance in most transmission system design and manufacturing activities.
Modern communications systems, especially state-of-the-art wideband systems, are particularly sensitive to signal distortion and its effect on performance. These systems and their designers are faced with ever-increasing demands for greater bandwidth in a finite spectrum and so, must contend with ever-tightening specifications associated with system element-related signal distortion. The ability to perform accurate measurement and characterization of the stimulus/response distortion effects of devices and elements used in the system is a vital consideration in determining the ultimate performance of the system.
Chief among the non-ideal characteristics exhibited by typical devices used in communications systems are non-linear effects. A non-linear effect is defined as the stimulus/response performance of a device that is not completely described by a linear equation. Generally, non-linear effects give rise to signal distortions in the form of a spurious frequency response. That is to say that the device by its operation introduces spectral components into the signal passing through the device that are unwanted and not consistent with the linear operation of that device. Generally, for devices that are classified as linear or quasi-linear, power levels associated with the spurious responses are much lower or smaller than that of the primary or linear response signal. For example, a third order spur from a two-tone measurement might be −60 dBc for a given signal power level relative to the linear response signal. In other words, the spur level is 1,000,000 times smaller than the desired, linear response signal. However, even though the spurious response of a given device is often very small compared to its linear response, the spurious response can have a profound effect on the performance of the system as a whole.
A number of conventional measurement methodologies are used to measure and characterize the non-linear performance of devices used in a communications system. Most of these measurement methodologies either attempt to directly measure an aspect of the non-linear performance of a device or attempt to infer the non-linear performance through an indirect means. Generally, the indirect methodologies focus on measuring the effect of the device non-linearities on some aspect of system performance and therefore, are often referred to as “system level” measurements. Among the direct measurement methodologies are the 1 dB compression point test, the two tone and multi-tone intermodulation response tests, and saturated power tests. Indirect or system level measurements include such things as the bit error rate measurement, eye patterns or eye diagrams, and the adjacent channel power ratio (ACPR). The ACPR is particularly important for modern, wideband code division multiple access (W-CDMA) systems.
The 1 dB compression point test measures the point at which an input or stimulus power level produces an output power level response that deviates from a linear response by 1 dB. The two-tone and multi-tone tests measure the relative level of a particular spurious response or set of responses compared to the level of the linear or fundamental response. These tests are used to predict or infer the so-called second order, third order, and n-th order intercept points in amplifiers. The saturated power test measures the performance of the device at very high input power levels. As pointed out above, all of these direct measurement methodologies attempt to focus on a particular non-linear characteristic (e.g. second order intercept point). Generally, the measured non-linear characteristics are used to infer the effect that the non-ideal performance of a device will have on a signal passing through the system incorporating the device.
In contrast, the indirect measurements focus on a system level performance parameter. In the indirect measurement methodologies, the sum-total of all of the non-linear performance characteristics of a device are tested or measured simultaneously in as much as they affect the performance parameter being measured. For example, a bit error rate test characterizes how a device or series of devices impacts the rate of bit errors at various stimulus signal-to-noise ratios (SNR) for a digital transmission system. The ACPR measures the amount of power that “leaks” from one channel of a system to an adjacent channel as a result of the non-ideal performance of a device under test (DUT). No attempt is generally made to identify what non-ideal performance effect of the DUT is causing the observed performance in the indirect measurements. On the other hand, the data generated by the indirect measurements are generally more closely related to the actually performance parameters of the system as a whole.
In both the direct and indirect measurement methodologies, the goal is to accurately measure the performance parameter and compare that measured value to a system specification or to predict system performance from the measured performance parameter. The sensitivity, dynamic range, and accuracy of such measurements are always of concern to system designers and system manufacturers.
The difficulty that is encountered with many measurement systems used to perform the direct and indirect measurements of device performance is that the measurement systems used to perform the measurements often exhibit inherent non-linear and/or spurious performance characteristics themselves. The inherent non-ideal performance of the measurement systems can limit the dynamic range and accuracy of the tests being performed.
For example, a pair of signal generators used in a two-tone test may generate spurious harmonic signals in the frequency range of the intermodulation product that is being measured. The presence of these spurious signals can limit the minimum level of a given intermodulation product that can be measured by the measurement system. Pre-amps and detectors used in the measurement system can have non-linear performance characteristics that produce spurious signals that interfere with the intended measurements. At the very least, it may be difficult or impossible to make accurate measurements of the amplitude or power level of small or very small signals in the presence of a large, linear response signal.
These inherent, non-ideal characteristics of the measurement system mean that the sensitivity or minimum level of the measurements taken therewith is instrument-limited. The ideal situation is to have measurements that are DUT-limited instead of instrument-limited since it is the non-ideal characteristics of the DUT that are of interest. The ultimate result of the presence of non-ideal characteristics in the measurement system is an effective limitation in the dynamic range of the measurement system, which thus limits the ability of the system to make accurate measurement of very low-level DUT-related distortion signals.
FIG. 1A
illustrates a block diagram of a conventional measurement system that can be used for either direct or indirect measurements. The measurement system compris
Bourde Christian A.
Taylor J. Barry
Agilent Technologie,s Inc.
Barlow John
Lau Tung S
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