Data processing: measuring – calibrating – or testing – Testing system – For transfer function determination
Reexamination Certificate
1999-09-10
2002-06-11
Hoff, Marc S. (Department: 2857)
Data processing: measuring, calibrating, or testing
Testing system
For transfer function determination
C702S108000, C702S182000, C702S057000, C702S070000, C702S075000
Reexamination Certificate
active
06405147
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and system for the accurate measurement of a transfer function or of signal transfer characteristics of signal transfer systems. In particular, this invention relates to a system and method that provide precision characterization of the amplitude and phase response of signal transfer devices, systems and channels.
2. Description of the Related Art
Modem analog, digital and mixed signal communication and radar systems can be classified generally as signal transfer devices, systems or channels. These systems transfer information in the form of a signal waveform from an input to an output. A signal is introduced into the system at an input. The signal passes through the components of the system and exits the system at an output. The information content of the signal is then extracted from the output signal by some appropriate means.
In the case of a typical communication system, a signal containing the information content or message to be communicated is introduced into the input of the system often called a transmitter. The signal exits the transmitter and passes through some type of communication channel such as a cable, the air, or possibly empty space to a receiver. The signal then passes through the receiver and is measured at the receiver's output to determine the message or information content of the signal. When viewed as a whole, the communication system comprised of the transmitter, communication channel and receiver is simply a device or channel for transferring the signal from the input to the output of the system.
Likewise, RADAR systems are essentially signal transfer devices. RADAR operates in a manner completely analogous to a communication system except that in the radar system the transmitter and receiver are often co-located and the information content of the signal is the effect that the radar's target has on the transmitted signal and not the signal itself. Still, the signal that is received contains information that is extracted at the RADAR receiver output.
The designer of signal transfer systems usually assumes that the components that make up the system are ideal, linear and time-invariant. The components are assumed to be ideal in the sense that they have a given amplitude and phase response over a given frequency range known as their operational band. The term linear means that the output and input of the devices are related by a linear function known as a transfer function, and time-invariant means that the transfer function is not a function of time. The amplitude and phase versus frequency characteristic of a device or channel is also called the spectrum or spectral response of the device or channel.
A deviation from the expected transfer function of a signal transfer system is generically known as distortion. The signal is distorted by the system. Distortion interferes with the accuracy of the signal transfer and generally increases the chances that the information content of the signal waveform will be corrupted. The corruption of the waveform by its passage through the system increases the chance that errors will be made in its interpretation at the output of the system. Sufficient errors can render a signal and the associated signal transfer system of little or no practical use.
Although systems are generally designed assuming that the components from which the system is constructed are ideal, it is impossible to manufacture ideal components; all real components are only approximations of the ideal components used in the design. The manufacturing goal is to produce components that are standardized within a given tolerance at an acceptable cost. Even if a given component is close enough to a given standard when manufactured, the component may change over time, thereby deviating from the standard. Components deviate from the desired performance characteristics both in terms of their individual complex frequency or spectral response and from unit to unit.
The signal path that connects the waveform source and receiver, be it wire, fiber optic cable or empty space, also can and does behave in a non-ideal manner. Therefore, the non-ideal characteristics of the channel will introduce distortion in the signal being transferred through the system. This distortion is in addition to the distortion introduced by the non-ideal components used to construct the system.
The non-ideal nature of manufactured components and the interconnecting channel leads to the need for obtaining precise measurements of the transfer functions that make up a system. The measurements provide data which can be used to accept or reject components, adjust the performance of the component or, in some cases, to equalize the component's or channel's non-ideal performance. Equalization is normally accomplished through the use of an equalizer designed from measurements of the non-ideal performance that is inserted into the signal path of the system. The ultimate performance of the signal transfer system is, therefore, integrally related to the ability to measure the system performance.
An example where precise measurement and subsequent equalization of system signal transfer characteristics are often required is matching multiple independent receiving channels for angle of arrival or antenna array applications. In general, individual receiver channels will have variations in their signal transfer characteristics resulting from normal manufacturing tolerance variations. These variations will directly effect the performance and accuracy of the system of multiple receivers and render the system unusable. However, if the transfer characteristics can be measured for each receiver channel with sufficient accuracy, a set of equalizer filters can be designed and placed in each receiver to effectively eliminate the variations between channels. Generally, the equalizers are used to flatten out the amplitude versus frequency variations and linearize the phase response of the individual channels as well as minimize the absolute differences between channels. Therefore, the precision of the signal transfer function measurement is an important consideration since it is these measurements that are used to create the equalizer designs.
In some cases such as modern communications systems that employ so called predistortion techniques to maximize data throughput, equalization is used to produce a transfer function that has neither a flat amplitude versus frequency characteristic nor a linear phase response. Precise measurement of the transfer characteristics of such systems is generally required to insure that the desired transfer function has been achieved by the designed equalizer as well as to provide necessary data for designing the equalizers. This is particularly true for systems that utilize wide bandwidth signals whose modulation features are particularly affected by slight undesired variations of group delay versus frequency response of the channel from a desired standard response.
The conventional technique for measuring the performance of a signal transfer system or channel is illustrated in
FIG. 1. A
signal source or signal generator
12
is used to generate a known stimulus signal. The stimulus signal thus generated is divided into two copies one of which is applied to an input of the system or device under test (DUT). The other copy of the stimulus signal bypasses the DUT and is collected by a signal measurement processor
16
. An output signal of the DUT produced in response to the applied stimulus signal is also collected by the signal measurement processor
16
. In the signal measurement processor
16
, the output signal of the DUT is compared to the copy of the stimulus signal that bypassed the DUT. The comparison in the signal measurement processor
16
of these two signals is used to determine the transfer function of the DUT. Since all systems have an associated time delay, this time delay as a function of frequency must be taken into account if an accurate transfer fun
Condor Systems, Inc.
Johnson J. Michael
Leitereg Elizabeth E.
North Shore Associates
Tsai Carol S.
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