Data processing: measuring – calibrating – or testing – Testing system – For transfer function determination
Reexamination Certificate
1998-08-28
2003-02-25
Wachsman, Hal (Department: 2857)
Data processing: measuring, calibrating, or testing
Testing system
For transfer function determination
C702S109000, C702S110000, C702S188000, C340S870140, C455S226100
Reexamination Certificate
active
06526365
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention generally relates to the area of electronic test equipment. More specifically, it pertains to devices that are capable of measuring “transfer functions”, i.e., functions that are an indication of a system's response to electromagnetic radiation. These test devices are commonly known as Network Analyzers.
2. Description of the Prior Art
We live in a sea of electromagnetic radiation. If we could visualize the radiation swirling about us, we would see every electrical circuit and every electrical system putting out its own radiation. Unfortunately, some of this radiation affects equipment that we depend on, such as our office computers, computer-controlled machines, and radio and television equipment. It is important, therefore, to be able to explore the radiation that will possibly impact our equipment and predict whether the equipment will be adversely affected by the radiation or whether the steps we have taken to protect the equipment, such as using conducting seals and grounding devices, will provide the necessary protection.
As our society becomes more laden with electronic wizardry, this task becomes even more important. Will your computer shut down when you turn on your new cellular telephone? Will your pacemaker malfunction when you turn on the microwave oven? With respect to electronic-intenisive equipment, such as aircraft and water craft navigational instruments, would nearby lightning or operating a computer on board adversely affect the usefulness of the equipment?
In order for industry to warrant the operation of their products, such as computers, computer-controlled machines, navigation instruments, cellular telephones and the like, the effects of external radiation on the particular device must be determined, and procedures or measures taken to prevent this radiation from corrupting the operation of the device. In order to market many electronic devices in the United States, the Federal Communications Commission (FCC) rules require certification against the external electromagnetic radiation effects. European Union rules and regulations require more stringent certification requirements for Electromagnetic Interference and Compatibility (EMI/EMC) applicable to all consumer and industrial equipment.
The interference caused by electromagnetic radiation can be characterized by measuring a quantity called. “transfer function” also known as “transfer characteristics” or “impulse responses”. Transfer functions, in general, characterize a system by relating the system's response (output) to a given excitation (input). This is depicted in FIG.
1
. Transfer function determination requires one signal (for exciting the system under test) and two measurements (the excitation and response). In
FIG. 1
, the excitation source or signal
1
(such as electromagnetic or acoustic radiation) is inputted to the system
2
under test (such as an aircraft, a computer, or a building) to bring about a response
3
(such as current or voltage induced on a cable or acoustic intensity in a chamber). The transfer function is defined as the ratio of the output signal response
3
to the input signal
1
, and must be determined for each excitation frequency. For example, a piano's transfer function can be determined by striking the keys. To make a complete transfer function, the piano response must be determined for each and every key; each key representing a different frequency. Examples of practical applications for transfer function measurements are to:
a) Characterize electronic filters;
b) Characterize mechanical vibrations of structures (such as automobile suspensions or bridges);
c) Characterize concert hall echoes;
d) Determine acoustic shielding effectiveness (blockage of sound) of enclosures; and,
e) Determine electromagnetic shielding effectiveness of automobiles, aircraft, buildings, missiles etc . . . .
The prior art utilizes two primary techniques in conjunction with a centralized data to acquisition and processing apparatus for measuring these transfer functions:
High-Pitier/Ultra-Wideband Test Technique
The high-power/ultra-wideband technique excites the system with a short duration pulse (called an “impulse”). Because any impulse has a frequency content that is extremely rich, i.e., contains many frequencies, one measurement contains transfer function information over a wide band of frequencies. The system transfer function is determined by computing the system's response to the impulse using a data processing technique such as the Fourier transformation technique. This is a fast measurement technique because the transfer function can be determined over a large frequency range using only one pulse.
Impulse excitation does not use up-conversion techniques to modulate the wideband pulse onto a carrier frequency. The pulse is used as the direct excitation source and the frequency range of the measurement is inversely proportional to the duration of the impulse. Therefore, increasing the frequency range of the measurement requires decreasing the impulse width. This places a limit on the maximum frequency that this method can be used because of practical limitations in narrowing the pulse due to electronic component limitations.
The processing method used for this technique (Fourier transform) does not give any data quality indication. This requires the operator to set the excitation power to levels that are higher than necessary in order to guarantee that noise does not corrupt the data. High-power amplifiers are required to boost the signal power of the impulse, because the transmitted pulse has a very short duration.
There are several drawbacks to this high-power/ultra-wideband method:
a) High-power amplifiers are expensive;
b) Ultra-wideband electronic components are unavailable or expensive;
c) High-power pulses can be hazardous to humans;.
d) High-power pulses can cause interference with other electronic systems;
e) The maximum frequency range is inherently limited; and,
f) Noise, which is a practical consideration in all measurements, can corrupt the transfer function, sometimes without the operator's knowledge.
Low Power/Narrow band or Continuous Wave (CW) Test Technique
The low-power
arrowband technique, commonly referred to as continuous wave (CW) testing, is the most popular technique used. This technique excites the system with a signal that is comprised of one dominant frequency (called a “tone”). This is similar to one piano key struck and your ear hearing only one dominant tone. The system transfer function is determined by measuring the system's response to the tone. A complete transfer function requires a tone to be transmitted at each and every frequency of interest.
The processing method used for this technique does not give any data quality indication. This requires the operator to set the excitation power to levels that are higher than necessary in order to guarantee that noise does not corrupt the data.
There are several drawbacks to this low-power
arrowband method:
a) Testing over a large frequency span is very slow due to the excessive number of individual tones that are required;
b) Transmitting tones can cause interference with other electronic systems;
c) Vital transfer function information can be missed due to the frequency stepping nature of the testing sequence; and,
d) Noise, which is a practical consideration in all measurements, can corrupt the transfer function, sometimes without the operator's knowledge.
Centralized Data Acquisition and Processing Apparatus
For large and complex systems under test, the prior art utilizes a centralized data acquisition and processing apparatus to implement the standard transfer function setup shown in FIG.
2
.
A signal generator
4
generates a controlled excitation signal
1
and sends it via an antenna
5
to radiate the system
2
under test to produce the system response
3
. An amplifier (not shown) may be used to increase the strength of the excitation signal. Sensors
6
and
7
(e.g
Marino Michael A.
Parhami Parviz
Robinson John T.
Murphey John J.
Scientific Applications & Research Assiociates, Inc.
Wachsman Hal
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