U.S. patent application number 09/975557 was filed with the patent office on 2002-04-18 for apparatus and method for measuring optical characteristics and recording medium.
Invention is credited to Imamura, Motoki, Kawazawa, Toshio, Kimura, Eiji, Nagumo, Satoru.
Application Number | 20020044274 09/975557 |
Document ID | / |
Family ID | 18792819 |
Filed Date | 2002-04-18 |
United States Patent
Application |
20020044274 |
Kind Code |
A1 |
Kimura, Eiji ; et
al. |
April 18, 2002 |
Apparatus and method for measuring optical characteristics and
recording medium
Abstract
To provide an apparatus for measuring chromatic dispersion when
modulating frequency of a variable wavelength light source is
different from modulating frequency of a fixed wavelength light
source for reference. This apparatus is provided with a variable
wavelength light phase comparator 24 for obtaining a phase
difference .phi.x-.phi.x' between a variable wavelength light
component and a signal having the first modulating frequency fmx, a
fixed wavelength light phase comparator 25 for obtaining a phase
difference between a phase difference .phi.0-.phi.0' between a
fixed wavelength light component and an electrical signal having
the second modulating frequency fm0, a phase difference converter
26 for converting the phase difference .phi.0-.phi.0' calculated by
the fixed wavelength light phase comparator 25 into .phi. ex, which
corresponds to the first modulating frequency fmx, and a true phase
difference calculator 27 for measuring a true phase difference
.phi. between the phase difference .phi.x-.phi.x' calculated by the
variable wavelength light phase comparator 24, and the converted
result .phi.ex of phase difference converter 26, and obtains the
true phase difference .phi. from which an affect of
contraction/extension of a DUT 30 is removed when the first
modulating frequency fmx and the second modulating frequency fm0
are different, thereby measuring wave dispersion.
Inventors: |
Kimura, Eiji; (Tokyo,
JP) ; Imamura, Motoki; (Tokyo, JP) ; Kawazawa,
Toshio; (Tokyo, JP) ; Nagumo, Satoru; (Tokyo,
JP) |
Correspondence
Address: |
LOWE HAUPTMAN GOPSTEIN
GILMAN AND BERNER LLP
SUITE 310
1700 DIAGONAL ROAD
ALEXANDRIA
VA
22314
|
Family ID: |
18792819 |
Appl. No.: |
09/975557 |
Filed: |
October 12, 2001 |
Current U.S.
Class: |
356/73.1 |
Current CPC
Class: |
G01M 11/338 20130101;
G01M 11/335 20130101; G01M 11/333 20130101 |
Class at
Publication: |
356/73.1 |
International
Class: |
G01N 021/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 13, 2000 |
JP |
2000-313392 |
Claims
What is claimed is:
1. An optical characteristics measuring apparatus for measuring
characteristics of light transmitted thorough a device under test
comprising: a variable wavelength light source for generating
variable wavelength light; a fixed wavelength light source for
generating fixed wavelength light; a variable wavelength light
modulating means for receiving a signal having a first modulating
frequency, and modulating said variable wavelength light with said
first modulating frequency; a fixed wavelength light modulating
means for receiving a signal having a second modulating frequency,
which is different from said first modulating frequency, and
modulating said fixed wavelength light with said second modulating
frequency; a composite light generating means for entering
composite light composed of said variable wavelength light and said
fixed wavelength light into said device under test; a wavelength
component extracting means for extracting a fixed wavelength light
component and a variable wavelength light component from
transmitted light transmitted from said device under test; and a
phase comparing means for measuring a true phase difference between
said variable wavelength light component having the first
modulating frequency and said signal having the first modulating
frequency based on said variable wavelength light component, said
fixed wavelength light component, said signal having the first
modulating frequency, and said signal having the second modulating
frequency; wherein characteristics of said device under test is
obtained from said true phase difference.
2. The optical characteristics measuring apparatus as claimed in
claim 1, wherein said phase comparing means is provided with: a
variable wavelength light phase comparing means for obtaining a
phase difference between said variable wavelength light component
and said signal having the first modulating frequency; a fixed
wavelength light phase comparing means for obtaining a phase
difference between said fixed wavelength light component and said
signal having the second modulating frequency; a phase difference
converting means for converting the phase difference calculated by
said fixed wavelength light phase comparing means to what
corresponding to said first modulating frequency; and a true phase
difference calculating means for calculating a true phase
difference from the phase difference calculated by said variable
wavelength light phase comparing means and the converted result of
said phase difference converting means.
3. The optical characteristics measuring apparatus as claimed in
claim 1 further comprising a characteristics calculating means for
calculating group delay or chromatic dispersion of said device
under test from said true phase difference.
4. An optical characteristics measuring method for measuring
characteristics of light transmitted thorough a device under test
comprising: a variable wavelength light generating step for
generating variable wavelength light; a fixed wavelength light
generating step for generating fixed wavelength light; a variable
wavelength light modulating step for receiving a signal having a
first modulating frequency, and modulating said variable wavelength
light with said first modulating frequency; a fixed wavelength
light modulating step for receiving a signal having a second
modulating frequency, which is different from said first modulating
frequency, and modulating said fixed wavelength light with said
second modulating frequency; a composite light generating step for
entering composite light composed of said variable wavelength light
and said fixed wavelength light into said device under test; a
wavelength component extracting step for extracting a fixed
wavelength light component and a variable wavelength light
component from transmitted light transmitted from said device under
test; and a phase comparing step for measuring a true phase
difference between said variable wavelength light component having
the first modulating frequency and said signal having the first
modulating frequency based on said variable wavelength light
component, said fixed wavelength light component, said signal
having the first modulating frequency, and said signal having the
second modulating frequency; wherein characteristics of said device
under test is obtained from said true phase difference.
5. A computer-readable medium having a program of instructions for
execution by the computer to perform an optical characteristics
measuring process for measuring characteristics of light
transmitted thorough a device under test, said optical
characteristics measuring process comprising: a variable wavelength
light generating processing for generating variable wavelength
light; a fixed wavelength light generating processing for
generating fixed wavelength light; a variable wavelength light
modulating processing for receiving a signal having a first
modulating frequency, and modulating said variable wavelength light
with said first modulating frequency; a fixed wavelength light
modulating processing for receiving a signal having a second
modulating frequency, which is different from said first modulating
frequency, and modulating said fixed wavelength light with said
second modulating frequency; a composite light generating
processing for entering composite light composed of said variable
wavelength light and said fixed wavelength light into said device
under test; a wavelength component extracting processing for
extracting a fixed wavelength light component and a variable
wavelength light component from transmitted light transmitted from
said device under test; and a phase comparing processing for
measuring a true phase difference between said variable wavelength
light component having the first modulating frequency and said
signal having the first modulating frequency based on said variable
wavelength light component, said fixed wavelength light component,
said signal having the first modulating frequency, and said signal
having the second modulating frequency; wherein characteristics of
said device under test is obtained from said true phase difference.
Description
BACKGROUND OF THE INVENTION
[0001] 1 . Field of the Invention
[0002] The present invention relates to measuring chromatic
dispersion characteristic of a DUT (Device Under Test) such as an
optical fiber, and more specifically relates to technology for
measuring without being influenced by a contraction/extension of
the DUT.
[0003] 2 . Description of the Related Art
[0004] When chromatic dispersion characteristic of a device under
test (DUT) such as an optical fiber are measured, it is desirable
to measure while eliminating the influence of a
contraction/extension of the DUT. A technology for measuring
without being influenced by the contraction/extension of the DUT is
described in Publication of Japanese Patent Laid-Open No.
H01-291141.
[0005] FIG. 4 shows a constitution of its measuring system. As
described in FIG. 4, the measuring system is divided into a light
source system 10 and a characteristics measuring system 20. A
variable wavelength light source 12 in the light source system 10
changes a wavelength to generate light with a wavelength of
.lambda.x (variable wavelength light). A fixed wavelength light
source 13 fixes a wavelength to generate light with a wavelength of
.lambda.0 (fixed wavelength light). .lambda.0 is a wavelength which
provides the minimum chromatic dispersion in a DUT 30. The variable
wavelength light and the fixed wavelength light are modulated with
a frequency of f respectively by an optical modulator 15a, and an
optical modulator 15b, and are composed by a multiplexer 16. The
frequency f is provided by power supplies for modulating, which are
omitted in the drawing.
[0006] Light composed in the multiplexer 16 enters into the DUT 30.
The light transmitted through the DUT 30 enters an optical
demultiplexer 21 of the characteristics measuring system 20. The
optical demultiplexer 21 separates the transmitted light through
the DUT 30 into light with the wavelength of .lambda.x and light
with the wavelength of .lambda.0. An optical/electrical converter
for measuring 22a and an optical/electrical converter for reference
22b respectively apply optical/electrical conversion to the light
with the wavelength of .lambda.x and the light with the wavelength
of .lambda.0, and a phase comparator 24 detects a phase difference
between an output from the optical/electrical converter for
measuring 22a and an output from the optical/electrical converter
for reference 22b.
[0007] The transmitted light with the wavelength of .lambda.x is
affected by the chromatic dispersion and the contraction/extension
of DUT 30. The transmitted light with the wavelength of .lambda.0
is affected only by the contraction/extension of DUT 30. This is
because .lambda.0 is the wavelength which provides the minimum
chromatic dispersion in DUT 30. Thus, detecting the phase
difference between the transmitted light with the wavelength of
.lambda.x and the transmitted light with the wavelength of
.lambda.0 removes the affect of contraction/extension of DUT
30.
SUMMARY OF INVENTION
[0008] However, it is required to provide the optical modulator 15a
and the optical modulator 15b with the same frequency for
modulating. In other words, it is impossible to set the frequency
for modulating in the optical modulator 15a and the frequency for
modulating in the optical modulator 15b different from each
other.
[0009] A purpose of the present invention is to provide an
apparatus and the like for measuring the chromatic dispersion when
the modulating frequency for the variable wavelength light source
and the modulating frequency for the fixed wavelength light source
for reference are different.
[0010] According to the present invention as described in claim 1,
an optical characteristics measuring apparatus for measuring
characteristics of light transmitted thorough a device under test
includes: a variable wavelength light source for generating
variable wavelength light; a fixed wavelength light source for
generating fixed wavelength light; a variable wavelength light
modulating unit for receiving a signal having a first modulating
frequency, and modulating the variable wavelength light with the
first modulating frequency; a fixed wavelength light modulating
unit for receiving a signal having a second modulating frequency,
which is different from the first modulating frequency, and
modulating the fixed wavelength light with the second modulating
frequency; a composite light generating unit for entering composite
light composed of the variable wavelength light and the fixed
wavelength light into the device under test; a wavelength component
extracting unit for extracting a fixed wavelength light component
and a variable wavelength light component from transmitted light
transmitted from the device under test; and a phase comparing unit
for measuring a true phase difference between the variable
wavelength light component having the first modulating frequency
and the signal having the first modulating frequency based on the
variable wavelength light component, the fixed wavelength light
component, the signal having the first modulating frequency, and
the signal having the second modulating frequency; wherein
characteristics of the device under test is obtained from the true
phase difference.
[0011] With the optical characteristics measuring apparatus
constituted as described above, the phase difference between the
phase of variable wavelength light component and the phase of
signal having the first modulating frequency includes the affect of
contraction/extension and the like of the device under test. On the
other hand, the phase difference between the phase of fixed
wavelength light component and the phase of signal having the
second modulating frequency includes only the affect of
contraction/extension and the like of the device under test. Thus,
it is possible to remove the affect of contraction/extension and
the like of the device under test from the phase difference between
the phase of variable wavelength light component and the phase of
signal having the first modulating frequency by taking into account
of the phase difference between the phase of fixed wavelength light
component and the phase of signal having the second modulating
frequency. In other words, a true phase difference is measured.
Also, the first modulating frequency may be different from the
second modulating frequency.
[0012] The "true phase difference" here is a phase difference when
the affect of contraction/extension of device under test is
removed.
[0013] The present invention as described in claim 2, is the
optical characteristics measuring apparatus as claimed in claim 1,
wherein the phase comparing unit is provided with: a variable
wavelength light phase comparing unit for obtaining a phase
difference between the variable wavelength light component and the
signal having the first modulating frequency; a fixed wavelength
light phase comparing unit for obtaining a phase difference between
the fixed wavelength light component and the signal having the
second modulating frequency; a phase difference converting unit for
converting the phase difference calculated by the fixed wavelength
light phase comparing unit to what corresponding to the first
modulating frequency; and a true phase difference calculating unit
for calculating a true phase difference from the phase difference
calculated by the variable wavelength light phase comparing unit
and the converted result of the phase difference converting
unit.
[0014] According to the present invention as described in claim 3,
the optical characteristics measuring apparatus as claimed in claim
1 further includes a characteristics calculating unit for
calculating group delay or chromatic dispersion of the device under
test from the true phase difference.
[0015] According to the present invention as described in claim 4,
an optical characteristics measuring method for measuring
characteristics of light transmitted thorough a device under test
includes: a variable wavelength light generating step for
generating variable wavelength light; a fixed wavelength light
generating step for generating fixed wavelength light; a variable
wavelength light modulating step for receiving a signal having a
first modulating frequency, and modulating the variable wavelength
light with the first modulating frequency; a fixed wavelength light
modulating step for receiving a signal having a second modulating
frequency, which is different from the first modulating frequency,
and modulating the fixed wavelength light with the second
modulating frequency; a composite light generating step for
entering composite light composed of the variable wavelength light
and the fixed wavelength light into the device under test; a
wavelength component extracting step for extracting a fixed
wavelength light component and a variable wavelength light
component from transmitted light transmitted from the device under
test; and a phase comparing step for measuring a true phase
difference between the variable wavelength light component having
the first modulating frequency and the signal having the first
modulating frequency based on the variable wavelength light
component, the fixed wavelength light component, the signal having
the first modulating frequency, and the signal having the second
modulating frequency; wherein characteristics of the device under
test is obtained from the true phase difference.
[0016] The present invention as described in claim 5, is a
computer-readable medium having a program of instructions for
execution by the computer to perform an optical characteristics
measuring process for measuring characteristics of light
transmitted thorough a device under test. The optical
characteristics measuring process includes: a variable wavelength
light generating processing for generating variable wavelength
light; a fixed wavelength light generating processing for
generating fixed wavelength light; a variable wavelength light
modulating processing for receiving a signal having a first
modulating frequency, and modulating the variable wavelength light
with the first modulating frequency; a fixed wavelength light
modulating processing for receiving a signal having a second
modulating frequency, which is different from the first modulating
frequency, and modulating the fixed wavelength light with the
second modulating frequency; a composite light generating
processing for entering composite light composed of the variable
wavelength light and the fixed wavelength light into the device
under test; a wavelength component extracting processing for
extracting a fixed wavelength light component and a variable
wavelength light component from transmitted light transmitted from
the device under test; and a phase comparing processing for
measuring a true phase difference between the variable wavelength
light component having the first modulating frequency and the
signal having the first modulating frequency based on the variable
wavelength light component, the fixed wavelength light component,
the signal having the first modulating frequency, and the signal
having the second modulating frequency; wherein characteristics of
the device under test is obtained from the true phase
difference.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a block diagram showing a constitution of an
optical characteristics measuring apparatus relating to an
embodiment of the present invention;
[0018] FIG. 2 is a drawing showing a principle of an operation of
the embodiment of present invention;
[0019] FIG. 3 is a flowchart showing an operation of the embodiment
of present invention; and
[0020] FIG. 4 is a block diagram showing a constitution of an
optical characteristics measuring apparatus of prior art.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0021] The following section describes an embodiment of the present
invention referring to drawings.
[0022] FIG. 1 is a block diagram showing a constitution of an
optical characteristics measuring apparatus relating to an
embodiment of the present invention. The optical characteristics
measuring apparatus relating to the embodiment of the present
invention includes a light source system 10 connected with one end
of a DUT 30, and a characteristics measuring system 20 connected
with the other end of DUT 30. The DUT 30 is what transmits light
such as an optical fiber.
[0023] The light source system 10 is provided with a variable
wavelength light source 12, a fixed wavelength light source 13,
power supplies for modulating 14a and 14b, optical modulators 15a
and 15b, and a multiplexer 16. The variable wavelength light source
12 generates variable wavelength light whose wavelength changes.
The variable wavelength light source 12 sweeps the wavelength
.lambda.x of variable wavelength light. The fixed wavelength light
source 13 generates fixed wavelength light whose wavelength is
fixed. It is desirable that the wavelength of fixed wavelength
light is fixed to the wavelength .lambda.0, which provides the
minimum chromatic dispersion in DUT 30. The power supply for
modulating 14a generates an electrical signal having a first
frequency of fmx. The power supply for modulating 14b generates an
electrical signal having a second frequency of fm0. The optical
modulator 15a modulates the variable wavelength light with the
first frequency fmx. The optical modulator 15a receives the
electrical signal generated by the power supply for modulating 14a
to obtain the first frequency fmx. The optical modulator 15b
modulates the fixed wavelength light with the second frequency fm0.
The optical modulator 15b receives the electrical signal generated
by the power supply for modulating 14b to obtain the second
frequency fm0. The optical modulators 15a and 15b include lithium
niobate (LN). As long as they can modulate light, they do not
necessarily include LN. The multiplexer 16 composes the variable
wavelength light with the fixed wavelength light to generate
composite light, and enters it into the DUT 30.
[0024] The composite light provided for the DUT 30 is transmitted
through the DUT 30. The light which has been transmitted through
the DUT 30 is referred as transmitted light.
[0025] The characteristics measuring system 20 is provided with an
optical/electrical converter 22, a detector 23, a variable
wavelength light phase comparator 24, a fixed wavelength light
phase comparator 25, a phase difference converter 26, a true phase
difference calculator 27, and a characteristics calculator 28. A
phase comparing means comprises the variable wavelength light phase
comparator 24, the fixed wavelength light phase comparator 25, the
phase difference converter 26, and the true phase difference
calculator 27.
[0026] The optical/electrical converter 22 applies
optical/electrical conversion to the transmitted light. The
detector 23 extracts a variable wavelength light component
modulated with the first frequency fmx, and a fixed wavelength
light component modulated with the second frequency fm0 from an
electrical signal obtained by applying optical/electrical
conversion to the transmitted light.
[0027] The variable wavelength light phase comparator 24 measures a
phase difference between a phase .phi.x of the variable wavelength
light component and a phase .phi.x' of an electrical signal
including the first frequency fmx generated by the power supply for
modulating 14a. The fixed wavelength light phase comparator 25
measures a phase difference between a phase .phi.0 of the fixed
wavelength light component and a phase .phi.0' of an electrical
signal including the second frequency fm0 generated by the power
supply for modulating 14b. The phase difference converter 26
converts the phase difference .phi.0-.phi.0' calculated by the
fixed wavelength light phase comparator 25 into what corresponding
to the first modulating frequency fmx. The true phase difference
calculator 27 obtains a true phase difference between the variable
wavelength light component having first modulating frequency fmx
and the electrical signal including the first modulating frequency
fmx from the phase difference .phi.x-.phi.x' calculated by the
variable wavelength light phase comparator 24, and the converted
result of phase difference converter 26. The "true phase
difference" here is a phase difference when the affect of
contraction/extension of DUT 30 is removed. The characteristics
calculator 28 calculates the group delay or the chromatic
dispersion of the device under test from the true phase difference.
A group delay characteristic is obtained from a relationship
between the true phase difference and the first modulating
frequency fmx. The chromatic dispersion characteristic is obtained
by differentiating the group delay characteristic by the
wavelength.
[0028] The following section describes a principle for how the
phase comparing means comprising the variable wavelength light
phase comparator 24, the fixed wavelength light phase comparator
25, the phase difference converter 26, and the true phase
difference calculator 27 obtains the true phase difference which
excludes the affect of contraction/extension of DUT 30 while
referring to FIG. 2.
[0029] The phase .phi. x of variable wavelength light component is
a sum of the phase .phi.x' of electrical signal including the first
frequency fmx generated by the power supply for modulating 14a, a
phase difference .phi. due to the chromatic dispersion, and a phase
difference .phi.ex caused by the contraction/extension of DUT 30.
.phi.xt is a phase of the variable wavelength light component when
there is no affect of the contraction/extension of DUT 30.
Relationship among .phi.x, .phi.x', .phi.xt and the like is
described in FIG. 2 (a). The variable wavelength light phase
comparator 24 is provided with .phi.x and .phi.x', and obtains
.phi.x-.phi.x'. However, .phi.ex is not obtained. Thus, the true
phase difference .phi. is not obtained.
[0030] On the other hand, .phi.e0, which is a difference between
the phase .phi.0 of fixed wavelength light component and the phase
.phi.0' of electrical signal having the second frequency fm0
generated by the power supply for modulating 14b is caused by the
contraction/extension of DUT 30, and is not affected by the
chromatic dispersion. This is because the wavelength .lambda.0 of
fixed wavelength light generated by the fixed wavelength light
source 13 is fixed to a value which provides the minimum chromatic
dispersion in DUT 30. FIG. 2 (b) shows a relationship among .phi.0,
.phi.0' and the like. The fixed wavelength light phase comparator
25 is provided with .phi.0 and .phi.0', and obtains
.phi.0-.phi.0'.
[0031] Both .phi.ex and .phi.e0 are caused by the
contraction/extension of DUT 30. Thus, they have the relationship
shown in the FIG. 2 (c), for example. The phase difference
converter 26 uses the relationship to calculate .phi.ex from
.phi.e0. It converts the phase difference .phi.0-.phi.0' calculated
by the fixed wavelength light phase comparator 25 to what
corresponding to the first modulating frequency fmx.
[0032] The true phase difference calculator 27 receives
.phi.x-.phi.x' from the variable wavelength light phase comparator
24, and .phi.ex from the phase difference converter 26 to obtain
the true phase difference .phi..
[0033] The flowchart in FIG. 3 describes the operation of
embodiment of present invention. The variable wavelength light
source 12 of light source system 10 changes wavelength to generate
light with the wavelength of .phi.x (variable wavelength light).
The fixed wavelength light source 13 fixes wavelength to generate
light with the wavelength of .lambda.0 (fixed wavelength light).
The .lambda.0 is a wavelength which provides the minimum chromatic
dispersion in DUT 30. The variable wavelength light is modulated by
the optical modulator 15a with the first frequency fmx, and the
fixed wavelength light is modulated by the optical modulator 15b
with the frequency fm0 (S10), and they are composed by the
multiplexer 16 (S12). The power supply for modulating 14a provides
the optical modulator 15a with the first frequency fmx. The power
supply for modulating 14b provides the optical modulator 15b with
the second frequency fm0.
[0034] The light composed by the multiplexer 16 is provided for the
DUT 30. The light transmitted through the DUT 30 is provided for
the optical/electrical converter 22 of characteristics measuring
system 20. The optical/electrical converter 22 applies
optical/electrical conversion to the transmitted light to provide
for the detector 23 (S14). The detector 23 extracts the variable
wavelength light component and the fixed wavelength light component
from the transmitted light which is applied with the
optical/electrical conversion (S16). The variable wavelength light
component is provided for the variable wavelength light phase
comparator 24. The fixed wavelength light component is provided for
the fixed wavelength light phase comparator 25. The electrical
signal generated by the power supply for modulating 14a is provided
for the variable wavelength light phase comparator 24. The
electrical signal generated by the power supply for modulating 14b
is provided for the fixed wavelength light phase comparator 25.
[0035] The variable wavelength light phase comparator 24 obtains
the phase difference between the phase .phi.x of variable
wavelength light component and the phase .phi.x' of electrical
signal including the first frequency fmx generated by the power
supply for modulating 14a (S20). Then, the fixed wavelength light
phase comparator 25 obtains the phase difference between the phase
.phi.0 of fixed wavelength light component and the phase .phi.0' of
electrical signal including the second frequency fm0 generated by
the power supply for modulating 14b (S22). The fixed wavelength
light phase comparator 25 provides the phase difference converter
26 with .phi.0-.phi.0', that is .phi.e0. The phase difference
converter 26 calculates .phi.ex from .phi.e0 (S24). The true phase
difference calculator 27 receives .phi.x-.phi.x' from the variable
wavelength light phase comparator 24, receives .phi.ex from the
phase difference converter 26, and obtains the true phase
difference .phi. (S26). Finally, the characteristics calculator 28
uses the true phase difference .phi. to obtain the group delay or
the chromatic dispersion of DUT 30 (S28).
[0036] With the embodiment of present invention, it is possible to
obtain the group delay or the chromatic dispersion of DUT 30 by
removing the affect of contraction/extension of DUT 30 and the like
when the first modulating frequency fmx and the second modulating
frequency fm0 are different from each other.
[0037] The embodiment described above is also realized as follows.
A computer provided with a CPU, a hard disk, and a medium reading
apparatus (such as a floppy disk and a CD-ROM) reads a medium
storing a program for realizing the individual parts described
above in the medium reading apparatus, and installs the program on
the hard disk. This method also realizes the function described
above.
[0038] With the present invention, it is possible to obtain
characteristics of a device under test by removing an affect of
contraction/extension of the device under test when the first
modulating frequency and the second modulating frequency are
different from each other.
* * * * *