U.S. patent application number 09/941145 was filed with the patent office on 2002-02-28 for waveform measuring apparatus.
This patent application is currently assigned to Anritsu Corporation. Invention is credited to Otani, Akihito, Otsubo, Toshinobu, Watanabe, Hiroto.
Application Number | 20020024458 09/941145 |
Document ID | / |
Family ID | 18751118 |
Filed Date | 2002-02-28 |
United States Patent
Application |
20020024458 |
Kind Code |
A1 |
Otani, Akihito ; et
al. |
February 28, 2002 |
Waveform measuring apparatus
Abstract
A reference signal generation portion generates a reference
signal independently of a repetition cycle of a signal under test.
A frequency measuring portion measures a repetition frequency of
the signal under test by using a reference signal from the
reference signal generation portion. A sampling frequency setting
portion computes and sets a value of frequency of a sampling signal
which can obtain a desired delay time with respect to a phase of
the signal under test based on a value of a repetition frequency
measured with the frequency measuring portion. The sampling signal
generation portion uses a reference signal from the reference
signal generation portion and the value of the frequency set by the
sampling frequency setting portion to generate a sampling signal
having a cycle corresponding to the frequency.
Inventors: |
Otani, Akihito; (Atsugi-shi,
JP) ; Otsubo, Toshinobu; (Atsugi-shi, JP) ;
Watanabe, Hiroto; (Atsugi-shi, JP) |
Correspondence
Address: |
FRISHAUF, HOLTZ, GOODMAN &
LANGER & CHICK, PC
767 THIRD AVENUE
25TH FLOOR
NEW YORK
NY
10017-2023
US
|
Assignee: |
Anritsu Corporation
5-10-27, Minamiazabu, Minato-ku
Tokyo
JP
106-8570
|
Family ID: |
18751118 |
Appl. No.: |
09/941145 |
Filed: |
August 28, 2001 |
Current U.S.
Class: |
341/155 |
Current CPC
Class: |
G01R 13/347 20130101;
G01R 19/2509 20130101 |
Class at
Publication: |
341/155 |
International
Class: |
H03M 001/12 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 31, 2000 |
JP |
2000-263592 |
Claims
What is claimed is:
1. A waveform measuring apparatus having sampling signal generation
means for generating a sampling signal having a cycle longer than
the repetition cycle of a signal under test, a sampling portion for
sampling the signal under test in synchronization with the sampling
signal from the sampling signal from the sampling signal generation
means and data processing portion for determining an envelope
waveform of a signal under test which is sampled with the sampling
portion, and determining a signal waveform of the signal under test
from this envelope waveform; the apparatus comprising: reference
signal generation means for generating a reference signal
independently of a repetition cycle of the signal under test;
frequency measuring means for measuring a repetition frequency of
the signal under test by using a reference signal from the
reference signal generation means; and sampling frequency setting
means for computing and setting a value of a frequency of a
sampling signal which can obtain a desired delay time with respect
to a phase of the signal under test based on a value of a
repetition cycle measured with the frequency measuring means;
wherein the sampling frequency generation means uses a reference
signal from the reference signal generation means and the value of
the frequency set by the sampling frequency setting means to
generate a sampling signal having a cycle corresponding to the
frequency.
2. A waveform measuring apparatus according to claim 1, wherein the
reference signal generation means includes a rubidium atomic
oscillator.
3. A waveform measuring apparatus according to claim 1, wherein the
reference signal generation means include a cesium atomic
oscillator.
4. A waveform measuring apparatus according to claim 1, further
comprising: a power divider for dividing a signal under test which
is optical signal into two directions when the signal under test is
an optical signal; and a photo detector for converting a signal
under test which is one optical signal which is divided with the
power divider into a signal under test which is an electric signal;
wherein a repetition frequency of the signal under test which is
converted into an electric signal with the photo detector is
measured with the frequency measuring means; and the measured value
of the repletion frequency of the signal under test measured with
the frequency measuring means is given to the sampling frequency
setting means.
5. A waveform measuring apparatus according to claim 1, further
comprising: a power divider for dividing the signal under test
which is an optical signal into two directions when the signal
under test is the optical signal; and a clock recovery for
converting a signal under test which is an optical signal into a
signal under test of an electric signal having a repetition
frequency and outputting the converted signal by detecting a clock
of the recovery cycle from one signal under test divided with the
power divider; wherein the repetition frequency of the signal under
test which is converted into an electric signal with the clock
recovery is measured with the frequency measuring means, and the
measured value of the repetition frequency of the signal under test
measured with the frequency measuring means is given to the
sampling frequency setting means.
6. A waveform measuring apparatus according to claim 1, further
comprising: a photo detector for receiving a signal under test of
an optical signal sampled with a sampling signal input from the
sampling signal generation circuit with the electro-absorption
modulator and converting the signal under test which is an optical
signal after being sampled into a signal under test which is an
electric signal when the signal under test is an optical signal and
the sampling portion is an electro-absorption modulator; an
analog/digital converter for converting a signal under test which
is converted into an electric signal with the photo detector into a
digital signal under test to send the digital signal under test to
the data processing portion; and a display device for converting a
magnification of a time axis in the envelope waveform determined
with the data processing portion into a magnification of the
original signal under test to display the magnification as a signal
waveform of the signal under test.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is based upon and claims the benefit of
priority from the prior Japanese Patent Application No.
2000-263592, filed Aug. 31, 2000, the entire contents of which are
incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a waveform measuring
apparatus, in particular, to a waveform measuring apparatus for
determining a signal waveform of a signal under test having an
arbitrary repetition cycle which is input.
[0004] 2. Description of the Related Art
[0005] Generally, a signal generator for generating a signal under
test such as an electric signal, an optical signal or the like
having an arbitrary repetition cycle incorporates a reference
signal oscillator for generating a reference signal having a
reference frequency fs, and a waveform pattern generation portion
for generating a waveform pattern of the signal under test.
[0006] Then, in such signal generator, a repetition frequency
signal having a designated repetition frequency fa is created by
using a reference signal output from the reference signal
oscillator while an electric signal and an optical signal are
created which have an arbitrary repetition cycle Ta by using this
repetition frequency signal and the waveform pattern output from
the waveform pattern generation portion.
[0007] The electric signal and the optical signal having a
repetition cycle Ta output from such signal generator are generally
incorporated in an information communication system and are used as
a signal under test of various communication devices including, for
example, optical transmission cable.
[0008] Therefore, it is necessary to measure in detail the
characteristic of the electric signal and the optical signal output
from the signal generator prior to the practice of the test of
various communication devices including the light transmission
cable incorporated in the information communication system.
[0009] As one characteristic of this electric signal and the
optical signal, the signal waveform is measured.
[0010] Conventionally, there are proposed various measuring methods
for measuring a signal waveform of the signal under test that is an
electric signal, an optical signal or the like having such an
arbitrary repetition cycle.
[0011] However, in the case of a high frequency signal having a
repetition cycle Ta, namely, a repetition frequency fa exceeding 10
GHz, the method for measuring the waveform of the signal under test
is used a sampling method.
[0012] A representative sampling method for measuring a signal
waveform of the signal under test which has this repetition
frequency fa exceeding 10 GHz will be explained by using FIGS. 6A,
6B and 6C.
[0013] As shown in FIGS. 6A and 6B, the signal under test "a" which
has this repetition cycle Ta (for example, a repetition frequency
fa=10 GHz) is sampled with a sampling signal b having a frequency
Tb (for example, repetition frequency fb=999,9 MHz) longer than a
repetition cycle Ta of this signal under test "a".
[0014] In this case, it is so constituted that as shown in FIGS. 6A
and 6B, the sampling position of the sampling signal b to the
signal waveform having the repetition cycle Ta of this signal under
test "a" is shifted by a small time .DELTA.T with the passage of
time by adjusting a relationship between repetition cycles Ta and
Tb with the result that the sampling position is delayed as seen in
.DELTA.T, 2.DELTA.T, 3.DELTA.T, 4.DELTA.T, 5.DELTA.T, 6.DELTA.T . .
. .
[0015] Consequently, the signal under test c after being sampled
with this sampling signal b comes to have a discrete waveform in
which a pulse-like waveform appears at a position synchronous with
the sampling signal b as shown in FIG. 6C.
[0016] Then, the envelope waveform of each pulse-like waveform
becomes a signal waveform d which is expanded in a direction of
time axis of the signal under test "a".
[0017] A waveform measuring apparatus for measuring the signal
waveform d of the signal under test "a" in the principle of
sampling technique shown in FIGS. 6A, 6B, 6C is constituted, for
example, as shown in FIG. 7.
[0018] The signal under test "a" which has a repetition frequency
fa (repetition cycle Ta) is input to a sampling cycle 1 and a
frequency divider 2.
[0019] The frequency divider 2 sends an output signal obtained by
dividing the repetition frequency fa of the signal under test "a"
to 1/n to the phase comparator 3.
[0020] The voltage control oscillator (VCO) 4 functions as a phase
locked loop (PLL) which generates a signal having a frequency
(fa/n) having a frequency of 1/n (n: positive integer) of the
repetition frequency to feed back the signal to the phase
comparator 3.
[0021] The phase comparator 3 which constitutes a phase locked loop
(PLL) together with the voltage control oscillator (VCO) 4 detects
a phase difference between the phase of the output signal of the
voltage control oscillator (VCO) 4 and a phase of the output signal
of the frequency divider 2 and sends the phase difference to the
voltage control oscillator (VCO) 4 as a phase difference
signal.
[0022] With this phase locked loop (PLL), the phase of the output
signal from the voltage control oscillator (VCO) 4 is synchronized
with the phase of the signal under test "a".
[0023] The frequency (fa/n) of the output signal having a frequency
(fa/n) output from the voltage control oscillator (VCO) 4 is
converted into a frequency of (fa/n) -.DELTA.f by a fixed dividing
rate of frequency divider 5a and a fixed multiplying rate of
frequency multiplier 5b to be input to the sampling signal
generation circuit 6.
[0024] Here, the sampling signal generation circuit 6 applies a
sampling signal b having a repetition frequency (fb) as shown in an
equation (1) which is synchronized with the output signal which is
input and a repetition cycle (Tb) as shown in the equation (2) to
the sampling circuit 1.
fb=(fa/n)-.DELTA.f (1)
Tb=(nTa)+.DELTA.T (2)
[0025] However, the relationship between .DELTA.f and .DELTA.T can
be approximately shown in the equation (3).
.DELTA.f/.DELTA.T=fa.sup.2/n.sup.2 (3)
[0026] Then, the sampling circuit 1 sends a signal under test c
which is sampled by sampling the signal under test "a" which has
been input in synchronization with the sampling signal b input from
the sampling signal generation circuit 6 to the next signal
processing/waveform display portion 7.
[0027] This signal processing/waveform display portion 7 calculates
an envelope waveform of the signal under test c after being sampled
while converting a magnification of the time axis of this envelope
waveform into the magnification of the original signal under test
"a" to be displayed and output as a signal waveform d of the
original signal under test "a".
[0028] In this case, the expansion ratio of the envelope waveform
measured with respect to the signal waveform d of the signal under
test "a" is (fa/n.DELTA.f).
[0029] Incidentally, in the case where the signal under test "a" is
not an electric signal but is an optical signal, this optical
signal is converted into an electric signal to be applied to the
frequency divider 2.
[0030] Furthermore, in the case where the signal under test "a" is
not an electric signal but is an optical signal, for example, an
electro-absorption modulator is used instead of the sampling
circuit 1.
[0031] This electro-absorption modulator is capable of sampling a
pulse-like signal under test "a" that is an input optical signal by
applying a pulse-like electric field that is a sampling signal to
the electro-absorption modulator.
[0032] Then, the signal under test c that is an optical signal
which is sampled is sent to the signal processing/waveform display
portion 7 after being converted into an electric signal.
[0033] However, the following problems to be settled are provided
even in a conventional waveform measuring apparatus using a
sampling technique shown in FIG. 7.
[0034] That is, an output signal from the fixed multiplying rate of
frequency multiplier 5b for creating a sampling signal b having a
repetition signal fb (fa/n)-.DELTA.f output from the sampling
signal generation circuit 6 is created with a phase locked loop
(PLL) circuit comprising a fixed dividing rate of frequency divider
2 for dividing the signal under test "a", the phase comparator 3
and the voltage control oscillator (VCO) 4.
[0035] In this manner, the sampling signal b is an equivalent to
that is created by processing the signal under test "a" which is an
object of measurement with the result that such sampling signal b
is constantly phase synchronized with the signal under test
"a".
[0036] Consequently, the jitter generation amount in the timing of
sampling to the signal waveform d of the signal under test "a" is
suppressed, so that the measurement precision of the signal
waveform d of the signal under test "a" is improved.
[0037] However, the repetition frequency fb of the sampling signal
b is represented in a function of a repletion frequency fa of the
signal under test "a" as apparent from the above equations (1) and
(2).
[0038] This fact means that the repetition frequency fb of the
sampling signal b cannot be arbitrarily set independently of the
repetition frequency fa of the signal under test "a" when using the
fixed dividing rate of frequency divider and the fixed multiplying
rate of frequency multiplier.
[0039] That is, in the conventional waveform measuring apparatus,
as shown in FIG. 7, when the repetition frequency fa of the signal
under test "a" changes, the time resolution of the signal waveform
d of the signal under test "a", namely the measurement precision
automatically changes.
[0040] So that the signal waveform d of the signal under test "a"
cannot be measured in an arbitrary time resolution.
[0041] Furthermore, since the sampling signal b is directly created
from the signal under test "a", there is a problem in that a
complicated circuit structure is required which comprises the
frequency divider 2, the phase comparator 3, the voltage control
oscillator (VCO) 4, the frequency divider 5a and the multiplier
5b.
BRIEF SUMMARY OF THE INVENTION
[0042] In view of the above situation, an object of the present
invention is to provide a waveform measuring apparatus which is
capable of improving a measurement precision of a signal waveform
of a signal under test and is capable of measuring the signal
waveform in an arbitrary resolution precision because a frequency
of a sampling signal for sampling the signal under test can be
arbitrarily set independently of a repetition frequency of the
signal under test by measuring the repetition and creating the
sampling signal by using a common reference signal.
[0043] The present invention can be applied to the waveform
measuring apparatus for sampling the signal under test which has an
arbitrary repetition cycle which is input with a sampling signal
having a cycle longer than the repetition cycle of the signal under
test to determine an envelope waveform of the signal under test
which is sampled, the apparatus determining the signal waveform of
the signal under test from this envelope waveform.
[0044] In order to attain the above object, there is provided a
waveform measuring apparatus (1) having sampling signal generation
means (16) for generating a sampling signal having a cycle longer
than a repetition cycle of a signal under test, a sampling portion
(12) for sampling the signal under test in synchronization with the
sampling signal from the sampling signal generation means and data
processing portion (23) for determining an envelope waveform of a
signal under test which is sampled with the sampling portion, and
determining a signal waveform of the signal under test from this
envelope waveform; the apparatus comprising:
[0045] reference signal generation means (14) for generating a
reference signal independently of a repetition cycle of the signal
under test;
[0046] frequency measuring means (15) for measuring a repetition
frequency of the signal under test by using a reference signal from
the reference signal generation means; and
[0047] sampling frequency setting means (20) for computing and
setting a value of a frequency of the sampling signal which can
obtain a desired delay time with respect to a phase of the signal
under test based on a value of a repetition frequency measured with
the frequency measuring means;
[0048] wherein the sampling signal generation means uses the
reference signal from the reference signal generation means and the
value of the frequency set by the sampling frequency setting means
to generate a sampling signal having a cycle corresponding to the
frequency.
[0049] In the waveform measuring apparatus which is constituted in
this manner, the reference signal generation means (14) generates a
reference signal independently of the repetition cycle of the
signal under test.
[0050] The frequency measuring means (15) measures the repetition
frequency of the signal under test by using a reference signal from
the reference signal generation means.
[0051] The sampling signal frequency setting means (20) sets a
frequency of a sampling signal which can obtain a desired delay
time with respect to a phase of the signal under test by using a
repetition frequency measured with the frequency measuring
means.
[0052] Consequently, the repetition frequency (repetition cycle) of
the signal under test is accurately measured with the frequency
measuring means.
[0053] Then, the sampling frequency setting means sets a frequency
of the sampling signal which can obtain a desired delay time with
respect to the phase of the signal under test by using the
repetition frequency measured with the frequency measuring
means.
[0054] Then, the sampling signal generation means creates a
sampling signal having a cycle of the frequency which is set so
that a desired delay time can be obtained with respect to the phase
of the signal under test.
[0055] In this case, since it is possible to set the frequency of
the sampling signal in an arbitrary relation with respect to the
repetition frequency of the signal under test so that the signal
waveform of the signal under test can be measured in an arbitrary
resolution.
[0056] Furthermore, the repetition frequency of the signal under
test is measured and the sampling signal is created by using a
common signal.
[0057] Consequently, with respect to the sampling signal, the set
state of the frequency set in advance with respect to the signal
under test can be accurately measured so that the precision in the
measurement of the waveform can be improved.
[0058] Furthermore, in order to attain the above object, according
to the present invention, there is provided a waveform measuring
apparatus (2) according to (1), wherein the reference signal
generation means includes a rubidium atomic oscillator.
[0059] Furthermore, in order to attain the above object, according
to the present invention, there is provided a waveform measuring
apparatus (3) according to (1), wherein the reference signal
generation means includes a cesium oscillator.
[0060] In order to attain the above object, according to the
present invention, there is provided a waveform measuring apparatus
(4) according to (1), further comprising:
[0061] a power divider for dividing the signal under test which is
an optical signal into two directions when the signal under test is
the optical signal; and
[0062] a photo detector for converting a signal under test which is
one optical signal which is divided with the power divider into a
signal under test of an electric signal;
[0063] wherein the repetition frequency of the signal under test
which is converted into an electric signal by the photo detector is
measured with the frequency measuring means,
[0064] the measured value of the repetition frequency of the signal
under test measured with the frequency means is given to the
sampling frequency setting means.
[0065] Furthermore, in order to attain the above object, there is
provided a waveform measuring apparatus (5) according to (1),
further comprising:
[0066] a power divider for dividing the signal under test which is
an optical signal into two directions when the signal under test is
the optical signal; and
[0067] a clock recovery for converting a signal under test which is
an optical signal into a signal under test of an electric signal
having a repetition frequency and outputting the signal by
detecting a clock of the repetition cycle from one signal under
test divided with the power divider;
[0068] wherein the repetition frequency of the signal under test
which is converted into an electric signal with the clock recovery
is measured with the frequency measuring means, and
[0069] the measured value of the repetition frequency of the signal
under test measured with the frequency measuring means is given to
the sampling frequency setting means.
[0070] Furthermore, in order to attain the above object, according
to the present invention, there is provided a waveform measuring
apparatus (6) according to (1) further comprising:
[0071] a photo detector (21) for receiving a signal under test of
an optical signal sampled with a sampling signal input from the
sampling signal generation circuit with the electro-absorption
modulator and converting the signal under test which is an optical
signal after being sampled into a signal under test which is an
electric signal when the signal under test is an optical signal and
the sampling portion is an electro-absorption modulator;
[0072] an analog/digital converter (22) for converting the signal
under test which is converted into an electric signal with the
photo detector into a signal under test to send the converted
signal to the data processing portion; and
[0073] a display device (24) converting a magnification of a time
axis in the envelope waveform determined with the data processing
portion into a magnification of the original signal under test to
display the magnification as a signal waveform of the signal under
test.
[0074] Additional objects and advantages of the invention will be
set forth in the description which follows, and in part will be
obvious from the description, or may be learned by practice of the
invention. The objects and advantages of the invention may be
realized and obtained by means of the instrumentalities and
combinations particularly pointed out hereinafter.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
[0075] The accompanying drawings, which are incorporated in and
constitute a part of the specification, illustrate embodiments of
the invention, and together with the general description given
above and the detailed description of the embodiments given below,
serve to explain the principles of the invention.
[0076] FIG. 1 is a block diagram showing a general structure of a
waveform measuring apparatus according to a first embodiment of the
present invention;
[0077] FIG. 2 is a view showing a signal waveform of a signal under
test measured with the waveform measuring apparatus according to
the first embodiment of the present invention;
[0078] FIG. 3 is a view showing a signal waveform of a signal under
test measured with the waveform device according to the first
embodiment of the present invention;
[0079] FIG. 4 is a block diagram showing a general structure of a
waveform measuring apparatus according to a second embodiment of
the present invention;
[0080] FIGS. 5A, 5B, 5C and 5D are waveform diagrams showing a
signal under test and a recovery clock signal for explaining an
effect of a clock recovery device incorporated in a waveform
measuring apparatus according to a third embodiment of the present
invention;
[0081] FIGS. 6A, 6B and 6C are views for explaining a principle for
determining the signal waveform of a conventional representative
signal under test with a sampling method; and
[0082] FIG. 7 is a block diagram showing a general structure of a
conventional waveform measuring apparatus.
DETAILED DESCRIPTION OF THE INVENTION
[0083] Reference will now be made in detail to the presently
preferred embodiments of the invention as illustrated in the
accompanying drawings, in which like reference numerals designate
like or corresponding parts.
[0084] Hereinafter, each of the embodiments of the present
invention will be explained by using the drawings of each of the
embodiments.
[0085] (First Embodiment)
[0086] FIG. 1 is a block diagram showing a general structure of a
waveform measuring apparatus according to a first embodiment of the
present invention.
[0087] In the beginning, for example, the signal under test "a"
which comprises an optical signal having a repetition cycle Ta
(repetition frequency fa) input from the input terminal (not shown)
is divided off into two directions with the power divider 11. One
of the signals is incident on an electro-absorption modulator 12 as
a sampling circuit while the other signal is incident on the photo
detector 13.
[0088] Incidentally, it is assumed that in the waveform measuring
apparatus according to the present invention, the repetition
frequency fa of the signal under test "a" is set to 10 GHz.
[0089] The reference signal oscillator 14 oscillates, for example,
a reference signal h having a reference frequency fs (=10 MHz) of
10 MHz, so that this reference signal "h" is applied to the input
terminal (REF1) of the reference signal of the frequency counter 15
and the reference signal input terminal (REF2) of the frequency
synthesized signal generator 17 in the sampling signal generation
circuit 16.
[0090] In this case, as the reference signal oscillator 14 which
serves as a reference signal oscillating means, a highly stable
oscillator such as a rubidium atomic oscillator having a stability
of oscillation frequency on the order of 3.times.10.sup.-12/sec and
a cesium atomic oscillator having a stability on the order of
8.times.10.sup.-12/sec or the like is used.
[0091] Then, the photo detector 13 converts the signal under test
"a" of the incident optical signal into a signal under test "a" of
the electric signal to send the signal to the frequency counter 15
as a frequency measuring means.
[0092] The frequency counter 15 uses the reference signal "h"
applied to the reference signal input terminal (REF1) to measure
the repetition cycle fa of the signal under test al of the input
electric signal.
[0093] Specifically, this frequency counter 15 counts the frequency
of the reference signal "h" to create the base measurement period
and count the number of clocks (number of waves) of the signal
under test input to this reference signal measurement period.
[0094] Then, the frequency counter 15 sends the repetition
frequency fa of the signal under test to the sampling frequency
setting portion 20.
[0095] The sampling frequency setting portion 20 in this control
portion 19 calculates the repetition frequency fb of the sampling
signal b output from the sampling signal generation circuit 16 by
using the above equation (1) from the repetition frequency fa of
the input signal under test "a" input from the frequency counter
15.
[0096] For example, in the case where the measured value under the
condition of n=10, and .DELTA.f=0.1 Mz is fa=10 GHz, the repetition
frequency of the sampling signal b is set to fb=999.9 MHz from
fb=(fa/n)-.DELTA.f.
[0097] Next, the sampling frequency setting portion 20 sets the
calculated repetition frequency fb to the frequency synthesized
signal generator 17 in the sampling signal generation circuit
16.
[0098] Here, the frequency synthesized signal generator 17 in the
sampling frequency generation circuit 16 comprises, for example,
frequency synthesizers. It is possible to create a signal having an
arbitrary frequency by multiplying or dividing a reference signal
"h" having a reference frequency of fs=10 MHz which is applied to
the reference signal input terminal (REF2).
[0099] Specifically, this frequency synthesized signal generator 17
creates a sine waveform signal having a frequency fb of the
repetition frequency designated in the equation (1) from the
sampling frequency setting portion in the control portion 19.
[0100] Then, the signal of the sine waveform having a frequency fb
output from the frequency synthesized signal generator 17 is
trimmed in the waveform into the sampling signal b with a pulse
waveform configuration having a repetition frequency fb (repetition
cycle Tb) as shown in FIG. 6B in the following waveform trimmer
circuit 18.
[0101] In this manner, the sampling signal b having a repetition
frequency fb output from the sampling signal generation circuit 16
is input to the electro-absorption modulator 12.
[0102] This electro-absorption modulator 12 samples the sampling
signal b obtained by inputting the signal under test "a" of the
incident optical signal from the sampling signal generation circuit
16 to emit the sampled optical signal to the photo detector 21.
[0103] This photo detector 21 converts the signal under test c of
the optical signal after incident sampling into the signal under
test c1 of the electric signal.
[0104] Then, the signal under test c1 output from the photo
detector 21 is A/D converted into a signal under test c2 which is
digitally sampled with the digital/analog (A/D) converter 22 to be
input to the data processing portion 23.
[0105] This data processing portion 23 calculates the envelope
waveform of the signal under test c2 which is input and sampled. As
shown in FIG. 2, the magnification of the time axis of this
envelope waveform is converted into the magnification of the
original signal under test "a" to be output and displayed on the
display portion 24 as a signal waveform d of the signal under test
"a".
[0106] At this time, the display control portion 25 in the control
portion 19 has a function of monitoring the signal waveform d of
the signal under test "a" which is displayed on the display portion
24 and automatically correcting the display position of the signal
waveform d to a normal position.
[0107] Specifically, the display control portion 25 in the control
portion 19 is such that the signal waveform d of the signal under
test "a" which is displayed on the display portion 24 is drifted on
the display screen resulting from the fact that the repetition
frequency fa of the input signal under test "a" and the repetition
frequency fb of the sine signal created at the frequency
synthesized signal generator 17 do not accurately satisfy the
equation (1).
[0108] Then, the display control portion 25 of the control portion
19 monitors the signal waveform d of the signal under test "a"
displayed on the display portion 24 and adjusts the scanning
(sweep) start position on the display screen of the signal waveform
d so that this drift is not ostensibly generated.
[0109] In the waveform measuring apparatus according to the first
embodiment which is constituted in this manner, the repetition
frequency fa (repetition cycle Ta) of the input signal under test
"a" is measured with the frequency counter 15.
[0110] Then, the sampling frequency setting portion 20 in the
control portion 19 calculates the frequency fb of the sampling
signal b by using this measured repetition frequency fa in the
above equation (1) to set the frequency fb to the frequency
synthesized signal generator 17 in the sampling signal generation
circuit 16.
[0111] In this case, it is possible to maintain the frequency fb of
the sampling signal b in an arbitrary relation with respect to the
repetition frequency fa of the signal under test "a" by
appropriately adjusting the values of n and .DELTA.f.
[0112] In other words, the frequency fb of the sampling signal b
can be set to an arbitrary value independently of the repetition
frequency fa of the signal under test "a".
[0113] Furthermore, the reference signal "h" from the reference
signal oscillator 14 is applied to the frequency counter 15 and the
frequency synthesized signal generator 17 in the sampling signal
generation circuit 16.
[0114] Consequently, the repetition frequency fa of the signal
under test "a" is measured and the sampling signal b is created by
using a common reference signal "h".
[0115] Consequently, since the set state of the frequency between
the frequency fb of the sampling signal b which is set in advance
and the repetition frequency fa is accurately maintained, the
precision in the measurement of the waveform can be improved.
[0116] Furthermore, as described above, in the state in which the
set state of the frequency between the frequency fb of the sampling
signal b and the repetition frequency fa of the signal under test
"a" is accurately maintained, the signal waveform d' shown in a
solid line displayed in the display portion 24 moves as in a signal
waveform d' shown in a dot line when the phase of the signal under
test "a" changes, as shown in FIG. 3.
[0117] Consequently, the phase change amount .phi. in the signal
under test "a" can be grasped by measuring this movement quantity
Td.
[0118] Furthermore, as described above, when the relationship
between the frequency fb of the sampling signal b and the
repetition frequency fa of the signal under test "a" does not
satisfy the equation (1), the signal waveform d displayed in the
display portion 24 continues to be drifted.
[0119] From the measurement start time, for example, only in the
initial measurement period of 5 through 10 sec, the frequency
counter 15 is driven to determine the frequency fb of an accurate
sampling signal b by using the equation (1) at the sampling
frequency setting portion 20 to be set at the sampling signal
generation circuit 16.
[0120] After the lapse of the initial measurement period, the
measurement in the frequency counter 15 and the sampling frequency
setting portion 20, and the calculation operation are suspended,
and the frequency fb of the sampling signal b is fixed to the value
determined at the initial measurement period.
[0121] Furthermore, the display adjustment operation of the display
control portion 25 described above is suspended.
[0122] In such a state, when the repetition frequency fa of the
signal under test "a" is frequency drifted from the initial period
measurement period, the signal waveform d displayed on the display
portion 24 continues to be drifted in correspondence to this
frequency drift.
[0123] Consequently, it is possible to grasp the frequency change
amount in a signal under test "a" by measuring the drift amount per
unit time of the signal waveform d displayed on the display portion
24.
[0124] (Second Embodiment)
[0125] FIG. 4 is a block view showing a general structure of a
waveform measuring apparatus according to a second embodiment of
the present invention.
[0126] In FIG. 4, like portions as the waveform measuring apparatus
according to the first embodiment shown in FIG. 1 are denoted by
like reference numerals and a detailed explanation on the
overlapped portions is omitted.
[0127] In the waveform measuring apparatus according to the second
embodiment, the signal under test "a" having a repetition frequency
fa input from the outside is incident on the electro-absorption
modulator 12 as it is while the signal is divided with the power
divider 11 to be incident on the clock recovery 26.
[0128] Conventionally, as well known, the clock recovery 26 detects
the start timing of the repetition cycle Ta, namely, the clock of
the repetition cycle Ta (frequency fa) to convert the signal under
test "a" that is an incident optical signal into a recovery clock
signal g as shown in FIG. 5D which is an electric signal having a
frequency fa (repetition frequency) and send the signal to the next
frequency counter 15.
[0129] This frequency counter 15 measures the frequency (repetition
frequency fa) of the recovery clock signal g of the input electric
signal to send the data of the measured repetition frequency fa to
the sampling frequency setting portion 20 of the control portion
19.
[0130] The following operation is the same as the operation of the
waveform measuring apparatus according to the first embodiment
shown in FIG. 1.
[0131] In the waveform measuring apparatus according to the second
embodiment which is constituted in this manner, the repetition
frequency fa of the signal under test "a" can be measured at the
frequency counter 15. Consequently, like the waveform measuring
apparatus according to the first embodiment, the signal waveform d
of the signal under test "a" can be measured in an arbitrary
resolution.
[0132] Furthermore, in the waveform measuring apparatus according
to the second embodiment, the clock of the repetition frequency Ta
(frequency fa) is detected by using the clock recovery 26 to send
the recovery clock signal g to the frequency counter 28.
[0133] That is, as shown in each of the waveforms shown in FIGS. 5B
and 5C, the waveform of the signal under test "a" has various
configurations. Like the waveform shown in FIG. 5A, it does not
always happen that a clear one peak waveform is present for each of
the repetition cycle Ta (frequency fa).
[0134] Consequently, even when the signal under test "a" having a
such a waveform is counted with the frequency counter 26 to
directly count the repetition frequency fa (repetition cycle Ta),
there is a problem that many peak waveforms and few peak waveforms
are counted so that an erroneous repetition frequency fa is
output.
[0135] Then, the clock of the signal under test "a" is reproduced
by using the clock recovery 26 to obtain a recovery clock signal g
as shown in FIG. 5D with the result that the repetition frequency
fa (repetition cycle Ta) of the signal under test "a" can be
detected at a high precision even when the signal under test "a"
has a complicated configuration as shown in FIGS. 5B and 5C.
[0136] Incidentally, the present invention is not limited to the
structure of the device according to the first and the second
embodiment.
[0137] For example, in each of the waveform measuring apparatus,
the repetition frequency fa of the signal under test "a" measured
with the frequency counter 15 is automatically set to the sampling
frequency setting portion 20.
[0138] However, the measured value (repetition frequency fa) of the
frequency counter 15 may be read with the eye and may be
automatically set in the sampling frequency setting portion 20 of
the control portion 19 in a manual operation by the operator.
[0139] Further, in each of the embodiments, the signal under test
"a" is assumed to be an optical signal.
[0140] However, the signal under test "a" may be a normal electric
signal.
[0141] In this case, instead of the electro-absorption modulator
12, a sampling circuit 1 which is used in the normal electric
signal shown in FIG. 7 is adopted and the photo detector 13 and 21
are removed.
[0142] As has been explained above, in the waveform measuring
device according to the present invention, the repetition frequency
of the signal under test is measured and the sampling signal is
created by using a common reference signal.
[0143] Thus, in the waveform measuring apparatus according to the
present invention, the frequency of the sampling signal for
sampling the signal under test can be arbitrarily set independently
of the repetition frequency of the signal under test while the set
state of the signal under test with respect to the repetition
frequency of the signal under test can be accurately maintained.
Consequently, the measurement precision of the signal waveform of
the signal under test can be improved while the signal waveform can
be measured in an arbitrary resolution precision.
[0144] Consequently, according to the present invention, the
frequency of the sampling signal for sampling the signal under test
can be arbitrarily set independently of the repetition frequency of
the signal under test by measuring the repetition frequency of the
signal under test and creating a sampling signal while the set
state with respect to the repetition frequency of the signal under
test can be accurately maintained. Consequently, it becomes
possible to provide a waveform measuring device which is capable of
improving the measurement precision of the signal waveform of the
signal under test and measuring the signal waveform at an arbitrary
resolution precision.
[0145] Additional advantages and modifications will readily occur
to those skilled in the art. Therefore, the invention in its
broader aspects is not limited to the specific details and
representative embodiments shown and described herein. Accordingly,
various modifications may be made without departing from the spirit
or scope of the general inventive concept as defined by the
appended claims and their equivalents.
* * * * *