Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
2001-08-28
2004-01-13
Porta, David (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S216000, C324S076390
Reexamination Certificate
active
06677577
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-263591, filed Aug. 31, 2000, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a waveform measuring method and apparatus. More particularly, the present invention relates to a waveform measuring method and apparatus for obtaining a signal waveform of a signal under test having an arbitrary repetition cycle outputted from a signal under test generator as a test object.
2. Description of the Related Art
In general, a signal under test generator for generating an electrical signal, an optical signal or the like each having an arbitrary repetition cycle incorporates a reference signal oscillator for generating a reference signal that has a reference frequency “fs” and a waveform pattern generating portion for generating a waveform pattern of a signal waveform.
In such a signal under test generator, a reference signal outputted from the reference signal generator is employed so as to generate a repetition frequency signal that has a specified repetition frequency “fa”. In addition, this repetition frequency signal and a waveform pattern outputted from the waveform pattern generating portion are employed so as to generate an electrical signal or an optical signal each having an arbitrary repetition cycle “Ta”.
The electrical signal and optical signal each having an arbitrary repetition cycle Ta outputted from such a signal under test generator are generally incorporated in an information communication system, and, for example, are employed as test signals of a variety of communication devices including an optical transmission cable.
Therefore, it is required to measure characteristics of the electrical signal or optical signal outputted from the signal under test generator in detail prior to testing a variety of communication devices containing the optical transmission cable incorporated in the information communication system.
One of the characteristic measurements of the electrical signal or optical signal is a signal waveform measurement.
Conventionally, there is proposed a variety of measuring techniques for measuring a signal waveform of a signal under test such as the electrical signal, optical signal or the like each having this arbitrary repetition cycle Ta.
However, in the case where the repetition cycle Ta of the signal under test, i.e., the repetition frequency “fa” is a high frequency signal that exceeds 10 GHz, the signal waveform of such a signal under test cannot be directly observed on a display screen such as oscilloscope. Thus, the selection range of the waveform measuring technique itself is limited.
A typical technique of measuring a signal waveform of a signal under test of which the repetition frequency “fa” exceeds 10 GHz will be described with reference to
FIGS. 7A
,
7
B, and
7
C.
As shown in
FIGS. 7A and 7B
, a signal under test “a” having a repetition cycle Ta (for example, repetition frequency “fa”=10 GHz) is sampled by a sampling signal “b” having a cycle Tb (for example, repetition frequency “fb”=999.9 MHz) longer than the repetition cycle Ta of this signal under test “a”.
In this case, a mutual relationship between the repetition periods Ta and Tb is adjusted, whereby the sampling position of a sampling signal “b” in the signal waveform in the repetition cycle Ta of the signal under test “a” is shifted by a differential time &Dgr;T together with an elapse of time, as shown in
FIGS. 7A and 7B
, and is provided so as to be delayed as &Dgr;T, 2&Dgr;T, 3&Dgr;T, 4&Dgr;T, 5&Dgr;T, 6&Dgr;T . . . .
Therefore, a signal under test “c” after sampled by this sampling signal “b” is obtained as discrete waveform in which a pulse shaped waveform is generated at a position synchronized with the sampling signal “b”, as shown in FIG.
7
C.
Then, the enveloped waveform of each pulse shaped waveform is obtained as a signal waveform “d” extended in a time axis direction of the signal under test “a”.
A waveform measuring apparatus for measuring a signal waveform “d” of the signal under test “a” is configured as shown in
FIG. 8
, for example, based on the principle of the sampling technique shown in
FIGS. 7A
,
7
B, and
7
C.
The signal under test “a” having the repetition frequency “fa” (repetition cycle Ta) is inputted to a sampling circuit
1
and a frequency divider
2
.
The frequency divider
2
delivers to a phase comparator
3
an output signal obtained by frequency dividing the repetition frequency “fa” of the signal under test “a” into 1
.
A voltage control oscillator (VCO)
4
functions as a phase locked loop (PLL) for generating a signal that has a frequency (fa
) of 1
(n: positive integer) of the repetition frequency “fa” so as to feed the signal back to the phase comparator
3
.
The phase comparator
3
configuring the phase locked loop (PLL) detects a phase difference between a phase of an output signal of the voltage control oscillator (VCO)
4
and an output of the frequency divider
2
, and delivers a phase difference signal to the voltage control oscillator (VCO)
4
.
A phase of an output signal from the voltage control oscillator (VCO)
4
is synchronized with a phase of the signal under test “a” by means of this phase locked loop (PLL).
A frequency (fa
) of an output signal having a frequency (fa
) outputted from the voltage control oscillator (VCO)
4
is converted into a frequency of (fa
)−&Dgr;f by means of a next stationary frequency divider
5
a
and a stationary multiplier
5
b
, and the converted frequency is inputted to a sampling signal generator circuit
6
.
Here, the sampling signal generator circuit
6
applies to the sampling circuit
1
a repetition frequency (fb) as shown in formula (1) synchronized with an inputted output signal and a sampling signal “b” having a repetition cycle (Tb).
fb
=(
fa
)−&Dgr;
f
(1)
Tb
=(
nTa
)+&Dgr;
T
(2)
However, a relationship between &Dgr;f and &Dgr;T is shown in formula (3)
&Dgr;
f/&Dgr;T=fa
2
2
(3)
Then, the sampling circuit
1
samples the inputted signal under test “a” by means of the sampling signal “b” inputted from the sampling signal generator circuit
6
, thereby delivering the sampled signal under test “c” to a next signal processing/waveform display portion
7
.
This signal processing/waveform display portion
7
calculates an enveloped waveform of the inputted signal under test “c” after sampled, and a scale of a time axis of this enveloped waveform is converted into a scale of the original signal under test “a”, whereby the signal waveform “d” of the original signal under test “a” is outputted to be displayed.
In this case, a magnification ratio of the signal under test “a” of the measured, enveloped waveform to the signal waveform “d” is obtained as (fa
&Dgr;f).
In the case where the signal under test “a” is an optical signal instead of an electrical signal, this optical signal is applied to the frequency divider
2
after converted into an electrical signal.
In addition, in the case where the signal under test “a” is an optical signal instead of an electrical signal, an electro-absorption modulator, for example, is employed instead of the sampling circuit
1
.
This electro-absorption modulator is applied a pulse shaped electric field that is a sampling signal, thereby making it possible to sample the pulse shaped signal under test “a” that is an optical signal that is inputted to the electro-adsorption modulator.
Then, the signal under test “c” that is the thus sampled optical signal is delivered to a signal processing/waveform display portion
7
after converted into an electrical signal.
However, a conventional waveform measuring apparatus employing a sampling technique shown in
FIG. 8
has the following problems to be solved.
That is, an output signal from a multiplier
5
b
for generating a sampling signal “b” that has a repeti
Otani Akihito
Otsubo Toshinobu
Watanabe Hiroto
Anritsu Corporation
Frishauf Holtz Goodman & Chick P.C.
Lee Patrick J
Porta David
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