Coded data generation or conversion – Converter calibration or testing
Reexamination Certificate
2001-08-28
2002-06-18
Tokar, Michael (Department: 2819)
Coded data generation or conversion
Converter calibration or testing
C341S118000, C341S115000, C341S155000, C324S076150
Reexamination Certificate
active
06407686
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-263592, 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 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.
2. Description of the Related Art
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.
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.
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.
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.
As one characteristic of this electric signal and the optical signal, the signal waveform is measured.
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.
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.
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
,
6
B and
6
C.
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”.
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 &Dgr;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 &Dgr;T, 2&Dgr;T, 3&Dgr;T, 4&Dgr;T, 5&Dgr;T, 6&Dgr;T . . . .
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.
6
C.
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”.
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
,
6
B,
6
C is constituted, for example, as shown in FIG.
7
.
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
.
The frequency divider
2
sends an output signal obtained by dividing the repetition frequency fa of the signal under test “a” to 1
to the phase comparator
3
.
The voltage control oscillator (VCO)
4
functions as a phase locked loop (PLL) which generates a signal having a frequency (fa
) having a frequency of 1
(n: positive integer) of the repetition frequency to feed back the signal to the phase comparator
3
.
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.
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”.
The frequency (fa
) of the output signal having a frequency (fa
) output from the voltage control oscillator (VCO)
4
is converted into a frequency of (fa
) −&Dgr;f by a fixed dividing rate of frequency divider
5
a
and a fixed multiplying rate of frequency multiplier
5
b
to be input to the sampling signal generation circuit
6
.
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
)−&Dgr;
f
(1)
Tb
=(
nTa
)+&Dgr;
T
(2)
However, the relationship between &Dgr;f and &Dgr;T can be approximately shown in the equation (3).
&Dgr;
f/&Dgr;T=fa
2
2
(3)
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
.
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”.
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
&Dgr;f).
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
.
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
.
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.
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.
However, the following problems to be settled are provided even in a conventional waveform measuring apparatus using a sampling technique shown in FIG.
7
.
That is, an output signal from the fixed multiplying rate of frequency multiplier
5
b
for creating a sampling signal b having a repetition signal fb (fa
)−&Dgr;f output from the sampling signal generation circuit
6
is created with a pha
Otani Akihito
Otsubo Toshinobu
Watanabe Hiroto
Anritsu Corporation
Frishauf Holtz Goodman & Chick P.C.
Mai Lam T.
Tokar Michael
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