Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Electrical signal parameter measurement system
Reexamination Certificate
2000-03-29
2002-10-01
Hoff, Marc S. (Department: 2739)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Electrical signal parameter measurement system
C375S226000, C375S359000, C375S360000, C375S371000, C324S622000, C331S004000
Reexamination Certificate
active
06460001
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an apparatus and a method that are applied to a measurement of a jitter of, for example, a microprocessor clock, and are used for estimating a fluctuation of a zero crossing interval of an input signal and a peak jitter (period jitter), particularly for estimating a worst value of peak jitter and its occurrence probability.
A clock frequency of microprocessor is doubled in every forty months. The shorter a clock period is, the more severe jitter measurement is required. This is because a timing error in the system operation must be avoided.
Incidentally, there are two types of jitters, i.e., a period jitter and a timing jitter. A period jitter becomes a problem since, in a computer clock for example, an upper limit of its operation frequency is determined by a period jitter. As shown in
FIG. 1A
, in a jitter-free ideal clock signal, for example, an interval T
int
between adjacent rising points is constant as indicated by a dotted line waveform, and in this case a period jitter is zero. In an actual clock signal, a rising edge fluctuates from an arrow toward leading side or trailing side, i.e., an interval T
int
between adjacent rising points fluctuates, and this fluctuation of the interval is a period jitter. For example, in the case of a sine wave that does not have a rectangular waveform like a clock signal, a fluctuation of an interval T
int
between zero-crossing points is also a period jitter. A period jitter becomes a problem of computer clocks.
On the other hand, a timing jitter is defined as the timing deviation from an ideal point in, for example, data communication. As shown in
FIG. 1B
, when a jitter-free square waveform is assumed to be a dashed line waveform, a deviation width &Dgr;&phgr; of an actual rising point (solid line) from a normal rising point (dashed line) is a timing jitter in the case of a jittery square waveform.
A conventional measurement of a period jitter is performed by a time interval analyzer (hereinafter, this measuring method is referred to as a time interval method or a TIA method). This is shown in “Phase Digitizing Sharpens Timing Measurements” by David Chu, IEEE Spectrum, pp. 28-32, 1988, and “Time Domain Analysis and Its Practical Application to the Measurement of Phase Noise and Jitter”, by Lee D. Cosart et al., IEEE Trans. Instrum. Meas., vol.46, pp. 1016-1019, 1997. This time interval method is a method in which zero-crossing points of a signal under test are counted, an elapsed time is measured, and a time fluctuation between zero-crossing points is estimated to obtain a period jitter. In this time interval method, it takes a long time to perform those measurements since data present between zero-crossings are not utilized for the measurements.
There is a method, as a conventional timing jitter measurement, in which a timing jitter is measured in frequency domain using a spectrum analyzer. Since, in this method, a low frequency range is swept to measure a phase noise spectrum, it takes approximately 10 minutes or so for the measurement.
From those view points, inventors of the present invention have proposed a method of measuring a jitter as described below in an article entitled “an application of an instantaneous phase estimating method to a jitter measurement” in a technical report “Probo” pp. 9-16 issued by Probo Editorial Room of ADVANTEST CORPORATION, Nov. 12, 1999.
That is, as shown in
FIG. 2
, an analog clock waveform from a PLL circuit under test (Phase locked loop)
11
is converted into a digital clock signal x
c
(t) by an digital-analog converter
12
, and the digital clock signal x
c
(t) is supplied to a Hilbert pair generator
14
acting as analytic signal transforming means
13
, where the digital clock signal x
c
(t) is transformed into an analytic signal z
c
(t).
Now, a clock signal x
c
(t) is defined as follows.
x
c
(
t
)=
A
c
cos(2
&pgr;f
c
t+&thgr;
c
+&Dgr;&phgr;(
t
))
The A
c
and the f
c
are nominal values of amplitude and frequency of the clock signal respectively, the &thgr;
c
is an initial phase angle, and the &Dgr;&phgr;(t) is a phase fluctuation that is called a phase noise.
The clock signal x
c
(t) is Hilbert-transformed by a Hilbert transformer
15
in the Hilbert pair generator
14
to obtain the following equation.
{circumflex over (x)}
c
(
t
)=
H[x
c
(
t
)]=
A
c
sin(2
&pgr;f
c
t+&thgr;
c
+&Dgr;&phgr;(
t
))
Then, an analytic signal z
c
(t) having x
c
(t) and {circumflex over (x)}
c
(t) as a real part and an imaginary part, respectively is obtained as follows.
z
c
(
t
)
=
x
c
(
t
)
+
x
^
c
(
t
)
=
A
c
cos
(
2
π
f
c
t
+
θ
c
+
Δφ
(
t
)
+
j
A
c
sin
(
2
π
f
c
t
+
θ
c
+
Δφ
(
t
)
)
From this analytic signal z
c
(t), an instantaneous phase &THgr;(t) of the clock signal x
c
(t) can be estimated by the instantaneous phase estimator
16
as follows.
&THgr;(
t
)=[2
&pgr;f
c
t+&thgr;
c
+&Dgr;&phgr;(
t
)] mod 2&pgr;
A linear phase is removed from this instantaneous phase &THgr;(t) by a linear phase remover
17
to obtain a phase noise waveform &Dgr;&phgr;(t). That is, in the linear phase remover
17
, a continuous phase converting part
18
applies a phase unwrap method to the instantaneous phase &THgr;(t) to obtain a continuous phase &thgr;(t) as follows.
&thgr;
c
(
t
)=2
&pgr;f
c
t+&thgr;
c
+&Dgr;&phgr;(
t
)
The phase unwrap method is shown in “A New Phase Unwrapping Algorithm” by Jose M. Tribolet, IEEE Trans. Acoust., Speech, Signal Processing, vol. ASSP-25, pp. 170-177, 1977 and in “On Frequency-Domain and Time-Domain Phase Unwrapping” by Kuno P. Zimmermann, Proc. IEEE. vol. 75, pp. 519-520, 1987.
A linear phase [2&pgr;f
c
t+&thgr;
c
] of a continuous phase &thgr;(t) is estimated by a linear phase evaluator
19
using a linear trend estimating method. This estimated linear phase [2&pgr;f
c
t+&thgr;
c
] is subtracted from the continuous phase &thgr;(t) by a subtractor
21
to obtain a variable term &Dgr;&phgr;(t) of the instantaneous phase &THgr;(t), i.e., phase noise waveform as follows.
&thgr;(
t
)=&Dgr;&phgr;(
t
)
The phase noise waveform &Dgr;&phgr;(t) thus obtained is inputted to a peak-to-peak detector
22
, where a difference between the maximum peak value max (&Dgr;&phgr;(k)) and the minimum peak value min (&Dgr;&phgr;(1)) of the &Dgr;&phgr;(t) is calculated to obtain a peak value &Dgr;&phgr;
pp
of timing jitters as follows.
Δφ
pp
=
max
k
(
Δφ
(
k
)
)
-
min
l
(
Δφ
(
l
)
)
In addition, the phase noise waveform &Dgr;&phgr;(t) is inputted to a root-mean-square detector
23
, where a root-mean-square value of the phase noise waveform &Dgr;&phgr;(t) is calculated using following equation to obtain a root-mean-square value &Dgr;&phgr;
RMS
of timing jitters.
Δφ
RMS
=
1
N
∑
k
=
0
N
-
1
Δφ
2
(
k
)
A method is called the &Dgr;&phgr; method since it estimates a peak value of timing jitters and/or a root-mean-square value of timing jitters from the phase noise waveform &Dgr;&phgr;(t). According to the &Dgr;&phgr; method, a jitter measurement can be performed in a test time of 100 millisecond order since measuring points are not limited to zero-crossing points. Further, in
FIG. 2
, the analytic signal transforming means
13
, the instantaneous phase estimator
16
and the linear phase remover
17
compose phase noise detecting means
25
.
As mentioned above, it is important, for example in manufacturing computers, to know whether or not the computer can operate even in the worst peak value case of period jitter in the operation clock, namely in the worst case condition that has both the maximum time interval between adjacent rising edges of a clock and the minimum time interval between adjacent rising edges of a clock. From this point of view, i
Ishida Masahiro
Soma Mani
Yamaguchi Takahiro
Advantest Corporation
Gallagher & Lathrop
Hoff Marc S.
Lathrop, Esq. David N.
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