Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Electrical signal parameter measurement system
Reexamination Certificate
2000-03-29
2004-05-11
Hoff, Marc S. (Department: 2857)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Electrical signal parameter measurement system
C702S066000, C702S072000, C702S079000, C702S189000, C702S010000, C702S010000
Reexamination Certificate
active
06735538
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an apparatus and a method for measuring a quality measure of a clock signal that drives, for example, a microprocessor.
A jitter has traditionally been used as a measure for estimating quality of a clock signal of a microprocessor. Incidentally, there are two types of jitters, i.e., a period jitter and a timing 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.
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 obtained to obtain a period jitter. In addition, a root-mean-square value of the period jitters is obtained.
There is a method, as a conventional timing jitter measurement, in which a timing jitter is measured by measuring a phase noise-spectrum in frequency domain, and those spectrums are summed to estimate a root-mean-square value of timing jitters.
The 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 analog-digital 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.
&AutoLeftMatch;
z
c
(
t
)
=
x
c
(
t
)
+
x
^
c
(
t
)
=
A
c
cos
(
2
π
f
c
t
+
θ
c
+
Δ
φ
(
t
)
+
j
A
c
sin
(
2
π
f
c
t
+
θ
c
+
Δ
φ
(
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;(
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.
Δ
φ
pp
=
max
k
(
Δ
φ
(
k
)
)
-
min
l
(
Δ
φ
(
1
)
)
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.
Δ
φ
RMS
=
1
N
∑
k
=
0
N
-
1
Δ
φ
2
(
k
)
A method for obtaining, in this manner, a peak value of timing jitters and/or a root-mean-square value of timing jitters from the phase noise waveform &Dgr;&phgr;(t) is called a &Dgr;&phgr; method. 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
.
In the case of a jitter that each rising edge of a clock signal fluctuates in the same direction with substantially same quantity, a microprocessor driven by this clock signal is not influenced so much by the jitter. In a design of a PLL circuit that generates a clock signal, a correlation coefficient between rising edges of the clock signal is important. The correlation coefficient takes a vale of −1 to +1. If, for example, this value is 0.6, it can be seen that the PLL circuit has room for improvement in correlation coefficient by 0.4. It can be deemed that a fluctuation between adjacent rising edges of a clock signal consists of a linear fluctuation (signal) in which a fluctuation of a following rising edge depends on a fluctuation of an immediately leading rising edge and a fluctuation (noise) in which a fluctuation of a following rising edge does not relate to a fluctuation of an immediately leading rising edge, whereby a
Ishida Masahiro
Soma Mani
Yamaguchi Takahiro
Advantest Corporation
Gallagher & Lathrop
Lathrop, Esq. David N.
West Jeffrey R
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