Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Phase comparison
Reexamination Certificate
2000-11-24
2003-02-25
Le, N. (Department: 2858)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Phase comparison
C375S226000
Reexamination Certificate
active
06525523
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a jitter measurement apparatus and a jitter measurement method.
A Time Interval Analyzer or an oscilloscope has conventionally been used in a period jitter measurement. Each of those methods is called a Zero-crossing Method. As shown in
FIG. 1
, for example, a clock signal (a signal under measurement) x(t) from a PLL (Phase-Locked Loop) under test
11
is supplied to a time interval analyzer
12
. Regarding a signal under measurement x(t), a next rising edge following one rising edge fluctuates against the preceding rising edge as indicated by dotted lines. That is, a time interval T
p
between two rising edges, namely a period fluctuates. In the Zero-crossing Method, a time interval between zero-crossings (period) is measured, a fluctuation of period is measured by a histogram analysis, and its histogram is displayed as shown in
FIG. 2. A
time interval analyzer is described in, for example, “Phase Digitizing Sharpens Timing Measurements” by D. Chu, IEEE Spectrum, pp.28-32, 1988 and “A method of Serial Data Jitter Analysis Using One-Shot Time Interval Measurements” by J. Wilstrup, Proceedings of IEEE International Test Conference, pp.819-823, 1998.
In addition, Tektronix Inc. and LeCroy Co. have recently been providing digital oscilloscopes each being able to measure a jitter using an interpolation method. In this jitter measurement method using the interpolation method (interpolation-based jitter measurement method), an interval between data having signal values close to a zero-crossing out of measured data of a sampled signal under measurement is interpolated to estimate a timing of zero-crossing. That is, a time interval between zero-crossings period) is estimated by a data-interpolation with a small error to measure a relative fluctuation of period.
That is, as shown in
FIG. 3
, a signal under measurement x(t) from the PLL under test
11
is inputted to a digital oscilloscope
14
. In the digital oscilloscope
14
, as shown in
FIG. 4
, the inputted signal under measurement x(t) is converted into a digital data sequence by an analog-digital converter
15
. A data-interpolation is applied to an interval between data having signal values close to a zero-crossing in the digital data sequence by an interpolator
16
. With respect to the data-interpolated digital data sequence, a time interval between zero-crossings is measured by a period estimator
17
. A histogram of the measured values is displayed by a histogram estimator
18
, and a root-mean-square value and a peak-to-peak value of fluctuations of the measured time intervals are obtained by an RMS & Peak-to-Peak Detector
19
. For example, in the case in which a signal under measurement x(t) has a waveform shown in
FIG. 5A
, its period jitters are measured as shown in FIG.
5
B.
On the contrary, we have proposed the &Dgr;&phgr; method for measuring a jitter by obtaining a variable component (phase noise) of an instantaneous phase of a signal under measurement. This &Dgr;&phgr; method is characterized in that an instantaneous phase of a signal under measurement is estimated using an analytic signal theory.
FIG. 6
shows a processing block diagram of the &Dgr;&phgr; method. An input signal is transformed into a complex analytic signal by a Hilbert pair generator
21
. An instantaneous phase of an input signal is obtained from the complex analytic signal by an instantaneous phase estimator
22
. A linear phase component is removed from the instantaneous phase by a linear trend remover
23
to extract a phase noise. With respect to this phase noise, a sample value closest to a zero-crossing point in a real part of the complex analytic signal is extracted by a zero-crossing resampler
24
to obtain a timing jitter sequence. A peak-to-peak value of the output of the zero-crossing resampler
24
is obtained by a &Dgr;&phgr;
PP
detector
25
as a peak-to-peak jitter &Dgr;&phgr;
PP
of the input signal. In addition, a root-mean-square value of the output of the zero-crossing resampler
24
is obtained by a &Dgr;&phgr;
RMS
detector
26
as a root-mean-square value &Dgr;&phgr;
RMS
of jitter of the input signal. Furthermore, a histogram of each sample value of the resampler
24
is displayed and estimated by a histogram estimator
27
. This &Dgr;&phgr; method is described in, for example, “Extraction of Peak-to-Peak and RMS Sinusoidal Jitter Using an Analytic Signal Method” by T. J. Yamaguchi M. Soma, M. Ishida, T. Watanabe, and T. Ohmi, Proceedings of 18th IEEE VLSI Test Symposium, pp. 395-402, 2000.
In the jitter measurement method by the time interval analyzer method, a time interval between zero-crossings is measured. Therefore a correct measurement can be performed. However, since there is, in this jitter measurement method, a dead-time when no measurement can be performed after one period measurement, there is a problem that it takes a long time to acquire a number of data that are required for a histogram analysis. In addition, in an interpolation-based jitter measurement method in which a wide-band oscilloscope and an interpolation method are combined, there is a problem that a jitter histogram cannot correctly be estimated and a jitter is overestimated (overestimation). That is, there is no compatibility in measured jitter values between this jitter measurement method and the time interval analyzer method. For example, a measured result of jitter measured by a time interval analyzer for a clock signal of 400 MHz is shown in
FIG. 7A
, and a measured result of jitter measured by an interpolation-based jitter measurement method for the same clock signal is shown in FIG.
7
B.
Those measured results are, a measured value by the time interval analyzer 7.72 ps (RMS) vs. a measured value by the interpolation-based jitter measurement method 8.47 ps (RMS), and the latter is larger, i.e., the measured value by interpolation-based jitter measurement method has overestimated the jitter value. In addition, the interpolation-based jitter measurement method cannot correctly estimate a Gaussian distribution having single peak.
It is an object of the present invention to provide a jitter measurement apparatus and its method that can estimate a jitter value having compatibility, similarly to the &Dgr;&phgr; method, with a conventional time interval analyzer method, i.e., a correct jitter value in a shorter time period.
SUMMARY OF THE INVENTION
The jitter measurement apparatus according to the present invention comprises: analytic signal transformation means for transforming a signal under measurement into a complex analytic signal; instantaneous phase estimation means for obtaining an instantaneous phase of the signal under measurement from the complex analytic signal transformed by the analytic signal transformation means; zero-crossing timing estimation means for obtaining a zero-crossing timing sequence of the signal under measurement from the estimated instantaneous phase; period estimation means for obtaining an instantaneous period sequence of the signal under measurement from the zero-crossing timing sequence estimated by the zero-crossing timing estimation means; jitter detection means to which the instantaneous period sequence is inputted for obtaining a jitter of the signal under measurement.
In addition, it is desirable that the jitter measurement apparatus further comprises cycle-to-cycle period jitter estimation means to which the instantaneous period sequence is inputted for calculating its differential sequence and for outputting a cycle-to-cycle period jitter sequence to supply it to the jitter detection means.
In addition, it is desirable that the zero-crossing timing estimation means comprises: instantaneous phase data interpolation means to which the instantaneous phase data are supplied for interpolating instantaneous phase data between a plurality of instantaneous phase data around a predetermined phase value in the instantaneous phase data; zero-crossing data determination means for determining a data closest to the predetermined value in the data-i
Ishida Masahiro
Soma Mani
Watanabe Toshifumi
Yamaguchi Takahiro
Advantest Corporation
Gallagher & Lathrop
Lathrop, Esq. David N.
Le N.
LeRoux Etienne P
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