Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Electrical signal parameter measurement system
Reexamination Certificate
2000-08-28
2003-07-22
Assouad, Patrick (Department: 2857)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Electrical signal parameter measurement system
C375S226000, C324S076770
Reexamination Certificate
active
06598004
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a jitter measurement apparatus and a jitter measuring 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
, a clock signal (a signal under measurement) x(t) from a PLL (Phase-Locked Loop) under test
11
, for example, 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 between two rising edges T
p
, namely a period fluctuates. In the Zero-crossing method, a time interval (period) between zero-crossings is measured, a relative 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.818-823, 1998.
On the other hand, 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 measuring method using the interpolation method, data around a zero-crossing are interpolated from measured data of a signal under measurement that is sampled at high speed to estimate a timing of zero-crossing, whereby a time interval between zero-crossings (period) is estimated 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 signal data sequence by an analog-to-digital converter
15
. A data-interpolation is applied to data around 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 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) is a waveform shown in
FIG. 5A
, its period jitters are measured as shown in FIG.
5
B.
In the jitter measuring 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 measuring 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 a jitter measuring method in which a wide-band oscilloscope and an interpolation method are combined, there is a problem that a jitter is overestimated (overestimation). That is, there is no compatibility in measured jitter values between this jitter measuring method and the time interval analyzer method. For example, a result of jitter measurement using a time interval analyzer for a clock signal of 400 MHz is shown in
FIG. 6A
, and a measured result of jitter measurement using an interpolation method for the same clock signal is shown in FIG.
6
B.
Those measured results are, a measured value by the time interval analyzer 7.72 ps (RMS) vs. a measured value by the interpolation method 8.47 ps (RMS), and the latter is larger, i.e., the latter has overestimated the jitter value.
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 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: band-pass filtering means for selectively passing therethrough components from which harmonic components of a signal under measurement have been removed; zero-crossing timing estimating means for estimating zero-crossing timings of the signal that has passed through the band-pass filter; period estimating means for obtaining an instantaneous period waveform, namely an instantaneous period value sequence of the signal under measurement, from those estimated zero-crossing timings; and jitter detecting means for obtaining jitters of the signal under measurement from the instantaneous period waveform.
This jitter measurement apparatus includes AD converting means (analog-to-digital converter) for digitizing an analog signal and for converting it into a digital signal, and an input signal or an output signal of the band-pass filtering means is converted into a digital signal.
In addition, in this jitter measurement apparatus, the zero-crossing timing estimating means comprises: waveform data interpolating means for interpolating waveform data around the zero-crossing of the signal that has passed through the band-pass filtering means; zero-crossing specifying means for specifying a waveform data closest to the zero-crossing in the data-interpolated signal waveform; and timing estimating means for estimating a timing of the specified data.
It is desirable that the waveform data interpolating means uses polynomial interpolation, cubic spline interpolation, or the like.
In addition, the zero-crossing timing estimating means may estimate a zero-crossing timing by inverse linear interpolation from the waveform data around the zero-crossing in the signal that has passed through the band-pass filtering means.
It is desirable that the band-pass filtering means comprises: time domain to frequency domain transforming means for transforming the signal under measurement into a signal in frequency domain; a bandwidth limit processing means for taking out only components around a fundamental frequency of the signal from the output of the time domain to frequency domain transforming means; and frequency domain to time domain transforming means for inverse-transforming the output of the bandwidth limit processing means into a signal in time domain.
In this band-pass filtering means, if the signal under measurement is long, the signal under measurement is stored in a buffer memory. The signal under measurement is taken out in the sequential order from the buffer memory such that the signal under measurement being taken out is partially overlapped with a signal under measurement taken out just before. Each partial signal taken out from the buffer memory is multiplied by a window function, and the multiplied result is supplied to the time domain to frequency domain transforming means. The signal inverse-transformed in time domain is multiplied by an inverse number of the window function to obtain the band-limited signal.
In addition, it is desirable that the jitter measurement apparatus includes cycle-to-cycle period jitter estimating means to which the instantaneous period waveform obtained from the period estimating means is inputted for obtaining, in the sequential order, differential values each being a difference between adjacent instantaneous periods having a time difference of one period therebetween to calculate a differential waveform, and for outputting a cycle-to-cycle period jitter waveform data.
In addition, it is desirable in this jitter measurement apparatus to remove amplitude modulation components of the signal under measurement by waveform clipping means.
The jitter detecting means is constituted by one or a plurality of means out of peak-to-peak detecting means for obtaining a difference between the maximum value and the minimum value of the instanta
Ishida Masahiro
Soma Mani
Yamaguchi Takahiro
Advantest Corporation
Assouad Patrick
Gallagher & Lathrop
Lathrop, Esq. David N.
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