Coded data generation or conversion – Digital code to digital code converters – Adaptive coding
Reexamination Certificate
2000-04-18
2001-08-07
JeanPierre, Peguy (Department: 2819)
Coded data generation or conversion
Digital code to digital code converters
Adaptive coding
C341S123000, C327S271000
Reexamination Certificate
active
06271773
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the coherent sampling method and apparatus.
The invention is particularly concerned with a digital sampling oscilloscope (DSO) employing the coherent sampling to measure input repetitive signals with fine time resolution.
The novel digital oscilloscope can acquire waveforms without waveform missing phenomena for a short time period by means of a equivalent time sampling.
2. Description of the Prior Art
The equivalent time sampling is widely employed in the digital oscilloscopes. The sampling is well known as the measures to obtain waveforms sampled with the finer time resolution than that of the time period of the sampling clock.
The equivalent time sampling includes three systems. The first is the sequential sampling. The second is the random sampling. The third is the coherent sampling.
The prior art of digital oscilloscopes and their related technics are disclosed as follows.
Prior Art 1; Picosecond Domain Waveform Measurements, N. S. Nahman, Time-Domain Measurements in Electromagnetics, Van Nostrand Reinhold
Prior Art 2; IEEE Standard for Digitizing Waveform Recorders, IEEE Std 1057-1994 pp. pp.
5 & 28
Prior Art 3; U.S. Pat. No. 5,708,432, Jan. 13, 1998, Coherent Sampling Digitizer System, Reynolds et al.
Prior Art 4; Electrical Test Instruments, —Theory and Applications—R. A. Witte RTR Prentice Hall, pp. 120-121
Prior Art 5; The Microwave Transition Analyzer; A new Instrument Architecture for Component and Signal Analysis, D. J. Ballo and J. A. Wendler, October 1992, Hewlett-Packard Journal
Prior Art 6; Japanese Provisional Publication No. 10-293140 Nov. 4, 1998, Random Sampling Holdoff Method and Circuit, Uchida et al.
Prior Art 7; Waveform Missing Mechanisms and a Countermeasure in a Random Sampling System, IEEE Instrumentation and Measurement Technology conference, St. Paul Minnesota, U.S.A. May 18-21, 1998. K. Uchida et al.
Prior Art 8; Acquisition Clock Dithering in a Digital Oscilloscope, D. E. Tpeppen, April 1997, Hewlett-Packard Journal
In the prior art 1, the sequential sampling and the random sampling are disclosed.
In
FIG. 1
, there is shown a circuit of the sequential sampling disclosed in the prior art 1. In the sequential method, the signal f(t) of the recurrent pulse generator
81
is passed through a delay line DL to allow time for the sampling pulse to be generated. On successive signal occurrences, the delay generator
82
shifts the sampling time in a known a priori way, usually uniformly. The successive samples are stored in the signal (vertical) channel f(t) memory
84
. The memory output
85
from the memory
84
is displayed in a order as indicated by the numbers at the points. No signal channel delay line may be required for triggerable signal sources if satisfactory electronic delay is available.
In
FIG. 2
, there is shown a circuit of the random sampling disclosed by the prior art 1. In the random method, the time value of each sample is not known a priori but is determined by measuring the relative time position (time point) between the start of the signal f(t) and the sampling pulse. The signal f(t) is generated by a recurrent pulse generator
81
. The value so determined is stored in the time-base memory
87
. Note that (1) the sampling pulse is not synchronized to the signal, (2) no signal delay line is thus required, and (3) non zero samples are obtained whenever the signal and sampling pulse occur simultaneously. The time ramp
86
could just as well be started by the signal f(t) and stopped by the sampling pulse
90
.
The sequential sampling shown in
FIG. 1
requires the delay line DL to obtain samples at trigger points or theretofore. The wider the bandwidth of the delay line DL is, the thinner it's diameter is. The thin delay line has the high cutoff frequency, however, the line is accompanied with much dissipation in a high frequency range. The bandwidth is, therefore, compromisingly limited by employing the delay line.
In order to obtain the fine time resolution of time in the operation of the sampling data acquisition, it is required to employ the time base with the fine time resolution and the wide bandwidth feature together. If no pretrigger signal is obtainable in the sequential sampling, the bandwidth and the fine time resolution are compromised because of the delay line.
In
FIG. 2
, the time ramp
86
is started by the start of the signal f(t) and stopped by the sampling pulse a(t). Thereby the outputs
85
of the memory
84
are displayed at the points indicated by the outputs n of the time base memory
87
in a random order as shown by the numbers from
1
to
13
. In this way, the signal f(t) is reproduced as f(n).
The random sampling shown in
FIG. 2
has the pretrigger ability to be able to sampling at points previous to the start of the signal f)t) to be measured. Accordingly, the random sampling requires no delay line which limits the bandwidth.
In the paragraph 4.1.5 of the prior art 2, the coherent sampling is disclosed as one of the equivalent time sampling. The coherent sampling is realized by setting the input signal's repetition frequency to that of sampling clock appropriately.
In the prior art 3, another coherent sampling is disclosed. Therein, the repetition frequency Fs of the sampling clock is set appropriately to the input signal's repetition frequency Ft in contrast with the prior art 2. The prior art 3 shows the condition to realized the coherent sampling. In the conditions, Ft/Fs=M/N, in which M and N are relatively prime integers. The integer N is the number of samples during a repetition cycle of the input waveform to be measured. The integer M is the number of cycles of the input waveform to produce data of N different time points representing one cycle of the input waveform.
In the prior arts 2 and 3, the coherent sampling is realized by means of appropriately setting one repetition frequency to another between the input signal and the sampling clock.
In the prior art 4, the random sampling is described as follows.
Since random repetitive sampling provides pretrigger information, it has largely displaced sequential sampling, except at microwave frequencies. At microwave frequencies, the time/division setting on the scope can be very small, causing the window of time that is viewed on the display to also be very small (perhaps 100 ps). The probability of a randomly acquired sample falling into the desired time window is so small that random repetitive sampling would take a long time to acquire the entire waveform. Alternatively, sequential sampling forces the sample points to occur within the desired time window so the entire waveform can be acquired quickly.
In the sequential sampling, the sampling frequency depends on the input signal's repetition frequency. When the repetition frequency becomes over several hundred kHz, the sampling frequency Fs of 100 kHz or so is generally employed. The sequential sampling's data acquisition time Tseq is given by
Tseq=
(
Tw/Tres
)(1
/Fs
) (1)
in which Fs is the sampling frequency, Tw is the time window and Tres is the time resolution.
The equation (1) means, for example, that the acquisition time
Tseq=
(100 ps/1 ps)(10 &mgr;s)=1 ms
in which the time window Tw is 100 ps, the time resolution Tres is 1 ps, and the sampling frequency Fs is 100 kHz or {fraction (1/10)} &mgr;s.
In the random sampling, the data acquisition time Tran is given by
Tran
={(
Tmh
)/(
FsTw
)}(
Tw/Tres
)
k
or
Tran
={(
Tmh
)/(
FsTres
)}
k
(2)
The Tmh is the hold time duration, (1/(FsTw)) is the reciprocal of the probability to sample the signal within the time window Tw by the sampling clock. The (Tw/Tres) is the number of acquisition data.
The reading, writing and other processes are executed during the hold time duration Tmh. The constant k is approximately given by 21 og(Tw/Tres)+1, which depends on the sampling uniformity. Because, the sampling are not executed uniformly for the limited tim
Jean-Pierre Peguy
Jeanglaude Jean Bruner
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Teratec Corporation
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