Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Electrical signal parameter measurement system
Reexamination Certificate
1998-02-10
2001-02-13
Hoff, Marc S. (Department: 2857)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Electrical signal parameter measurement system
C345S182000
Reexamination Certificate
active
06188966
ABSTRACT:
FIELD OF INVENTION
This invention relates generally to electronic instrumentation for signal sampling, such as digital oscilloscopes, and more specifically to connection of samples for display of waveforms where the samples are taken during repetitive sampling.
BACKGROUND OF THE INVENTION
Digital oscilloscopes (and other digital signal acquisition instruments) sample analog input signal voltages, convert each analog sample to a digital value with an analog-to-digital converter, store the digital sample values in a memory for processing, and display the processed digital sample values. A waveform record is a one dimensional array of digital sample values. Typically, the array is a time ordered list, with a fixed time increment between adjacent samples. Modern digital oscilloscopes typically have three sampling methods for acquiring a waveform and filling in the waveform record: single-shot sampling, sequential repetitive sampling, and random repetitive sampling.
In single-shot sampling (also called real time sampling), an entire waveform record is captured from one trigger event. Enough samples occur within the time interval of interest to adequately represent the analog signal that was sampled. The time between adjacent samples is the reciprocal of the sample rate.
In sequential repetitive sampling, one sample is typically captured from each trigger event. The time from the trigger event to the sample is typically incremented by a small amount for each subsequent trigger event. The waveform record is filled sequentially in time order, requiring one trigger for each record entry. The time between adjacent samples in the waveform record is much shorter than the reciprocal of the sample rate.
In random repetitive sampling, multiple samples are captured from each trigger event, but not enough to fill the waveform record. The samples are taken at random times with respect to the trigger event. The time from the trigger event to each sample is accurately measured and each sample is placed at the proper time slot in the waveform record. With multiple trigger events, the waveform record is gradually filled. The time between adjacent samples in the waveform record is much shorter than the reciprocal of the sample rate.
When a waveform record is drawn or displayed, individual samples may be represented simply as dots or pixels. Alternatively, it is often useful to connect dots. One straightforward algorithm for connecting dots is to simply connect the dot for sample (N) to the dot for sample (N+1). For single-phase waveforms, for example a sine wave or a square wave, simple sequential connection of dots enhances the display. However, some waveforms are multi-phase. That is, they may have multiple values at a given delay time from a trigger event. One common example is the output of a digital gate that at a given delay from a clock edge may have a voltage corresponding to a logical one or a voltage corresponding to a logical zero. For sequential repetitive sampling or random repetitive sampling, sample (N) may come from a different trigger event than sample (N+1). Therefore, for example, sample (N) may be from a logical one state and sample (N+1) may be from a logical zero state. A human observer viewing just dots or pixels is capable of appropriately interpreting the two separate states. However, simply connecting adjacent record values, where adjacent record values randomly represent one of two states, can result in a chaotic display. There is a need for an improved method of connecting samples of multi-phase signals when drawing or displaying samples from sequential or random repetitive samples.
SUMMARY OF THE INVENTION
The system has an inherent limit, called the maximum slew rate (volts/second), on the rate of change of an input signal that can be accurately sampled. The system (hardware or software) compares the slope of a connection between two samples to a slope corresponding to the maximum slew rate. If a connection exceeds the maximum slew rate, the connection is not drawn.
Three example embodiments are presented. In a first example embodiment, the system decides whether or not to connect to the next (or previous) consecutive sample, based on the outcome of comparison of the magnitude of the slope to a slew rate threshold. In the second example embodiment, the system looks ahead (or behind) up to M samples to see if any samples represent valid slew rates, and connects to the first sample representing a valid slew rate (or does not draw a connection if no sample represents a valid slew rate). In the third example embodiment, the system looks ahead (or behind) M samples and connects to the sample in the next M samples that represents the smallest valid slew rate (or does not draw a connection if no sample represents a valid slew rate). In each example embodiment, if no sample represents a valid slew rate, no connection is drawn. Direction of connecting is arbitrary. That is, the system may connect “forward” in time by looking at subsequent samples or “backward” in time by looking at previous samples.
The maximum slew rate number may be adjusted to compensate for noise, possibly increasing the number of misconnections but decreasing the number of situations in which no connection is drawn where a connection should be drawn. In the second and third example embodiments, by looking ahead (or behind) M samples, the system can potentially resolve a waveform with up to M phases and can accommodate trigger placement uncertainty. When looking ahead or behind multiple samples, the threshold may be made a function of the time between samples (as measured in the waveform record).
REFERENCES:
patent: 4455613 (1984-06-01), Shoemaker
patent: 2155738 (1985-09-01), None
Holcomb Matthew S.
Timm Daniel P.
Agilent Technologies
Barbee Manuel L.
Hoff Marc S.
Winfield Augustus W.
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