Sampling technique for waveform measuring instruments

A new sampling technique for waveform measuring instruments (including methods and circuits for implementing same) comprises the step of processing a series of digital signal samples through a decimator to extract a decimated sample value from each decimated sample interval in a series of decimated sample intervals. The series of digital signal samples is simultaneously processed through a digital peak detector to extract maximum and minimum values (peak detect sample values) from each decimated sample interval. For a given decimated sample interval, a difference between the maximum and minimum sample values for the interval is calculated. If the difference exceeds a glitch detect threshold value, the maximum and minimum sample values for the given decimated sample interval are transferred to a video sample memory. If not, the decimated sample value for the given decimated sample interval is transferred to the video sample memory.

BACKGROUND OF THE INVENTION 
This invention pertains to waveform measuring instruments, and more 
particularly, the sampling technique used by a waveform measuring 
instrument (e.g., a digital oscilloscope or similar device) to acquire and 
graphically display the waveform of an input signal. 
A waveform measuring instrument generally comprises three elements: an A/D 
converter, a memory, and a video display. Such an instrument acquires and 
displays an input signal's waveform by sampling the input signal, storing 
the acquired samples in memory, and then displaying the stored samples on 
its video display. Ideally, the instrument's A/D converter should always 
sample at its maximum sample rate, since a faster sampling rate is more 
likely to capture an accurate representation of the input signal's 
waveform. However, with present A/D converters sampling in the range of 
several Gigasamples/second, sampling a waveform at a low time base setting 
can require several Gigabytes of memory. This presents two problems. 
First, memories capable of storing several Gigabytes of data are costly. 
Second, video processors only operate at speeds of several 
Megabytes/second. Thus, displaying a waveform comprising several Gigabytes 
of information can cause a significant delay between consecutive sample 
set acquisitions by an A/D converter. 
There have been many attempted solutions to the above problems. A first 
solution is to reduce the sampling rate of the A/D converter. However, 
such a solution introduces two additional problems - aliasing, and missing 
short duration excursions of a waveform such as narrow pulses and 
glitches. Aliasing is a phenomenon caused by violation of the Nyquist 
principle, which states that a true representation of a waveform cannot be 
acquired if an input signal is sampled at a frequency of less than twice 
its maximum frequency. 
A second solution is to allow the A/D converter 100 to sample at its 
maximum rate, and then systematically reduce the number of samples that 
are stored to memory 104. One way to accomplish this task is to use a 
decimator 102 (FIG. 1). However, using a decimator 102 is similar to 
reducing the A/D converter's sample rate in that every Nth sample is 
stored to memory 104 and the rest are ignored. 
One of the most effective prior solutions of systematically reducing the 
number of samples to be stored in memory 112 is to use a digital peak 
detector 110. For every N samples acquired by the A/D converter 108, a 
digital peak detector 110 stores a maximum and a minimum sample value 
(FIG. 2). Thus, if a short-duration event or glitch occurs during a sample 
interval, the event is captured (provided that the maximum A/D sample rate 
is fast enough to sample it). A digital peak detector 110 systematically 
retains essential data while maintaining the maximum sample rate of an A/D 
converter 108. However, a disadvantage to using a digital peak detector 
110 is that it exaggerates and displays the worst case noise performance. 
"Noise" consists of signals internal and external to the oscilloscope which 
are picked up by a sensitive A/D converter 100, 108 and appear 106, 114 
superimposed on an input signal's waveform. For example, FIG. 8 shows an 
input signal's "true" waveform, and FIG. 9 shows the input signal's 
waveform with noise. Since a digital peak detector 110 looks for any peaks 
in a signal's waveform, regardless of whether they are caused by noise, 
"Peak Detect" mode is usually an optional feature of a waveform measuring 
instrument. 
As a result of the foregoing disadvantages to existing sampling techniques, 
it is a primary object of this invention to provide a sampling technique 
for waveform measuring instruments in which the Peak Detect acquisition 
mode can be used at all times, without causing degradation of a waveform 
due to the exaggeration of noise. 
It is also an object of this invention to provide a sampling technique 
which makes efficient use of sample memory. 
It is a further object of this invention to provide circuits which may be 
used in implementing the new sampling technique. 
It is yet another object of this invention to provide a sampling technique 
which allows for the use of anti-aliasing techniques such as that 
disclosed in U.S. Pat. No. 5,115,189 to Holcomb. 
SUMMARY OF THE INVENTION 
In the achievement of the foregoing objects, a new sampling technique for 
waveform measuring instruments is presented (including methods and 
circuits for implementing same). 
The technique comprises the step of processing a series of digital signal 
samples through a decimator to extract a decimated sample value from each 
decimated sample interval in a series of decimated sample intervals. The 
series of digital signal samples is simultaneously processed through a 
digital peak detector to extract maximum and minimum values (peak detect 
sample values) from each decimated sample interval. For a given decimated 
sample interval, a difference between the maximum and minimum sample 
values for the interval is calculated. If the difference exceeds a glitch 
detect threshold value, the maximum and minimum sample values for the 
given decimated sample interval are transferred to a video sample memory. 
If not, the decimated sample value for the given decimated sample interval 
is transferred to the video sample memory. 
The above sampling technique allows regular (or even continuous) use of a 
digital peak detector, without the worry of noise exaggeration. 
The technique also allows for a more efficient use of memory. Peak detect 
sample values, which require twice the amount of storage space as a single 
decimated sample value, are stored less frequently. 
These and other important advantages and objectives of the present 
invention will be further explained in, or will become apparent from, the 
accompanying description, drawings and claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A new sampling technique for waveform measuring instruments is generally 
described below, and is diagramed in FIG. 3. The technique comprises the 
step of processing 116 a series of digital signal samples through a 
decimator 140 to extract a decimated sample value from each decimated 
sample interval in a series of decimated sample intervals. The series of 
decimated sample intervals comprises a sample set. Simultaneously with the 
above step, the series of digital signal samples is processed 118 through 
a digital peak detector 142 to extract maximum and minimum sample values 
(peak detect sample values) from each decimated sample interval. For each 
decimated sample interval, a difference between the maximum and minimum 
sample values for the interval is calculated 120. If the difference 
exceeds a glitch detect threshold value 122, 124, the maximum and minimum 
sample values for that interval are transferred 126 to a video sample 
memory 144 for future graphical display; if not, the decimated sample 
value for that interval is transferred 128 to the video sample memory 144. 
Having thus described the sampling technique in general, the technique will 
now be described in further detail. 
Before proceeding, it is important to note that a waveform measuring 
instrument may comprise several channels. Although the following 
description is made in conjunction with a single channel, the disclosed 
technique may readily be applied to a multiple channel instrument. In such 
a case, the technique would be applied in a similar manner to each channel 
of the instrument. 
A first preferred embodiment of the FIG. 3 sampling technique is shown in 
FIG. 4. The technique begins as an analog-to-digital (A/D) converter 146 
of a waveform measuring instrument samples an analog input signal and 
creates a series of digital signal samples 132 (hereinafter "samples"). 
Reference may be made to FIG. 5, which shows a circuit 200 of a waveform 
measuring instrument incorporating the technique presented in the flow 
chart of FIG. 4. The A/D converter 146 should preferably operate at its 
highest sampling rate so that the samples it acquires will completely and 
accurately represent an input signal's waveform. 
Once acquired, the samples are simultaneously processed by a decimator 140 
and a digital peak detector 142. The decimator 140 extracts every Nth 
sample (where N represents the number of samples in a decimated sample 
interval) and discards all other samples. The decimated sample interval is 
normally set using the "time base" control of a waveform measuring 
instrument (such controls are standard on oscilloscopes), but could be set 
explicitly in an instrument's firmware. 
The digital peak detector 142 extracts minimum and maximum sample values 
("min/max sample data" or "a min/max sample pair") from each decimated 
sample interval (the same interval of N samples referred to above). 
In creating a graphical display, a glitch detect threshold circuit 150 
causes either the min/max data or the decimated sample data for a given 
decimated sample interval to be transferred to a video sample memory 144. 
The contents of the video sample memory 144 may be displayed 138 on a 
video display 152. 
The glitch detect threshold circuit 150 functions as follows. Min/max 
sample data is received from the outputs of the digital peak detector 142 
so that a difference between the maximum and minimum sample values of each 
decimated sample interval may be calculated 120. Once calculated, the 
min/max differences are compared 122 to a specified glitch detect 
threshold value (GDTV). If the min/max difference of a given decimated 
sample interval exceeds the GDTV 124, the min/max sample data for that 
interval is stored 134 in a sample acquisition memory 148. If the min/max 
difference of a given decimated sample interval is less than the GDTV 124, 
the decimated sample data for that interval is stored 136 in the sample 
acquisition memory 148. After the samples of all decimated sample 
intervals comprising a sample set have been processed through the glitch 
detect threshold circuit 150, the sample acquisition memory 148 will 
comprise a combination of decimated and min/max sample data. To display 
the contents of the acquisition memory 148 to video, its contents are 
transferred to a video sample memory 144 and then displayed 138 on a video 
display 152. 
The GDTV should be selected so that it is just large enough to eliminate 
min/max sample data which is a reflection of noise peaks. Thus, only 
min/max data which represents a significant deviation in a waveform, and 
which exceeds the defined GDTV, will be displayed. To achieve this result, 
the glitch detect threshold circuit 150 may be responsive to a GDTV 
selector 154. This selector 154 may comprise a manually adjustable knob, 
slide or the like located on an exterior of an instrument. Alternatively, 
the GDTV selector 154 can be implemented in the firmware of a waveform 
measuring instrument. If embodied in firmware, the GDTV selector may 
comprise either a constant or adjustable value. If adjustable, an adaptive 
filter within the firmware may monitor a waveform's characteristics, and 
automatically adjust (or set) the GDTV in response to changes in a 
waveform. 
The glitch detect threshold circuit 150 may be fashioned in numerous ways, 
one of which is shown in FIG. 5. The circuit 150 comprises a mathematic 
logic circuit 156, a comparator 158 and a multiplexer (MUX) 160. The 
mathematic logic circuit 156 receives the minimum and maximum sample 
values from the outputs of the digital peak detector 142. For each 
decimated sample interval, it calculates 120 the difference between the 
maximum and minimum sample values of that interval. The results of its 
calculations are output as a series of min/max differences. The min/max 
differences are received by the comparator 158 and compared to the GDTV. 
As earlier stated, the GDTV may be provided by an external or internal 
selector 154. The comparator 154 outputs a signal which is indicative of 
whether the min/max difference of a decimated sample interval exceeds the 
GDTV. This signal is received as the multiplexer's 160 select input (SEL). 
If the select input indicates that a min/max difference exceeds the GDTV, 
the multiplexer data lines carrying minimum and maximum sample values are 
fed to the acquisition memory 148. If the select input indicates that a 
min/max difference does not exceed the GDTV, a multiplexer data line 
carrying a decimated sample value is connected to the acquisition memory 
148. Operation of the decimator, digital peak detector, and glitch detect 
threshold circuit is synched via appropriate clock signals (not shown 
since their implementation is `de minimis` to one skilled in the art). 
The first preferred embodiment of the technique concludes as the contents 
of the acquisition memory 148 are transferred to a video sample memory 144 
for display on a video display 152. 
In the first embodiment, data is first processed 120, 122, 124 through the 
glitch detect threshold circuit 150, and then stored 134, 136 to memory 
148. This may be referred to as a "decide-then-store" approach. In a 
second embodiment of the sampling technique (FIGS. 6 & 7) , data is first 
stored 162, 164 to memory 166, 168 and then later processed 120, 122, 124 
through the glitch detect threshold circuit 150 (a "store-then-decide" 
approach). In the second embodiment 202, the single sample acquisition 
memory 148 which follows the glitch detect threshold circuit 150 in FIG. 5 
is replaced with a pair of acquisition memories 166, 168 at the front end 
of the glitch detect threshold circuit 150. The reason for using a 
"store-then-decide" approach, thus requiring both a decimated sample 
values acquisition memory 168 and a peak detect sample values acquisition 
memory 166, is to accommodate A/D converters 146 which operate at very 
high clock speeds. When using a very high-speed A/D converter 146, signal 
samples may be acquired and stored much faster than the glitch detect 
threshold circuit 150 can process them. If the additional data processing 
which occurs in the glitch detect threshold circuit 150 were to be 
performed as a signal was being acquired, it would interfere with the 
fast, efficient acquisition of samples. By first storing the decimated and 
min/max sample data to memory 166, 168, data can be processed by the 
glitch detect threshold circuit 150 between sample set acquisitions, thus 
increasing a wave measuring instrument's overall efficiency. If desired, 
the decimated and peak detect sample value memories 166, 168 may merely 
comprise divisions of a single memory. 
Note that a "store-then-decide" approach requires twice as much sample 
acquisition memory as a "decide-then-store" approach. Since memory is 
costly, instruments which do not incorporate high-speed A/D converters 146 
can eliminate use of the additional memory, process data through the 
glitch detect threshold circuit 150 as it is acquired, and store a reduced 
size sample data set in a single sample acquisition memory 148. The 
content of the acquisition memory 148 may be transferred to a video sample 
memory 144 between the acquisition of consecutive sample sets. 
The significance of the above described sampling technique can be 
appreciated with reference to FIGS. 8-13. FIG. 8 portrays the waveform 170 
of a 5-volt DC input signal having a glitch 172. FIG. 9 portrays the same 
input signal with a "noise" component 174. An A/D converter 146 sampling 
the input signal 174 of FIG. 9 might produce the digital signal samples 
176 plotted in FIG. 10. 
Since memory is expensive, and displaying too many samples can cause system 
delay, the samples 176 of FIG. 10 must be thinned out. Using a simple 
decimator as in FIG. 1, or as incorporated into FIGS. 5 & 7, the samples 
178 plotted in FIG. 11 are acquired. If these samples 178 were to be 
displayed to video 152, the displayed waveform 178 would fail to capture 
the input signal's glitch 172. 
If the A/D converter's samples 176 are processed through a digital peak 
detector as in FIG. 2, or again, as incorporated into FIGS. 5 & 7, the 
samples 180 plotted in FIG. 12 are acquired. Note that this sample data 
set 180 catches the input signal's glitch 172. However, it also reflects a 
lot of the input signal's 174 noise component. 
Using the disclosed sampling technique, and the circuit of FIG. 5 or 7, a 
plot of the samples 182 acquired for the input signal 174 of FIG. 9 would 
appear as in FIG. 13. The plotted samples 182 not only pick up the input 
signal's glitch 172, but they also eliminate much of the input signal's 
174 noise component. Of all the waveforms pictured in FIGS. 9-13, the 
waveform of FIG. 13 is the most accurate depiction of the input signal's 
"true" waveform 170 (FIG. 8). 
As mentioned in the Background of the Invention, supra, the primary problem 
with using an oscilloscope's Peak Detect mode is that it exaggerates 
signal noise by saving any samples which represent a deviation in a 
signal's waveform. In using the above described sampling technique, only 
those samples which represent a significant deviation in a signal's 
waveform will be saved to memory, thus resulting in a displayed waveform 
which more accurately depicts an input signal. 
The term "decimator", as used herein, encompasses any device which extracts 
a single sample value from a decimated sample interval of N samples. The 
"decimator" disclosed in U.S. Pat. No. 5,115,189 to Holcomb is 
particularly useful when used in conjunction with the sampling technique 
disclosed herein. Holcomb discloses a concept now referred to as low 
frequency dither. A low frequency dither "decimator" randomly extracts 
1-in-N samples (rather than every Nth sample) from a decimated sample 
interval. In doing so, it helps to prevent the effects of aliasing. 
While illustrative and presently preferred embodiments of the invention 
have been described in detail herein, it is to be understood that the 
inventive concepts may be otherwise variously embodied and employed and 
that the appended claims are intended to be construed to include such 
variations except insofar as limited by the prior art.