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Keysight Technologies
Three Compelling Reasons for
Deep Acquisition Memory
Application Note
Introduction
Oscilloscope vendors specify memory depth as a primary attribute for specific oscilloscope models.
This value equals the number of samples that are stored with each acquisition. Memory depth is
typically specified as a per-channel value. Some oscilloscopes have interleaved memory architectures.
For example, memory may have a specific value when all channels are turned on and double the value
when only half of the channels are enabled.
In addition to standard acquisition memory specified, most oscilloscopes have additional memory
loaded on the acquisition board with this additional depth licensed separately. While acquisition
memory depth is often used as a primary purchase consideration, the associated benefits and
tradeoffs of deep memory require additional thought to fully appreciate. Here are three areas
where deep memory adds value.
1. Capture Longer Period of Time
The most obvious benefit of deep applications that require deeper
acquisition memory is the capture memory.
of longer periods of time at a fixed
sample rate. Deep memory helps in
instances where the cause and effect
may be separated by a significant
time period and plays a key role
in viewing events that simply take
longer time to transpire.
Acquisition Time Window = Memory
Depth / Sample Rate. As shown in
Figure 1, with 1 Mpts (mega points)
per channel an oscilloscope can
capture .1 ms of time with 10 GSa/s
sample rate. As shown in Figure 2,
if the same oscilloscope instead
had 100 Mpts per channel, it could
capture on 10 ms of time at the same
sample rate. There are wide varieties
of applications and tests that benefit Figure 1. Acquisition Time Window = Memory Depth/Sample Rate. With 1 Mpts (mega
from longer time captures. Having points) per channel an oscilloscope can capture .1 ms of time with 10 GSa/s sample rate.
deep memory provides more flexibility To capture longer periods of time, sample rate is decreased.
for engineers when they encounter
Figure 2. Time Window = Memory Depth / Sample Rate. An oscilloscope with 100 Mpts
per channel can capture on 10 ms of time at 10 GSa/s. Additional depth provides more
time capture at a fast sample rate.
3
All advanced oscilloscopes from
major vendors include a mode that
allows memory to be partitioned
into smaller segments. The Keysight
Technologies, Inc. oscilloscopes
refer to this as segmented memory.
The user specifies how many seg-
ments the memory should be divided
into with each segment having
equal length. When the oscilloscope
sees the first trigger event, it stores 12.5 Mpts to capture 3 pulses 300 Kpts to capture 3 pulses
sample points until the first segment
of acquisition memory is full. It then Figure 3. In this example, capturing three consecutive pulses requires 12.5 Mpts memory
looks for the next occurrence of the in real-time mode. The oscilloscope can capture all three segments in segmented mode
trigger event. When this trigger event using just 300 Kpts memory. Each trigger point creates a trigger point. The oscilloscope
stores memory around each trigger point and uses a time tag to keep track of how much
occurs, it stores samples to the next
time occurs between segments.
segment of memory. The process
repeats until all the segments are
full or until the user tells the oscil-
loscope to stop looking for additional
trigger events. Segmented mode is
particularly useful for capturing bursts
of activity surrounded by long periods
of dead time. Many serial busses,
optics, and communication signals fit
in this category. By using segmented
memory, oscilloscopes can maintain
fast sample rates, capturing time
windows that span seconds, hours,
or even days.
How does deep memory enhance
segmented memory? First, users
can choose to use the additional
memory to capture an incremental
number of segments. For example,
on Keysight's Infiniium oscilloscopes,
users can specify that the memory
be divided into up to 131K segments.
That is a lot of segments. Second, for
a given number of segments, users Figure 4. Using segmented memory with a USB 2.0 bus, the oscilloscope can be made
can increase the memory depth of to track the entire enumeration process using just a few segments. The oscilloscope
each segment, providing the ability to triggers each segment acquisition on a setup packet that signals the enumeration
process is ongoing. In this example, the oscilloscope captured the entire 90 ms of
see more signal activity around each
enumeration using 10 segments each with 1.6 Mpts of memory for a total memory of
trigger point. With 1 Gpts of memory 16 Mpts. Without segmented memory, capturing the enumeration process would have
per channel and 1000 segments, each required 225 Mpts, or 14X more memory, at the same sample rate.
segment can be 1 Mpts of acquisition
memory. That is a decent amount
of memory and time capture per
segment.
4
2. Maintain Faster Sample Rate (over a constant period of captured time)
The second primary advantage of
deep acquisition memory is the
ability to maintain a fast sample rate
over a larger number of timebase
settings. Engineers tend to think of
oscilloscope specifications as values
that remain constant independent
of how oscilloscope controls are
set. Unfortunately, this isn't the
case. Let's take a quick look at how
memory depth can impact the sample
rate and overall bandwidth of an
oscilloscope as the user changes
the horizontal timebase. Most
oscilloscope users don't think about
this aspect of acquisition memory.
However, it makes a huge differ-
ence in the oscilloscope's ability to Deep memory, full scope bandwidth
maintain its other key specifications,
sample rate and bandwidth.
Shallow memory can reducte scope bandwidth
Figure 5. An oscilloscope with adequate memory can retain full bandwidth at
slower horizontal settings as shown in the right-hand image. An oscilloscope
with shallow memory will reduce sample rate, limiting overall oscilloscope
bandwidth and incorrectly displaying signals as shown on the right hand display.
5
Here's an example to illustrate the
point. An engineer chooses a 4 GHz
bandwidth oscilloscope equipped
standard with 10 Mpts of memory per
channel and a maximum sample rate
of 10 GSa/s. These specifications
appear bullet-proof, and the engineer
begins using the oscilloscope. The
engineer chooses a fast 10 ns/div
timebase setting. The oscilloscope
samples at 10 GSa/s and uses just
1 Kpts of memory. The entire 4 GHz of
effective bandwidth is preserved as
the engineer anticipates.
Remember the formula, (Acquisition
Time Window) = (Memory Depth)/
(Sample Rate).
The engineer needs to see more Figure 6. Sample Rate = Memory Depth / Time Window Captured. Having deeper memory
time on screen and thus turns the enables the oscilloscope to retain a faster sample rate and full bandwidth as the user
horizontal timebase knob to slower adjusts the timebase control to slower settings.
settings; the engineer chooses
200 us/div. To fill 10 horizontal
divisions sampling at 10 GSa/s,
By changing the horizontal time- What if the oscilloscope had 1 Gpts
the oscilloscope needs 20 Mpts
base control with a fixed amount memory instead of 10 Mpts? At the
of memory. Since only 10 Mpts
of memory, their oscilloscope slower timebase setting of 10 ns/
are available, the oscilloscope
compensated by decreasing sample div, the oscilloscope would retain the
compensates by dropping the sample
rate as well as the oscilloscope's maximum sample rate of 10 GSa/s
rate by a factor of two to 5 GSa/s.
overall effective bandwidth. The and hence would not alias signals
The oscilloscope can now acquire
front end of the oscilloscope was still up to the oscilloscope's rated 4 GHz
10 Mpts of samples and fill all 10
letting frequencies up to 4 GHz pass bandwidth. At the slower sweep
horizontal divisions. The engineer
through. However, the oscilloscope speed of 1 ms/div even up to
needs to see a bigger time window
with slower sampling is now subject 5 ms/div, the oscilloscope can still
and changes the oscilloscope's
to aliasing issues, as it is no longer acquire at 10 GSa/s and hence will
timebase to 1 ms/div. As the
sampling fast enough to support accurately represent signals up to the
oscilloscope has just 10 Mpts of
the frequencies passed through the oscilloscope's full 4 GHz bandwidth.
acquisition memory, to capture 10 ms
front end. The user reduced effective Surprisingly, memory depth is indeed
of activity, the oscilloscope must
bandwidth from 4 GHz to 400 MHz by linked with effective oscilloscope
drop its sample rate again, this time
changing the timebase setting. This bandwidth when horizontal timebase
to 1 GSa/s. When the user changed
measurement error was facilitated by settings are changed. Oscillocopes
the timebase setting, the memory
the oscilloscope having only 10 Mpts with deeper memory depth preserve
limitation slowed the oscilloscope's
of maximum memory. high sample rate and the ability to
sample rate and overall bandwidth.
use its full-rated bandwidth over a
larger range of timebase settings.
6
3. Get Better Measurement and Analysis Results
The third primary value of deep
memory is quality of measurements.
Have you ever considered if taking
rise time measurements on 1,000
acquisitions each with 1K memory
yields better results than taking
rise time measurements on a single
acquisition of 1 Mpt (1K and 1000)?
Ever wonder how an FFT with 10 Kpts
of memory differed from one with
10 Mpts? Ever considered how deep
versus shallow memory impacts jitter
characterization? These are great
questions, and deep memory has an
impact on all of them.
How much memory the oscilloscope
Figure 7. FFTs and other functions yield better results when more points are used. For
uses for analysis is a balance. If
FFTs, Resolution Bandwidth = Sample Rate / Points in FFT. So, to achieve more precise
memory depth is set too low, the
resolution bandwidth, the oscilloscope must acquire and process more samples. This
oscilloscope will have difficulty screen shot shows the difference between an FFT made on a 100 kpts acquisition and
providing meaningful analysis. For one made on a 10 Mpts acquisition of the exact same signal.
example, when doing eye recovery
and jitter, if the memory is set too low
the oscilloscope can't do PLL clock
recovery because it won't see enough
edges. Also, shallow memory inhibits
statistically valid analysis such as
FFTs and histograms because the
functions won't have enough points
on which to operate. If the memory is
set too high, the tradeoff is that the
oscilloscope's responsiveness suffers
as processing times for analysis
increase significantly.
What's the downside of having deep
memory all of the time? Deep memory
decreases update rate because the
oscilloscope must process additional
information before rending the result
on the display. Deep memory slows
this task, resulting in bigger delays
between when the oscilloscope can Figure 8. An oscilloscope's architecture determines how responsive it is when deep
trigger; sluggish user control; and memory is enabled. Update rate, how many waveforms per second an oscilloscope can
inability to show subtle signal detail. capture and process per second, is a good measure objective measure of how well the
Update rate with deep memory varies oscilloscope handles deep memory. As noted in the graphic with update rates shown on
dramatically by oscilloscope vendor a log scale, there is a huge variation in update rate between oscilloscopes when deep
as shown in Figure 8. memory is turned on.
7
Summary
When choosing an oscilloscope, pick and oscilloscope bandwidth when
one that has sufficient memory to capturing longer time windows. Also,
cover your future needs. While you deeper memory can help your oscil-
may not want to have deep memory loscope achieve better measurement
turned on all the time, having the and analysis results. Segmented
additional memory available on your memory enables a oscilloscope to
oscilloscope is a huge benefit for better utilize acquisition memory and
times you need it for debug and test- is effective for signals that have long
ing. The additional memory can be dead times between periods of activ-
enabled when you run into situations ity. The primary downside to deep
where it is needed. The benefits memory is a slowing update rate.
you will derive from the additional Oscilloscope update rate performance
memory include capturing longer time varies dramatically between vendors
windows with a given sample rate when deep memory is enabled.
and retaining a faster sample rate
8
09 | Keysight | Three Compelling Reasons for Deep Aquisition Memory - Application Note
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