The simple lesson with sample rate is that more is better.
But did you know that the method of achieving that high sample rate can actually distort
your signal and throw off your measurements?
Hi I'm Erin East with Keysight.
This episode of exposing signal integrity Myths is all about sample rate.
We all know sample rate as one of the banner specifications of an oscilloscope.
We've been taught that the higher the sample rate, the better the measurement will be.
Don't get me wrong, this still holds completely true.
However, there is something to be warry of.
Do you know exactly how your oscilloscope is achieving that high sample rate?
That could make a huge difference in your measurements.
Before we dive into that, let's take a step back.
Why do we even care about Sample rate?
Modern oscilloscopes convert your analog signal input into a digital signal so that you can
see it on the display.
The conversion works by taking data points, or samples, of your analog signal and using
those points to reconstruct the waveform digitally.
You can see the path that your signal takes through the oscilloscope in this animation.
Once your signal reaches the front end of the oscilloscope, the sampler will collect
data points.
Those data points feed into the analog to digital converter (ADC) where they are converted
to digital values.
They are stored in memory and eventually processed then displayed on the screen.
Simply put, the sample rate is just the rate at which the sampler collects samples.
The sample rate is measured in samples per second.
This rate ultimately affects the signal you see on screen.
In a well-designed oscilloscope, samples will occur at a consistent rate with no variation.
We'll learn about why that's important in a little bit.
And it's important to remember that memory and sample rate are closely linked.
As you increase the timebase, more of your waveform will display.
This means that more memory is being used to store that long timespan of data points.
Well, this will effectively decrease the sample rate because the oscilloscope can only store
a certain number of sample points in memory.
When you adjust the timebase, notice how it changes the sample rate.
Too low of a sample rate makes your signal look choppy, causes you to miss some signal
detail, and skews your measurements.
Always be sure to use the correct scaling for the analysis you are trying to perform.
So, what does all of this mean to your measurements?
Let's take a look what a signal looks like with a high sample rate vs. a low sample rate.
You can see with a high sample rate of 20GSa/s, we see a nice smooth representation of our
signal and can make accurate measurements on it.
With a low sample rate of 20 mega samples per second, we can see that the signal looks distorted because there
are not enough samples available to correctly interpolate the waveform.
If you were to perform a peak to peak measurement on this, your results would be completely
inaccurate.
Additionally, if your sample rate isn't high enough, you'll miss signal detail, glitches,
or errors because elements can occur between collected samples.
Since glitches and spurs can mean the difference between a device functioning correctly or
not, it's critical to detect them in the design phase.
Undersampling could easily cause you to think that your signal is clean, when in fact it
has several issues.
So, the key point here is the higher the sample rate, the higher the resolution of the trace.
This will allow you to make more accurate measurements of your signal and catch any
errors that may exist.
Learn about how to determine what sample rate is adequate for your signal, check out the
Everything You Need to Know About Sample Rate whitepaper at the link on screen or in the
description.
In that whitepaper you can also learn about some of the different acquisition modes that
can be used to analyze your samples in different ways.
Now, on to the big problem.
I mentioned that high sample rates are ideal, but that you have to be warry of the method
being used to achieve that high sample rate.
This method can make or break your measurements, quite literally.
The problem I'm referring to is called Interleave Distortion.
Some oscilloscopes can reach incredibly high sample rates, like 40 GSa/s and higher.
However, to achieve these rates, they have to use two or more ADCs, synchronized with
clock phase delays in a "one fires then the other fires" pattern.
This is a very common practice and it effectively doubles the maximum sample rate.
At a high level, this technique seems useful.
After all, the faster the sample rate, the higher the resolution.
However, there is a catch.
If we go back to Nyquist's theorem, it states that the samples must all be equally spaced
in time to produce an accurate trace.
While this is relatively easy to do with a single ADC, it is much more difficult to get
the phase delay clocks of the multiple ADCs perfectly synchronized.
To achieve this, the synchronized ADCs must have the same vertical gain, offset, and frequency
response.
If these interleaving requirements aren't satisfied, the phase delay clocks will not
be aligned causing inconsistent spacing between samples, like you see here.
You can see how the misaligned clocks cause the samples to be collected at varying intervals.
The processor in the oscilloscope assumes that all samples are collected at a consistent
rate.
This means if samples are collected at uneven intervals, when the waveform is digitally
reconstructed it will appear distorted.
Much like the distorted signal with too low of a sample rate that we saw earlier, any
measurements you make on a signal like this will be completely inaccurate.
So, in this case, the higher sample rate actually produces a less accurate waveform.
Typically, a higher sample rate is preferred, but when improper interleaving technology
enters the process, it brings enough inaccuracy to negate this method of achieving a faster
sample rate.
This is especially true when working with extremely high-frequency signals since even
the smallest misalignment in clocking has a significant effect on the waveform.
However, there are many cases where interleaving is implemented correctly, like with the Keysight
S-Series oscilloscopes we're using today.
Interleaving isn't something to be afraid of, just something to be cautious of.
There are also some extremely precise oscilloscopes out there, like the Keysight UXR-Series, that
can reach incredibly high sample rates without interleaving ADCs.
This will, of course, give you even more accuracy and higher signal integrity.
Aside from visually seeing the distortion in the trace on screen, there's also a test
you can perform to see if interleave distortion exists in your oscilloscope and if it's responsible
for any inaccuracies.
When you see distortion on your signal, you can only guess at whether the distortion is
caused by ADC interleaving or if there's another reason.
Use a fast Fourier transform (or an FFT) to quickly and easily evaluate the nature of
the distortion.
With an FFT, you can see the various frequency components of your signal.
You should be able to look at the harmonics of your signal, but any irregular spikes found
between these harmonics could be identified as harmonic distortion, noise, or interleave
distortion.
So, let's look at this FFT display of a 2.5 GHz sine wave at 40 GSa/s.
The fundamental frequency stands out here, and you can look at the harmonics at consistent
intervals.
There are no unexpected frequency components between any of the harmonics or fundamental
frequency, so this signal is not likely subject to interleave distortion.
Now on this screenshot you see, an FFT is performed on the same signal using a different
oscilloscope at 40 GSa/s.
There are multiple significant spurs outside the fundamental frequency and harmonics.
The largest is at 7.5 GHz, which is 10 GHz below the fundamental.
The next largest is at 12.5 GHz, which is 10 GHz above the fundamental.
The four ADCs in the oscilloscope sample at 10 GSa/s, so it is evident that the interleaved
ADCs are directly responsible for this distortion.
It is not uncommon for scopes with extremely high sample rates to produce distorted signal
reconstructions on screen.
It's really important to be aware of the possible downsides of faster sample rates and always
check for those in any oscilloscope you're using.
So, In summary, always be cautious when you are working with both low and high sample
rates.
This is a key specification to monitor when you're making measurements.
Remember that the sample rate can automatically change as you adjust the time base settings,
so be sure you are using the appropriate settings for the measurements you need to make.
The key things to remember about sample rate in your testing are:
Sample rate refers to the rate at which the oscilloscope can collect data points, in Sa/s
Too low of a sample rate can cause distortion and skew your measurements
And lastly, Interleaved ADCs will present an impressive sample rate specification; however,
if they are not properly synchronized this method can distort your signal.
The sample rate is more than just an internal working of your oscilloscope.
It has a profound impact on the trace you see.
Sample rate determines whether you see an accurate representation of your signal, or
a distorted one.
As an engineer creating new digital designs in a world of increasingly fast data rates
and high-precision devices, you don't have any room for unnecessary development delays.
Don't let your oscilloscope's internal errors affect your measurements.
In addition to sample rate, there are a few other key considerations to ensure your oscilloscope
gives you the highest signal integrity and best measurement accuracy.
To learn about the other important aspects of signal integrity, download the How to Determine
Oscilloscope Signal Integrity eBook.
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