Hi everyone and happy holidays! It's really important to analyze the
frequency components in your signal because they can often cause noise in
your design which can end up causing your device to malfunction. You can
really easily do this using a fast Fourier Transform or FFT.
If your design has excess noise it can interrupt the functionality of components by exceeding
certain tolerances or even causing voltage spikes that can damage your
parts. This could be a problem that your customer ends up finding if you don't
test correctly during your design phase. Analyzing the frequency content in your
design is going to be a critical step in ensuring your device is gonna work
properly. You may not always associate frequency domain analysis with an
oscilloscope because oscilloscopes are typically used to
analyze the time domain. However, an oscilloscope is a really convenient tool for measuring
and analyzing your frequency content as well. In this episode of
Scopes University you'll learn all about what an FFT is and how to use it on an
oscilloscope so you can test your device as thoroughly as possible.
You may remember learning about fast Fourier transforms, or FFTs, in your
engineering classes. Very simply put an FFT is basically just a math operation
that splits up the signal you're seeing in the time domain, translates that
information into the frequency domain, and displays it on the oscilloscope in a
graph showing amplitude versus frequency. You can see in this image visually how
the time domain in the frequency domain correlate which makes this all a little
easier to understand. You can see that the time domain signal is split out into
each of the different sine waves that it's composed of, and each of those waves
occur at a certain frequency and amplitude. There can be many
circumstances where you find there are added frequency components in your
design that shouldn't be there, or you find there's something like crosstalk
that's causing issues with your components. All of these can be problems
that cause your device to break or not work properly. These are problems that
you can't often see in the time domain, so having the ability to view the
frequency domain can reveal anomalies and glitches that you didn't even know
were there. Or perhaps they are problems that you can see in the time domain, but
you need to understand a little bit more about the frequency and power of that
part of your signal. This will allow you to figure out what's causing that error
glitch so you can go back in your design and correct it. No matter what the case
is for you, you can see how valuable this analysis is and understanding your
device more. Using an FFT on an oscilloscope is really pretty easy and I
think it's a fundamental skill that's really important
to learn. An FFT is just another math operation on the oscilloscope. So if
you already have familiarity with setting up math operations, the process
for setting up an FFT is very similar. Most of the infiniivision oscilloscopes
have an FFT button directly on the front panel, but for the ones that don't you
can just go into the math menu, press display to turn it on, and change the
operator to FFT. Then set up the sources with the channels that you're
trying to analyze. The signal we're looking at here is just a normal clock
signal that appears to have some high frequency noise riding on it, so we're
going to identify the source of this noise. The first thing I want to do is
zoom out on the time base of the signal a little bit. Like I said, an FFT is just
a math function and the more data that math function has to work with, the more
accurately it can calculate. Once you have the signal all set up with the FFT
turned on you can clearly see the spikes in frequency. You can auto scale the FFT,
or you can play around with the centre and span settings to customize your view.
The span setting is the FFT frequency width and the center setting is the
frequency in the middle of the span. So a 1 kHz span with a 10 kHz
center frequency will show you the spectra from 9 kHz to 11 kHz.
So to analyze the frequency peaks, you can just press the search front panel
button and then change it to frequency peaks. And now you can scroll through the
various frequency peaks on-screen and verify that what you expect to see is
actually there and debug any components that you were seeing that shouldn't be
there. We can see the spike for the high frequency noise riding on the signal
right here and we can see it occurs at 19.5 MHz. Now that
we know what the frequency of the noise, we can identify what it is that's
causing it and figure out how to eliminate it. This could be happening for
many reasons, but one of the main causes that we've been seeing more and more
with modern technology as chips get smaller is crosstalk. If components or
traces are placed too close together on a board, then the signals coming from
them can end up interfering with each other causing things like noise like you
saw in our signal we were working with here. So, the solution could be something
as simple as moving components around on your board. This is going to be different
for every design, but having insight into the frequency helps a lot in debugging.
FFT gives you a simple way to analyze the frequency without having to isolate
certain parts of your signal or make any additional measurements.
And you're able to do that alright on the screen of the oscilloscope while you're
already doing all your other measurements. To learn more details about
how to make advanced FFT measurements, make sure you check out the application
note, How to Make FFT and Pulsed RF Measurements. Thank you everyone for
joining me! Make sure you subscribe to our YouTube channel so that you can
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Happy Holidays!