Today we're going to learn about what frequency response analysis is and how
to set it up on an oscilloscope.
Hi, I'm Erin East and welcome to Scopes University.
In this episode we're going to learn a little bit about the basics
of frequency response analysis. And the name itself is actually pretty
self-explanatory. It enables you to analyze how your device is going to
respond to various input frequencies. So, basically you input a sine wave of
various frequencies into your device and then you compare and measure the gain
and phase of the input of the system versus the output of the system. The
result is a bode plot, which you may be familiar with, but a bode plot plots the
frequencies on the x-axis and the gain and phase on the y-axis. The gain is just
the ratio of the input voltage versus the output voltage and the phase is the
phase shift seen between the input and the output. This will help you verify
that your device is going to operate correctly when signals of different
frequencies are input into the device. So, this is really useful to anyone working
with amplifiers, filters, power supplies, audio systems, or anything like that and
it's really convenient to have this tool built right into the oscilloscope
because you can further characterize the device you're working with while you're
already doing your other characterization on the oscilloscope. So,
let's head to the lab to learn about the device we're going to be characterizing
today and learn how to do that on the oscilloscope.
So, for this exercise we're
just going to build a very basic bandpass filter. This is made out of all
passive components. All you need is a capacitor, inductor, and a resistor to
build this. So you can really easily make this on a breadboard. And once I have
this all set up on my protoboard, I can take a look at how to set this up with
the channels on the oscilloscope. Most of the InfiniiVision oscilloscopes have
this optional built-in frequency response analysis application and what
it does is uses the built-in waveform generator to sweep a sine wave in a
certain range of frequencies and then measure on the input and output of the
device. So let's open up the application and the first thing I want to do is hook
up the waveform generator to the input of the device that I'm working on. Next,
we're going to probe channel 1 on the input of the device so we can measure
the input and then we want to probe channel 2 on the output of the device.
This will allow us to probe on the input and output
and compare those. And we can see the first menu is the Setup & Apply
menu. This is where you can configure the settings for the specific tests that
you're working on and we can see here you can set up a frequency sweep which
is most typical but you can also change the frequency mode to single just to
test at a specific frequency. And this is helpful when you know there's a problem
area in your device. You can also adjust the start and stop frequencies
that the WaveGen will sweep between along with the source and the amplitude
of the signals. On the touchscreen InfiniiVision oscilloscopes you can
also specify the amplitude profile of the sweeping sine wave. Theoretically,
the test amplitude shouldn't really matter since the gain is a ratio of the
voltage input and the voltage output. However, when you're testing sensitive
electronic circuitry, if the input test amplitude gets too high, then nonlinear
distortions can sometimes occur during some frequency test ranges. So the
amplitude profile allows you to optimize the input test amplitudes between
different decades of frequencies in order to maximize the dynamic range of
the measurement results without distortion. Once you have all of these
settings configured properly for your specific test you can press run analysis.
Now you can see the sine waves actually changing in frequency on the screen and
then you can see each of the data points starting to be plotted on the bode plot
as it runs. Then once it's finished running you can use the touchscreen to
move around the orange triangular markers on the chart and you can see the
data for these markers down here in the bottom right corner. The scope will
already auto scale the bode plot when the test is complete, but to control the
scaling even further you can press this arrow in the top right corner of the
plot to change the scaling in the offset of both the gain and the phase.
Lastly, you can also press this table icon on the left-hand side to see all of
the data that we see plotted in the bode plot in an organized table instead and
this can all be exported right into a CSV file that can be used for further
analysis offline of the scope later on. One last thing, you can also select
one of the rows in the table and a dialog box will pop up allowing you to
run a single test at that specific setup frequency to do further analysis at that
frequency point. So this can be really helpful if you see something that is
abnormal at a certain frequency in your bode plot and you want to analyze that
further and see what's going on at that specific frequency.
The frequency response analysis application is also available on the low price 1000 x-series
oscilloscope. So this is a lot of performance to get in this little scope.
The features of the application are a little bit more limited, for example you
can't set the custom amplitude profile, but it gives you the gain and phase data
in the form of a bode plot on the screen so you're able to make these types of
measurements on this class of an oscilloscope. You can see how
convenient it is to have this type of analysis built right into the oscilloscope.
You're able to do that frequency response analysis
characterization of your device while you're already doing other
characterization with the oscilloscope. And having that bode plot right on
screen makes it really easy to see how your device is responding to various frequencies.
Thank you everyone for joining me today.
If you want to learn a little bit more about frequency response analysis and some more details of the
application, make sure you check out the FRA datasheet, which you can find on the
scopes university webpage. You have a link to that in the description below
and you'll see it in a few seconds on the screen here. As always, just leave a
comment below if there's a specific topic you'd like to see me cover in
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