Hi, my name is Brad Frieden
I'm a product marketing engineer with Keysight Technologies.
This video is a part of a series of videos looking at how
Infiinium oscilloscopes can be used to make a variety of RF measurements.
We're going to be looking at the S-Series oscilloscope. This scope offers 8GHz of bandwidth.
It's been redesigned with a new front end and time base.
and combined with amplitude and phase correction
on the channel response, these scopes function as an
excellent RF receiver for a variety of applications.
We will look at a digital communications example, wideband communications today.
In this video, we're going to make some basic FFT measurements on
a simple input signwave.
We'll look at some of the details for how to set that up.
We're using the S-Series oscilloscope with 8GHz bandwidth.
This scope has been redesigned from previous generations with the new time base,
a new front end, giving it very low close in
phase noise as well as very linear amplitude response as well as phase response.
All of that results in very high quality RF measurements.
We're using a PSG single generator to create this 1 GHz sine signal.
We're bringing it through a very narrow
filter, so the signal that I'm going to be bringing over to this oscilloscope
is a very pure 1 GHz sine.
Now I'm gonna take the signal generator and adjust the signal up
so that I'm right around 0 dBm coming into the oscilloscope
and we will start making our measurements from there.
After a default setup
and an auto scale
we should see a nice sine wave appear, and we do
and we'll do a quick measurement here in time
and take a look in and see that that frequency is in fact 1 GHz.
And I can also make a
peak to peak measurement on the signal and one mW 0 dBm ought to be a be about 630 mV
peak to peak, and sure enough it's right there.
so we've got some expected measurements to confirm we do have a 1GHz sine wave
Now I'd like to see a spectral view
so from math spectral
I can bring up an FFT showing me this
time domain signal, what I see on screen here,
now turn into an FFT for viewing in the frequency domain.
Notice I have an access here from 0 Hz up to 10 GHz
the top of the screen is at 2 dBm- a little bigger than the 0 dBm signal that we have.
down here to minus 78 dBm.
Notice that the signal is very wide
in the frequency domain even though we know it really is just
a single frequency of 1 GHz and should be looking like a spike.
What we need to do is adjust the
resolution bandwidth of the FFT
This defines how wide the frequency buckets are across this
axis that our signal is dropping into for display.
So if we click on this and start bringing that resolution bandwidth
making it more fine: 200 MHz,
100 MHz, 50 MHz, notice how
we're changing time per division and bringing more time capture on screen
which results in more
cycles of the sine wave. Notice how this is becoming more narrow I'm getting a
finer resolution, and I'm able to distinguish
some different things here and also notice
how the noise floor is going to be dropping. Here we are at -40
-50 so we're just skirting around -50 dBm.
As we continue to bring down the resolution bandwidth,
here we are now down to 1 MHz
resolution bandwidth, notice I see a very clear
1 GHz spike corresponding to our main signal
I also see some smaller spikes here there's one
at the second harmonic 2 GHz
and a third harmonic at 3 GHz.
So we're seeing that at around -50 dBm
that is about our third harmonic
distortion level. That's really a lot of nice range there
and then if I check mark peaks
it finds that top peak and tells me that that's at basically 1 GHz
and right hovering near its 0 dBm.
So this is what we would expect for a basic measurement.
In many measurements, one wants to zoom into
frequency of interest and look with a much more narrow span around that point.
In this case, we'd like to
look right at 1 GHz and perhaps look at around 100 MHz
span around that point and look with a higher resolution.
So let's do that.
If we look at the FFT controls, I have things like Start Frequency, Stop Frequency, Center Frequency.
The span we were using resolution bandwidth just a moment ago
let's change the center frequency from being
5 GHz to instead being 1 GHz.
So now my spike is in the center screen; let's change our span
so instead of it being 10 GHz, let's make it 100 MHz.
This should really zoom us in here now to the signal.
Now, I see width. Looks like I need
even more fine resolution bandwidth, but I can do that.
Now I notice that my noise is now down near the bottom of the screen
at -80. We can change this to
20 dB per decade to give us a little more room on screen of seeing what's happening.
And now let's us start driving that resolution bandwidth
more and more narrow so we'll go to 500 kHz
down to 100 kHz and 20 kHz
finally I go to 10 kHz.
Notice how my noise floor has been dropping I've got that
measurement happening on 1 GHz sine,
and you can see it really is a pure signal, even at this close in view
we're really just seeing a 1 pixel here of the signal.
We've seen how an 8 GHz bandwidth S-Series oscilloscope
was able to make a nice FFT view
a spectral view of the signal.
We've also seen how the
flat response of the oscilloscope both in amplitude and phase
and low phase noise contributed to a measurement like this.
And, we could have also looked at broadband signals with a wide kind of spectral content.
That's the topic of another video.
If you would like more information about what you've seen today
and this application of using FFTs, please go to the URL on your screen
and thanks for watching.