Today we will be discussing frequency domain versus time domain.
My name is-
What's up everyone? The name is Nick Ben and I'm an engineer here at
Keysight and welcome to this episode of What the RF! In the last episode we discussed what
a spectrum analyzer or signal analyzer is and the various measurements you can
make with it. Today we'll go into more detail about the difference between the
frequency and time domain. Alright, so previously we had briefly
mentioned the time domain and frequency domain but what exactly is the difference?
And what about the frequency domain tells us more about our signal? Lucky for
us I have this set up here where I'm sending the same signal into the oscilloscope
and the signal analyzer. The oscilloscope and signal analyzer are
tools to visualize electrical signals in the time and frequency domain
respectively. As you may remember, an oscilloscope displays signals with
respect to time versus amplitude- or in the time domain. Here we're outputting a
10 MHz signal from the oscilloscope's waveform generator to channel one of the
oscilloscope, and visually this is how it appears on the oscilloscope, as a
sinusoidal waveform with a period of about 100 ns. Now if we were to
adjust the output from the waveform generator to a 20 MHz signal, then this is
how it would look on the oscilloscope. Once again, as a sinusoidal waveform with a period
of about 50 ns. Basically the higher the frequency, the more waves we
see in the same span on our oscilloscope. Now in my last video I got a question
from one of you asking what the advantage of a signal analyzer is if
you're already own an oscilloscope with built-in FFT. Well I'm glad you asked!
Signal analyzers have lower noise floors this means they are much more sensitive
to frequency components and are better at showing you low-level signals. Signal
analyzers also have better spurious free dynamic range, or SFDR, than
oscilloscopes. Alright so now we have this signal analyzer and as you may
remember signal analyzers display signals with respect to frequency
versus amplitude, or the frequency domain. If we were to output the same 10
MHz signal from the waveform generator that we had seen earlier on
the oscilloscope to the analyzer, this is how it would appear visually- just a
single peak at 10 MHz. And if we were to adjust the output of the waveform
generator to be a 20 MHz signal, then it would be a peak that appears at
20 MHz. So yeah, in the time domain signals appear as sinusoidal waves and
in the frequency domain they appear as a distinct impulses. Ok well now this begs
the question of "why the heck do we care to use a signal analyzer?" Is it just to
show a wave as a line or something? Well, in a perfect world we would see the
sinusoidal waveform like the ones we saw on our oscilloscope at frequencies of 10 MHz
and 20 MHz. Unfortunately we don't live in a perfect world. When
dealing with various devices, it's often you see a not-so-perfect sine wave with
many ripples, like this one that I've now set up by feeding two sine waves from
the waveform generator into the oscilloscope. You can say that a real
world signal can be represented as a sum of different sinusoidal signals, rather
different frequencies. Now let's say you're designing a product and your
product can only operate in a specified bandwidth and it can't be emitting in
other bandwidths. Then you must determine at what other frequencies do the other
signals exist that are corrupting the signal you want from your device. And
that's where our signal analyzers come in they help separate and display this
combination of different sinusoidal signals into their distinct frequency
components. So that if you're expecting your device to operate at let's say 10 MHz
you can see all the other frequencies that are messing with your
device- for example this 20 MHz signal. And once that's figured out you
can use a band pass filter to tune out those extra annoying signals you weren't
expecting. Alright that's it for this episode. Thank you so-