bode plot
gain and phase measurements are often
used to characterize the frequency
response
of a broad range of devices and circuits
including analog
filters as well as power supply feedback
networks
they are also performed extensively in
university teaching labs
to help teach engineering students about
the concepts
of frequency dependent variables
including the impedance of inductors and
capacitors
hi i'm johnny hancock product manager
for keysight's
infiniivision x-series oscilloscopes
let's get started by showing how to
create a bode
plot of a bandpass filter the old
traditional way using an oscilloscope
and an external function generator
so i have a bandpass filter here this is
actually a
training board that ships with this
particular product the 1000 series
oscilloscope
and i'm have my external generator
connected to the input i'm probing
channel one on the vn pin
channel two on the v out pin of this
filter
and i'm going to go through the
measurement technique that say a student
in the university would do
the way i did it 45 years ago they're
doing it the same today now if you're an
industry
maybe your company can afford to buy a
network analyzer or frequency response
analyzer
but in the university environment they
typically use
just oscilloscopes and an external
generator so
i've got the scope set up at 100 hertz
one volt peak to peak i've got or the
generator i've got the scope set up
and it's measuring uh channel vn
channel 2 v out i'm going to make
measurements at 100 hertz 200 hertz 500
hertz
one kilohertz i'm going to step all the
way up to
uh 10 megahertz so it's going to be a
lot of measurements so
the first measurement here it measures
vn measures
uh 0.99 volts i'll write that down
v out measures about 28 29 millivolts
and the phase measures 88 degrees
now compute the uh the gain 20 log v
out over vn so i got 29
e 3 minus enter
0.99 divided by
log 20 times minus 30.7 db this is my 15
c engineering calculator app on my
mobile phone
so now i'm going to change the frequency
to 200 hertz
and repeat
[Music]
that was a lot of work so here's all the
data i took
here's my plot it's not all that at that
accurate
in just a minute i'll show you how it
can be done much
quicker with the infiniivision
oscilloscope but first i want to have a
little talk about
bodhi the last time i did a video on
body plots
i got lots of comments down there in the
comments section
so calling me an idiot saying i don't
know how to
pronounce bodhi they say it is with a
long o
and a silent a i'm here here to tell you
technically it's na it's it's uh body
plots are named after dr
hendrick wade boda it's a dutch
name and the dutch pronunciation is voda
his family preferred the dutch
pronunciation his colleagues
including claude shannon and harry
nyquist at bell labs
they called him bodhi all my professors
called it a bodhiplot
so it's a bodhiplot now let's do it a
much more efficient
way so now i'm going to plug in my
bandpass filter
directly into the generator output
of the oscilloscope go into the analyze
menu
and select frequency response analysis
which means bode plot
and i'm going to set it up to start
sweeping at 100 hertz stop
at 10 megahertz like we did manually
and let's increase my amplitude so that
we're doing exactly
uh like we did with this scope and
just say go and so it automatically
scales the signal
it measures using the frequency domain
it measures the magnitude of both vn and
v
out begins plotting it the blue plot
that's
creeping across screen here that's the
gain plot
the orange or the reddish looking plot
is the phase plot it started up around
90 degrees
the gain plot started around -30 which
is what i determine
manually and it goes up to about 0 db
and there you can see it then trailing
off exactly the characteristics that you
would see from a bandpass filter
at this point i could uh
make measurements with the markers and
over here it tells me
exactly what the gain phase and
frequency
is at any point i could export the data
and analyze it offline if i like now
that was a lot faster
a lot easier and a whole lot more
accurate
automatic bode plot measurements are
standard on the 1000
x-series oscilloscopes that have the
built-in generator
and is an option on the 3000t
4000 and 6000 x-series oscilloscopes
if you think you might have learned
something in this short video to make
you more proficient
in performing oscilloscope measurements
i invite you to view our other
infiniivision oscilloscope measurement
tips videos
to learn more about keysight's
infiniivision x-series oscilloscopes
go to the url listed on your screen or
contact one of keysight's
authorized distributors thanks and
semper fi