In this video today I'm going to be
demonstrating how to perform a power
supply rejection ratio measurement using
an InfiniiVision X-Series oscilloscope.
This type of measurement sometimes is
simply called a PSRR measurement. You
typically use either a network analyzer
or frequency response analyzer to
perform this measurement. What it
measures is it measures a power supply's
ability to reject garbage or signals
that aren't supposed to be on the input.
Do they sneak through to the output so
it measures the power supply's rejection
capability.
The way the
measurement is performed is the input DC
input is modulated, it requires a summing
circuit and then you measure V-in, the DC
input, with the modulation and V-out and
find out how much then you compare V -n
and V-out. Now in a normal bode plot
it's computed as 20 log V-out over V-in
but in a power supply rejection ratio it is
the inverse of that. Rejection is the
inverse of gain so it's 20 log V-in
over V-out. So what I'm going to be
testing today is a evaluation board from
pico test. This happens to be a linear
power supply, has a very very low noise
output, virtually no output ripple sub
millivolt. For the summing device I'm
going to be using an accessory again
from pico test- is called the J 2120 a
line injector and so it has a DC input
and then it has the AC input which I
have connected to the wave gen. Now I'm
probing the input signal with a
10-to-1 passive probe, this standard
probe that comes with the scope, but on
the output I'm probing with a 1-to-1
passive probe. The output signal level is
going to be really really small and so
you need to use a 1-to-1 probe. Now
this is not the best probing techniques
because this long ground lead is going
to pick up other noise, environmental
noise. A better probing technique would
be to use an accessory that comes with
your probe that will get you a very
short ground like this. So this will give
you the best response, but for today I
found out that I can use the standard
ground lead so that's what I'm going to
do. So let's go ahead and get started
with the demonstration. Well I'm going to
start with a default setup so we can
set everything up from scratch.
Next I'll go to the analyze menu, select
the power application and then select
the analysis menu. Now in this menu we
can select a variety of power related
measurements. If the scope is licensed
with power options such as power quality,
current harmonics, switching loss, output
ripple, but if we scroll down here to the
bottom, we have two frequency response
measurements which are quite unique for
an oscilloscope. So we're going to
perform the power supply rejection ratio
measurement. Next I'll go into the setup
menu and before I set things up, notice
the question mark over here, that's a
help menu. If I press that it brings up a
diagram that shows me how the
measurement should be set up, and this is
basically how I have it set up. So let's
set it up here. First up at the top we
have frequency mode sweep which is the
normal mode for doing this type of test.
There's also a single. We will show that
later. Next is the start frequency. The
lowest it can go is 20 Hz, the highest
is 20 MHz. We're just going to use
the defaults. Then points per decade we will
use the default 10 points per decade. You
can go up to 50. Then you can define
which channel the scope is the input,
which is the output. I'm using the
default channel 1 and channel 2 and then
the most difficult thing to set up is
what test amplitude do you use? Now you
may not
think that it matters since it's just a
relative or ratio type measurement but
it's actually a very critical
measurement or critical setup. So this is
200 mV peak to peak relative to
a 50 ohm termination. I'd like to point
out that most frequency response
analyzers either specify the test
amplitude in RMS or they specify it in
dBm so just note the difference there.
Let's go ahead and just use all the
default settings here. So one advantage
of the scope based solution- first of all
you can see the waveforms during the
test. So the reason that's important is
you can monitor for any distortion that
may be occurring. Now it looks like we're
getting a pretty good measurement simply
at the 200 mV peak to peak input.
You notice it is a little wiggly in
different areas. That is a signal to noise
ratio problem. If we pumped up to the
amplitude, we might be able to smooth
that out. So we might do that in just a
minute. First of all like to show you
there's two independent cursors that you
can move and measure the rejection and
frequency and then you can see the
scaling on the left side here. It also
automatically scales as it's performing
the measurement. You can also scale
things manually if you like- move it up
and down or you can enter a value. So
let's go back and let's perform another
measurement. Let's pump up the amplitude
to 700 mV peak to peak. That
should give us a little more resolution
on the measurement. Now the green wave
form is V-out. Although you can't see
what the volts per division setting is
but all the way across this measurement
it's set at 1 mV per division so
it's a very very low level signal. Now we
just passed a frequency, I don't know if
you knows noticed it, where there was some
distortion now. Something to
note, now this is a much smoother we have
much better signal-to-noise ratio
down here at the lower frequencies but
remember that distortion you saw. Let's
go back and perform a single measurement.
Now I could do it right. I could select
single and enter the frequency I want to
test at. But you can also go to the
tabular view that shows all of the
measurements that are performed. The last
measurement frequency, amplitude and the
amount of rejection. What I want to
select is I'm going to perform a test at
approximately 500 kHz and there
you can see the PSRR at 500 kHz
and there's the waveforms. Now notice
channel one is a 1 mV per
division. It's a very low level signal. I can
improve it somewhat by turning on
averaging, but the thing to notice though
is it is not a sine wave. It is very
distorted so the 700 mV peak to
peak is simply too high of a test signal.
What you're going to find is that some
frequencies, a lower level signal works
better, and let's go into the wave
gen and let's manually reduce that until
V-out begins to look like a sine wave.
There you see it's a very very low level
signal. Now it may help to turn on FFT
and look at the purity of that sine wave
to make sure there's no distortion.
Anyway, what you're going to find is it
some frequencies a higher amplitude
works, improves signal and noise, and at
other frequencies you need to use a
lower amplitude signal so that you don't
induce distortion. So I've already gone
through the process of finding out what
the optimum amplitudes are and let's go
back to sweep mode, and now I'm going to
select a feature called "amplitude
profile." This is where I can optimize the
amplitude at each test frequency. So I'm
going to set it at 100 Hz, input 500
mV and then from 100 Hz to 1
kHz, I'm going to keep it constant
at 500 mV.
At 10 kilohertz I'm going to ramp up to
1V and then at 100 kHz I'm
going to ramp back down to 500
mV and then at 1 MHz I'm
going to ramp down to 250 mV, 10
MHz, same thing, 250 mV all
the way up to 20 MHz. So let's run
using these amplitudes which I think are
optimized. So now we have good signal
noise ratio at the low end. We can see a
few little wiggles up here. We're really
stressing the scope as far as how much
it can actually measure and now let's
move the cursors and you can see up here
at around 25 kHz, the scope was
able to measure about 95 dB of rejection
That's pretty impressive for a scope. At
the high end, 20 MHz, it's as low as
about 40 dB. There's an inflection that
occurs around 2.5 MHz
at about 42 dB. Now this was a linear
power supply, very low noise output and
we were able to get some pretty
impressive rejection figures, 95 dB. If
you're testing a switch mode power
supply, you're going to be battling all
that output ripple, and I think about the
best the scoping is going to be able to
do in that case, is probably about 60 dB.
Well thanks for watching this
demonstration today on performing power
supply rejection ratio measurements
using InfiniiVision X-Series
oscilloscopes. If you'd like to see the
other videos on the other power
measurements, if they have the power
option, as well as download an
application note on this particular
application I show today, go to the URL
listed on your screen. Thanks.