Hi, I'm Kenny Johnson, R&D engineer for Keysight Technologies. I want to take a couple minutes
today to tell you about a new probe that I just worked on, the N7028 power rail probe.
Now, this probe is very useful for making power integrity measurements. When we say
power integrity, what we mean is the measurement or the study of the conversion and delivery
of power within a system. Now, the type of measurements that users make when they're
making power integrity measurements include supply drift, supply compression, PARD which
is commonly known as periodic and random disturbances, static and dynamic load response. People
also want to look at how their product behaves over temperature. And then finally, high frequency
transients and noise on their supplies that can cause clock and data jitter in their system.
So when users approached us asking for a tool that can help them do these kind of things,
what they said they needed is four things in a probe. They wanted low noise because
you don't want the noise of your measurement system to overshadow the noise that you're
trying to trace down. Second, they told us that what they wanted was support for the
popular supply rails that they encounter on a daily basis within new systems. Third, they
said they want low loading. They want a system that when they hook it up to their, they want a measurement system that when they hook it up to their system,
it doesn't distort their system or make it misbehave. And then finally, they want high
bandwidth. The troubling part for many users is chasing down these high frequency noise
and transients that can cause clock and data jitter. Many, many folks have told us that's
the single biggest cause of clock and data jitter in their system is the high bandwidth
transients and noise. So let's take a couple of minutes now and show you how the probe works.
So we talked about that one of the features that users are looking for in a measurement
solution for power integrity was low noise. Now, one of the things I want to share with
you is a common pitfall that many of our users fall into. And that is that they choose to
use the one mega ohm input to their scope rather than the 50 ohm input. When you're
choosing a probe to hook up to your scope or even just cabling straight into your scope,
you can get the lowest noise with the scope typically from the 50 ohm input. I want to demonstrate
that to you here real quick. What I've done is I've got the scope just in maximum sensitivity on 50 ohms.
I'm going to go ahead and I'm going to change it to one mega ohm and you'll be able
to see the trace get fatter, and you'll also be able to see the measurements jump up. Now
that's what happens when you, when you're using the one mega ohm path. Many times users
think the one mega ohm path is the right choice because it, its specifications are more utilitarian.
And they think that that lends itself better to power rail measurements. You can be hooking
up to high voltages. You're better off doing that. What we want to tell you today is that
there's better ways to do it, and it's probes and accessories that hook up to the 50 ohm
input to the scope so that you can get the benefits of the much lower noise floor of the 50 ohm channel.
Recall that the second thing that users said that was important to
them was the ability to have support for the voltage rails that they're going to encounter
on a daily basis. That could be things from 0 all the way up to 24 volts has been very
popular with users that we've talked to. Now why that's important to them is that what
they're trying to do is look at the very small AC signals riding on top of their DC supplies.
Believe it or not, what you see back there on that scope is a 1.8 volt DDR memory. Now
what I've done is I've double probed the system, same probe point with two different inputs
to the scope. One is going directly into 50 ohm path which we said is the low noise path
so of course, we want to do that. The other I'm probing it with the N7020A power rail probe.
And I'm making use of probe offset. So with probe offset we can subtract out the
DC content of the signal and zoom in. Most scopes of a 50 ohm path don’t have enough
offset range to cover all the supplies that people want to see. So if you look at these
signals, you can see on channel 2 there, the green trace, we zoomed in on it as much as
we possibly can on the 50 ohm channel. You can see some wiggles and the remnants of the
switching of the, of the power supply. On channel 1, though, there's clearly a lot more
detail in the wave form itself. But if you look closely at the measurements at the bottom there,
you'll see that the peak-to-peak voltage for the zoomed in channel, that's the probe
with offset with the N7020A, we're seeing about 80 millivolts of peak-to-peak noise
on that supply. Where we don't have the offset, we're actually seeing nearly 25 percent more,
over 100 millivolts of peak-to-peak ripple on that supply. So what happens then is by
not using and offset being able to zoom in on the probe, we don't have enough sensitivity,
we're not using the full dynamic range of the input to the A to D. And the punch line
is that as a user you could be throwing away margin then if you're not making use of probe
offset and you're making measurements on power rails. Remember the third feature that we
talked about that users told us that was important in a power rail measurement tool was low loading.
Now when we talk about loading, this is a pretty typical thing in oscilloscopes. When
you hook up the oscilloscope and it's probed to your circuit, they've become intimate.
It's an electrical contact, and it's now part of your circuit. And it's gonna change the
way your circuit behaves. There's no way around that. We just wanted to change its behavior
as little as possible, and that's called low loading. So a very common tool that we’re
using from users that approached us, was they were making their power rail measurements
going into 50 ohms. Remember, we talked about that being the nice low loading path. And
in some cases, they had enough offset range that they could zoom in on the top of the
waveform like we're showing you in the previous example. But in this particular case, what
happens is that 50 ohm DC load on their supply is just too extreme for their targets. They
tell me that that can be as much current draw as maybe their microcontroller is using or
in cases where they have a power management IC, the PMIC actually goes off into other states
because it thinks things are, other things are happening in the system. So when we designed
the N7020A power rail probe, we gave it a 50k ohm DC input impedance to minimize the loading.
Now in this quick little demo what I've got here is I've got a 3.3 volt supply.
And I've taken and right at the probe point I've put in a sense resistor. And I'm making
use of our N2820 high sensitivity current probe. And so I'm going to measure the current
that is being synced into the scope. This is the load of the probe, if you probed with 50 ohms.
So when I plug on the cable, I set it so it'll trigger on me plugging in the 50 ohms.
When we see the voltage go live, we'll trigger the scope and you'll see the
current spike that happens, too. The loading that's going to happen on the target.
So from this, it's pretty obvious that what happens then is when I take and I put 50 ohm DC load
onto the 3.3 volt supply, I'm sinking 66 million amps of current into the scope. That's just
too extreme for a lot of applications. So maybe a better choice than just going straight
into 50 ohms would be to use the N7020A power rail probe.
So to close things out, we're going to talk about bandwidth. Remember that I said the fourth thing that users were asking
us for was a high bandwidth solution. It turns out that in the high-speed data systems that
we have today, our high-speed digital systems is that the clock and the data are highly
susceptible to noise, riding on the supply. I said earlier that the noise on the supply
can be the number one cause of clock and data jitter in the system. And so the bandwidth
that people need to actually track these things down is happening at the switching currents
in their systems. And they want very, very high bandwidth to be able to capture these,
these transients. So what I've got here is it's actually a supply, a 1.5 volt supply.
And I'm double probing it. I'm probing it with the N7020A power rail probe. And then
I'm also probing it with a very, very popular one-to-one passive probe that goes into the
one mega ohm input. Now I chose that one mega ohm input and one-to-one passive probe because
I've encountered many, many users who like that probe and they use it for looking at
their power integrity solutions. What I wanted to point out here was two things. First, that
you're going to miss the high frequency transients that are causing the issues. And second – and
this is extra credit at the end there and everything – I'm gonna show you how extra
noise comes into that one mega ohm path. So if you look at the scope here, I'm going to stop it for a second.
And you can see these, these spikes and these transients that are showing
up on channel 1. That's the N7020A power rail probe. That's the higher bandwidth probe.
It's capturing these transients that clearly are missing from the 20 megahertz path. It
seems obvious when you think about it that the lower bandwidth probe is going to filter
out those transients. As I understand it, what happens is, is the supply drops on things
like that is it changes the propagation delay through gates in the devices. And that can
start creating timing errors that ultimately can even lead to bit failures. So those are
the kind of things that users are trying to track down on these high-speed systems. Now
for the extra credit assignment when I was talking about the noise coupling in, watch
as I raise up the trigger level on channel 2. And I go up here a little bit higher and
start capturing some of these transients. Now these aren't real transients. What that
really is, is that is noise from the outside environment coupling into the probe. The mega
ohm input to a scope and mega-ohm high impedance probes make a great antenna. They're gonna
pick up all sorts of magnetic coupling. So the switches that are in your system, they're
going to couple into those high impedance paths. That's another reason why not to use the high
impedance path when you're looking at the power rail measurements. So in closing, let's
go over what we discussed. Remember that users that came to us asking for a specialized tool
for making power integrity measurements had four key things that they were looking for.
First, low loading. With the N7020A power rail probe we accomplished this by keeping
the probe as a one-to-one attenuation probe so we've got good signal-to-noise ratio. In
addition, the scope is terminated into the 50 ohm input of the scope. And you recall
our demonstration where that is the lower noise input to the scope. Second, these users
were asking for support for the popular supply voltages that they're gonna encounter within
their systems. The N7020A power rail probe has plus/minus 24 volts of probe offset, so
you can zoom in on the top of that DC supply and see all those AC signals that you're looking for.
Third, users wanted low loading. We accomplished that with this probe by, by having a 50k ohm
DC input impedance. Remember the example I showed you with the – going into 50 ohms
and the excessive loading and everything. This probe gets around that by having 50k
ohms of DC. And finally, high bandwidth so that people can track down those troublesome
and bothersome high frequency noise and jitter than can disturb their clock and their data.
During the examples that I showed you today we’re using the Keysight S-Series 10 bit
high definition – high definition oscilloscope. That would be the tool of choice. Or you can
also use our f-, InfiniiVision X-Series scopes which are also compatible with the N7020A
power rail probe. I want to thank you for taking the time. And if you want to learn more about
this product, please just go to Keysight.com and search on N7020A. Thank you very much.