Hi, My name is Jeff Most, and I'm an engineer here at Keysight. Customers have
told us some of their biggest signal integrity measurement challenges are
removing the effects of the probes and fixtures that they need to use when
connecting to a DUT that doesn't have standard instrument connectors at some
of the ports.
They tell us that what they really want are the measurement results of just
the DUT and not the DUT plus those probes and fixtures. In the video today,
I'm going to show you an easy method of accomplishing exactly that when making
measurements with the TDR. And there's three steps to the process.
The first step is to calibrate the TDR. The second step is to use the calibrated
TDR to characterize the probes and fixtures that you need to use to connect
to your DUT. And the last step is to measure the DUT plus the probes and
fixtures, and then remove the effects of those probes and fixtures through
de-embedding. And this process works for a time domain measurement like an
impedance profile and frequency-domain measurements like insertion loss or
return lost or crosstalk. And while the process that i'm going to show you is
pretty easy to implement, the results are still quite accurate.
Let's take a close look at the DUT. It is a two-port device with no connectors.
Because there are no connectors to characterize the device, I've built it into
this validation board with the connector and a connector launch on each port.
The actual DUT is this small device inside the red box in the center of the
validation board. Also on the validation board, I built a replica of the fixture
with connectors. To properly characterized the DUT, I will need to
characterize the fixture and then de-embed the fixture at each port of the
DUT from the combined measurement of the DUT plus fixtures.
Here you can see the DUT that I'll actually measure, and by de-embedding the
fixtures, I can determine the measurement results of just the DUT without the
fixtures.
Let's take a close look at the instrumentation we're going to be using to
make measurements today.
This is the Keysight 86100D DCA-X mainframe, and this is the N1055A
50 GHz TDR/TDT module.
This one happens to have four channels, but you can also get them with two
channels, and each channel has a remote head like this one with a 1.5 millimeter
connector at the end.
The three steps in the process we're going to follow today is first I'll set
up the TDR to make a two-port measurement since our DUT has two ports.
Next I will use this ECal module (DC to 67 GHz) to calibrate the TDR system
right at the connector of the remote head. So that's where our reference plane
will be. An important note is you can use the TDR to make measurements without
calibration, but to maximize accuracy like we want to do today and to take
advantage of advanced features like the embedding and AFR, you need to do a
calibration. The second step is we'll use the calibrated TDR to characterize our
fixtures using a new tool available on the TDR called automatic fixture removal
or AFR. In this case, one-port AFR. And, just a note:
Keysight introduced one-port AFR a couple of years ago as part of our
N1930B PLTS software, and there will be some more information at the end of this
video that shows you where you can get some additional details on AFR if you
would like them.
The final step we're going to go through today is using the TDR to measure the
DUT plus fixtures and probes and then remove the effects of those fixtures and
probes with de-embedding.
The first step in our process today is to set up the TDR for a two-port
measurement and then perform a TDR calibration at the reference plane, which
is the connector on the remote head.
So the best way to start is just with an instrument default set up.
Next, we'll go into TDR mode, and we will choose a two-port DUT since that's what
our devices that we're going to measure today. At this point, you're ready to make
measurements with your TDR. You can turn the steps on and off with these
controls. You can turn the receivers on and off with these controls. But if you
want the best accuracy and if you want to be able to use advanced features like
AFR and de-embedding, you need to perform a TDR calibration.
So we can select a new calibration. There are several steps here. The first one is
to just confirm the setup is what you want (and it is), then you have a choice on
the type of calibration you're going to do. Today we're going to do
ECals - we select that. We have to answer some questions about the type of
connectors on our DUT, and we have 2.4 millimeter males.
We set up the averaging we want (and that's good),
and then we can start the calibration measurement wizard.
This tells us where to connect the heads on our ECal module, which we will do next,
and then we can begin to measure.
Now that we've calibrated the TDR, we're going to use the calibrated TDR to
characterize our fixture. And we could start back in the DUT setup. We call up
the de-embedding menu. and that lets us launch the automatic fixture removal
wizard here.
This asks us a few questions about the fixture, and it is a single-ended
fixture. And we're going to measure one port because they only have access to one
port.
This lets us tell AFR where they're going to be measuring an open or short.
And our fixture is open. And finally it's time to make the measurement.
So we click measure, and now we can just save the file.
Essentially, one-port AFR measures from one port of the device. Or you would
typically just get return loss or s11. AFR extracts the full two-port
S-parameters (the s2p file), and a good practice to see how effective that
extraction was is to look at the return loss and insertion loss of the s2p file
that AFR created.
I've just put the insertion loss and return loss of the fixture on the screen,
and the thing to look for is the frequency where the return loss and
insertion loss are the same because the AFR model does not work well beyond any
frequency where the insertion loss and return loss are the same. All the way up
to 50 GHz, the insertion loss and return loss don't ever touch each other,
so this extraction worked well to be on 50 GHz. If you're performing this
and you run into a problem where your insertion loss and return loss graphs
cross each other at a low frequency,
there's a couple of alternatives you can do. Instead of just measuring into an
open, you can measure into both an open and a short, and you may get a better model.
And if that doesn't work for you,
the best thing to do is to not use one-port AFR, but to make a two times through
and characterize that.
So the last step is to measure the DUT with the fixtures
connected and to de-embed the fixtures.
This is our set up. I have my two TDR heads connected to my DUT.
First let's go and take a look at the measurement results that we want, and
we're looking for some frequency domain data. For those of you who have seen the
DUT, you may have already figured out that it's actually a band pass filter. So we
just want to take a look at the insertion loss s21, and there it is.
You can see the pass band at about 25 GHz.
And this is the insertion loss of our DUT with the fixtures in place. So now
let's de-embed those fixtures.
We go back to the setup and into our de-embed setup.
We just need to now find the S-parameter file for our fixtures.
So we're going to load that S-parameter file, which is on the desktop.
We're going to select that. And now I can put this for each port.
Port one of the fixture is the connector IS port, which is connected to the TDR.
So over on this port, we need to change the configuration of the fixture so port one
is connected to the TDR and port 2 is connected to the DUT. And it's very easy
to reverse those.
So there you go. By checking this box, we can remove the fixtures by unchecking it.
We'll have the fixtures in place. Let's leave it unchecked for now. We'll just
slide this out of the way, and you can see our insertion loss.
I can just save that.
So there we have the insertion loss saved with the fixtures in place. And now
we will de-embed the two fixtures, and you can see the difference.
I have much less insertion loss in the pass band. Using fixtures and probes with
the TDR is fast, easy and accurate. If you have a high performance instrument, a
robust calibration process, and AFRs, you can characterize probes your fixtures
with non-standard instrument connectors.
If you already have a DCA-X-based TDR system like this one, you can add AFR
with a user-installable license. And if you don't have one but would like to
learn more, please contact your local field engineer
Thanks for spending time with us today.