It is11:10, so we can go ahead and get started. The topic of this particular session is methods
for optimizing vector modulation, and we are talking about IQ modulation here. I will talk
a little about alternative or some more modern methods that are emerging that that apply
to some applications, not all, but in terms of IQ modulation. We want to be able to optimize
that signal because vector signals made up of a phase angle, of course, and a magnitude
that is translated from those polar coordinates to the rectangular coordinate system of IQ.
So there is a deterministic link there between those two, and we will probably keep that
in mind as we go through this. The IQ modulator, fairly simple in basic structure, although
a little bit difficult to optimize. That is what we are going to talk about here in the
coming slides. We have an LO input that is shared between two mixers. We split that and
shift one 90 degrees from the other. That gives us, we call that in quadrature, the
two signals are in quadrature providing us with two RF signals, RF one, RF two, and here
is our I and Q input. Which, of course, are the analog baseband signals that represent
our vector modulation when combined in this method. And out comes our IQ input, this goes
onto the RF for upconversion filtering, but here there is quite a bit that can go on within
the IQ modulator that we need to really consider and optimize. The first thing, and probably
most important, as signals become more and more wideband the effect of frequency response
of the IQ modulator has some dramatic effects on our system performance. If we have an IQ
modulator that has a frequency response that looks something like this across a 4 GHz frequency
span. Even if we are looking at a 200 MHz channel in here, you can see there is quite
a bit of variation in just the magnitude and that is what this particular chart shows,
is magnitude only. There is, of course, the other component to consider, that is the phase
distortions or group delay. It will have a similar frequency response as well. We need
to be able to, mitigate the effects of this frequency response through adjustments within
the IQ modulator or the I and Q baseband files themselves. Here is a representation, I hope
you can see this, of a multitone signal. We have 20 tones here, spanning 2 GHz. I guess
maybe I should qualify some of the bandwidths here, in that a lot of commercial communication
systems, we are talking 20 MHz or maybe up to 160 MHz with some WLAN technologies. Of
course in aerospace and defense applications where you are using radar or you have a satellite
with multiple transponders filling up a very large segment of bandwidth, these kinds of
issues become even more acute with the extension of those bandwidths. So radar, electronic
warfare, satellite, all use bandwidths or need to be tested and characterized in bandwidths
in the GHz range. So, of course, Agilent has products that can generate and analyze these
as well. Getting back to the flatness, both amplitude and phase, and of course phase is
hard to visualize here, so we are going to focus a lot on amplitude. And you can see
the roll off of the frequency response of our system across the 2 GHz bandwidth. So
what do we do to mitigate that? We need a nice, flat bandwidth. We need to rid ourselves
of those phase distortions. Well, in what I call the classic methodology for getting
rid of these is where we take the signal, the IQ waveform in a digital file, and load
it into an arbitrary waveform generator. We can up convert that, of course, with a signal
generator that has a wideband arbitrary IQ modulator in it. Then take a spectrum analyzer
or some other digitizing platform, it could be an oscilloscope. A spectrum analyzer, of
course, gives us the best dynamic range and correct the signal. We make measurements,
narrow band measurements, all along the bandwidth of interest. We pre-distort, correct, or calibrate
our waveform and then we download that corrected waveform into the arbitrary waveform generator
and that provides us with the ability to pre-distort that signal to achieve a very flat magnitude
and phase response across the entire bandwidth. Here, you can see we are down to a less than
0.5 dB across a 2 GHz spectrum with these multitone signals. This can be applied not
just to multitone signals, but also any kind of digital modulation or vector modulation.
Well, is there a better way to do this? Yes, of course this is kind of a little bit of
a commercial. We have a signal generator MXG that does all this correction in real-time
and it uses a couple of different ways to do that. There is a fully temperature compensated
calibration done at the factory, of course, so it knows exactly the operating point of
the entire block diagram of the system at any particular temperature or operating condition.
And in this case, we can apply some baseband processing to flatten the spectrum across
160 MHz of bandwidth in real-time, so there is no more changing or altering the waveform
file. It is all done in real-time for you within the system and it is done at baseband.
I will not dwell on this slide, but we have baseband corrections that are calculated at
boot up or when a cal is run in an RF correction. And these are convolved with one another in
order to get that flat phase and magnitude response. It is good to note you can also
turn this off. You do give up a few microseconds, maybe tens of microseconds of switching speed
when you invoke the real-time corrections under your waveform, so that is the little
bit of a tradeoff. But, of course, the flexibility we give you is to be able to turn this off
if you have other means for correcting your signal or you do not need this kind of accuracy.
Any questions so far with that? So I would like to jump into some of the imperfections
in the IQ modulator and how we can mitigate those issues. The first one you see usually
is an LO feed through and of course it is caused by the paths being not identically
matched in terms of a DC offset between our I and Q signals. So the DC offset between
the I and Q allows some of the LO to bleed through to the RF signal. Now, there are a
couple of things we can do. We can apply some pre-distortion, that is similar to what I
just talked about where we go in and in this case, we take the waveform file. We take a
tone 180 degrees out of phase with our LO and we place it right at the LO in order to,
kind of noise canceling headphones, to take that or reduce that effect. We can also adjust
the I and Q path DC offset level and here it is visually. The I signal a little bit
of a voltage difference between the Q at the input. Here you can see the difference. DC,
before offset adjustment, we got a 55 dBc LO feed through. After a DC offset adjustment
within the signal generator, we get -66. Of course we could reduce this more if we used
a pre-distortion technique to put a tone in here to cancel this particular tone. That
is another method that you can use. Of course, most modern, including Agilent's signal generators,
have this capability to adjust DC offset. The PSG here, I will just mention very quickly
why you see two screens, here is the PSG actually has two IQ modulators internal to it. One
is a narrow band that operates with the internal, arbitrary waveform generator and the other
one is a wideband IQ modulator that can accommodate signals up to 4 GHz between 2 and 4 GHz depending
on where in the spectrum you want to modulate these signals. We can adjust either path,
either IQ modulator path. Like I mentioned, most modern signal generators have this capability.
But a lot of people are not aware that you have the ability to optimize your signal in
this method. So here is the effect of using a pre-distortion. Here is the -55 LO feed
through again, and if we put a tone 180 degrees out of phase with the LO, we can get it down
to -73 dBc. So, let's move onto some other IQ modulator imperfections that you may encounter
and one of those is quadrature error, where the signals that are split from your LO or
not exactly 90 degrees phase difference between them. That gives you an imperfection that
hits the mixers and mathematically and deterministically you can figure out exactly what kind of image
or distortion tone that you are going to get at this particular frequency. I will not go
through the trigonometry here, it is pretty straightforward. As you have an error term
here in your phase, you develop an unwanted image out here due to your quadrature error.
Again, this is a PSG, but this applies to any modern signal generator. You do have that
adjustment to adjust the I and Q quadrature within your signal generator. Another imperfection
that you will see within the IQ block of your signal generator is gain imbalance, and timing
skew. This is where you have a delay between the I and the Q signals as they approach the
mixers, so you have a difference in time called the timing skew and you may also have a gain
imbalance. One may have more gain or loss within the path than the other. This is something
that signal generator manufacturers take very seriously when they design and lay out their
signal generator in order to make sure those paths, the I and the Q path are identical.
If they are not, you are resulting in what is called the IQ images and I think I have
a picture of that here. Here you can see a curve, kind of a bird or bat wing effect of
the images showing up here in this multitone signal. Here, we are using 100 MHz of bandwidth
to illustrate this. But it is hard to see, but you can see a lot of these images or tones
showing up there. Basically, yeah, similar to this. Now, we can adjust that gain balance
and timing skew again within our signal generator. Let me go back, sorry about that. Here, within
the user interface. We can reduce those images across the entire modulation bandwidth and
here, when we make that adjustment, we have gone from images up to 23 dBc all the way
to -40. All the way down to -40. Here is another representation of that where we have suppressed
the upper tones and you can see all these distortion products where we have misadjusted
the timing skew by 400 picoseconds, so it is actually quite a bit and you can see how
much in the effect of these images. We have gotten rid of the desired tones and here we
can just see the distortion products. If the skew is properly adjusted, you can see that
these distortion products disappear. So any questions about some of the adjustments that
we can make and the different anomalies you might encounter with the IQ modulator? Okay.
Let's talk about IQ modulation, different techniques for IQ modulation. You can see,
once again, we have an analog IQ modulator here and you can see some of the distortion
products and effects of the analog IQ modulator in the picture, or maybe you cannot. It is
a little washed out, but there is a lot of images in there from all the different imperfections
that I have listed before. Whether it is timing skew, gain imbalance, DC offset, they give
you these effects. When you go and you do the modulation in a digital method and classically
that could be done in the IQ waveform, so you do the little bit of extra math in the
software and instead of creating two waveforms with I and Q, you basically create an IF waveform,
basically, you know, frequency and magnitude that you can put directly into or load directly
into an arbitrary waveform generator and outcomes instead from a two channel arbitrary waveform
generator, you have IQ. You have one channel that has the IF signal coming out of it, so
it is basically doing the IQ modulation in software stored within your waveform file.
Simple as that, but that is not often convenient. It has some problems in up conversion. You
have a lot of insertion loss when you try to mix this up, but what you do have at the
IF output is a very clean spectrum without all these sins of the IQ modulator. These
have gone away and as I mentioned, this is done non-real-time is what I have just described
and that is a little inconvenient. What if we could do IQ modulation digitally or with
math and software in real-time? That is really where we want to get to. Here is an example,
of course, of no in band images, no feed through. Also, we can flatten. You still have the effect
of some of the magnitude frequency response here, but the other distortions of the IQ
modulator are disappeared. But let's get to his particular slide where we have described
this analog methodology here already We just talked briefly about doing this in software,
creating the waveform file that has already done the IQ modulation mathematically. What
if we could do this in real-time? There are a couple of benefits to that because you can
interpolate your sample rate here within the arbitrary waveform generator perhaps to save
you some waveform memory, which you cannot do with this methodology. We can do this of
course with the Agilent M8190 arbitrary waveform generator. We have the ability to do that
IQ modulation directly within FPGA, right here. And we can lower the sample rate of
the I and Q data and up convert basically or interpolate the sample rate up in order
to get the full IQ modulation bandwidth. We also have a DDS engine, which I invite you
to come over to the booth and talk to me about. It gives you a lot of flexibility in creating
different scenarios within your waveform on top of your IQ modulation file. Okay, I think
we will leave it at that because I am out of time. Are there any questions? Okay. Well,
we have this set up over in the Agilent booth and I would like to talk to you about it further.