Should I go ahead and get started? All right. So who here is actually just looking for a
seat to rest on or is just interested in phase noise? You are interested in phase noise?
Excellent. My name is Riadh Said. This is a little short presentation that Ben Zarlingo
and I put together. So bear with me. I am a little sick, but if you have any questions,
we only have 15 minutes, but you can stop me and ask. If we do not have time to go through
the whole thing, it is probably okay as well. Okay? Basically, one of the things we want
to do is cover phase noise. It is always a complex measurement that may or may not affect
your tests, and a lot of engineers I speak to do not always understand where it matters
and where it does not, and hopefully today you will be able to have some tools to where
you can actually figure out on your own whether you need the best phase noise or you need
something a little less than that. We are going to do a quick overview of signal generation
choices and trade-offs. As an example, we are going to talk about when phase noise matters
and when it does not, and we will do a couple examples – one from a radar perspective
and one from an OFDM modulation perspective for communications based. And then we’ll
talk about some resources you have to improve phase noise and signal generation. So in not
every case you need the best phase noise. Sometimes you need it the very best. Maybe
you are working on a radar system and you need the absolute best at every situation,
and sometimes you do not. Maybe you need to balance the cost of the phase noise together
with the performance of the device, and you are trying to optimize the performance. Right?
And at other times it just does not even matter. It has no impact on your measurement, and
you are trying to figure out, okay, which one is the best. What are the trade-offs?
Is it the cost of the instrumentation or the LO or the VCO? Maybe you need switching speed
for production throughput testing; maybe that is a factor in the signal source, and there
is other things, such as broadband noise, which also has an impact on your choice of
signal generation or technique. So what are some terms? So a common metric for phase noise
is single sideband phase noise. Phase noise, what it is, it is a short-term stability phenomenon,
usually in seconds. Sometimes referred to as jitter. In this case we have the script
LMF curves, which are single sideband phase noise plots at a particular frequency, and
this phase noise is a function of both your signal generator, your synthesizer, your VCOs
and your system. And then there is what is called long-term stability so that could be
minutes or years. That is typically determined by your reference oscillator. So as a design
area, you need to understand both your short-term and your long-term needs and how it impacts
you and your device so you can make the best choices. For example, here we have our PSG
signal generator phase noise plot, and what we have here on these blocks are areas where
how the synthesizer actually impacts the performance of phase noise. So in this first block here,
you can see this is at 10 GHz. Between one Hz and one kHz offset a lot of that phase
performance is actually determined by your reference oscillator either inside the instrument
or external to the instrument. This flat portion here is usually what we call the pedestal
region, and that is primarily determined by your synthesizer. All your phased-lock loops
and your synthesizers determine that noise floor, and that is typically between one kHz
and 100 kHz offset. Then this last little portion here between 100K and a meg, that
is typically determined by your oscillator used in the system. So whether it is a YIG
or a VCO the roll-off here will be determined by that choice. And last but not least we
have the broadband noise so we are talking a few MHz offset all the way out to hundreds
of MHz offset. That is typically determined by the front end or the RF chain of your instrumentation.
Any questions on that? No. So narrowing the hardware choices, that was just a quick overview
of where the phase noise is coming from, but when you are choosing a signal generator,
there are things that affect each one of those categories. For example, we have a lot of
signal generators out there that have both analog only or there is digital modulation
as well. So when you go to digital modulation, typically you have IQ modulators, and the
IQ modulator is usually broadband so that has a tendency to degrade the broadband noise.
Also, depending on how your front end is optimized, maybe you are optimized for more linearity,
but in terms of CW operation, maybe you just want to operate at max power. In that case,
you are willing to trade off maybe harmonics for improved broadband noise, those sorts
of things. Not only that, there is different synthesizer architecture so there is single-loop
synthesizers versus triple-loop, double-loop synthesizers, and that will affect the pedestal
region of phase noise, and that also is a trade-off in cost, typically. And, of course,
like we described earlier, phase noise performance levels can be either done with a VCO, voltage
controlled oscillator, or a YIG, and again, that determines the roll-off in that pedestal
region at those 100 kHz to one MHz offsets. And then in this case, if you have a YIG it
usually affects switching speed. So there are trade-offs with speed in the signal generator.
Here is an example of our new X-Series signal generators. We have our EXG here, which has
a single loop architecture, and you can see with our new MXG we have what’s called a
triple phase locked-loop architecture, and fundamentally what happens here is our pedestal
region drops by ten to 25 dB, going with that new triple looped synthesizer architecture.
Another example is our PSG, and here we are showing a frequency versus phase noise relationship,
and so the general trend here is as you go in increasing frequency, the phase noise degrades
proportionately; however, it is not always a simple relationship.
This next plot kind of shows, with our MXG, for example, across the frequency range from
300 MHz to five GHz. It looks pretty symmetrical, but then you look at broadband noise. It does
vary with frequency a bit so it is not always a direct relationship. It can be complex,
and you might want to actually take a spot measurement of your frequency of interest
to see what the true phase noise is at that point. So in terms of where does phase noise
matter and where does not it matter, so here are some examples. OFDM systems have a tendency
to be sensitive with phase noise because they’re trying to be orthogonal with the adjacent
carriers. You have oscillator substitution. Usually you want a very clean oscillator so
you would know it is not contributing to your measurements in your system. Folks working
on ADCs, analog to digital converters, trying to get the best signal in the noise ratio,
so any jitter on their clocks or their analog signals that they’re using to characterize
that could affect the sensitivity characterization of that device. Of course, if you are working
on Doppler radar, phase noise has a direct impact on that, and we’ll give you an example.
Things where it does not matter, though, again going back to OFDM, OFDM is kind of tricky.
In some cases it matters and in some cases it does not. If you know what the trade-offs
are you can make the right decision. If you are working on very wideband signals, like
QPSK over several megaHz, typically not very sensitive to phase noise, if you are working
on just basic CW harmonic testing, amplifier gain testing, again, phase noise really does
not come into play in those types of measurements. So if you are paying a lot of money for phase
noise and you are just doing these types of tests, you are wasting your money. So one
of the things we have as a tool, if you do not have a good feel of how phase noise impacts
your design, we have a technique in some of our signal generators to actually inject phase
noise. So in this case what we do is we digitally add a 20 dB per decade slope both to the close
in phase noise and broadband phase noise. You define F1 and F2, and you can tell us
what phase noise level you want, and by just doing that you can then inject a specified
and controlled amount of phase noise and characterize how your device behaves under those conditions,
and that way you can optimize your design for not over designing the reference section.
So how much phase noise can you get away with? So there are some tools to do that. Here is
an example plot. Here we have the standard phase noise of their signal generator, and
we have two different levels of phase noise that have been injected with different offsets
and DVC ratios, and again, you can try these different levels on the fly and see which
one is most effective for you, and that way you can optimize your design instead of guessing
at what you think is needed. So how do you actually go about figuring out what exactly
your system needs? Here we have a radar example. It is a fairly straightforward equation. You
are trying to figure out the Doppler offset frequency because that is what you want to
detect in your radar system. Right? In this case it is two times the fundamental frequency
you are transmitting at over the speed of light times the radio velocity of the return
signal. In this case let’s talk about airport surveillance radar, maybe at 3 GHz. You are
seeing 100 mph airplane traveling through the air. Its Doppler return signal offset
is going to be 900 Hz so right there you know the phase noise at the 900 Hz offset is likely
going to matter to you as the designer. If you are working on military applications,
maybe you are looking at Mach 1.5 in the x-band. Maybe it is the 33 kHz offset in this case.
So now you are starting to define where it matters to you. The next step would be to
consider other effects so a lot of synthesized signal generators have Frac-N loops or phase
locked loop Frac-N loops that inject spurs of random nature, and so you are going to
want to choose a signal generator that is not going to impact your system. In this case,
if you have a radar return signal that is at FD-1 or FD-2, and you have spurs coming
from your LO adjacent to those return signals, that could confuse your radar system and ultimately
degrade the sensitivity of your whole system. So those are other considerations in phase
noise. So you’ve talked about your Doppler offset or your frequency offset. We’ve talked
about, maybe, spurs. Your next question is what’s the absolute level that you need
to get to? In this example it is from a book called Introduction to Radar by Skolnik. If
you want to understand where all this came from, that is a good reference. We have it
in the back of this presentation. And here it has the calculation for radar at one GHz
Doppler radar with a bandwidth of ten kiloHz, and we assume a target surface area of one
square meter and a minimum velocity. In this case, if you do all the math, what that translates
to is a minus 120 dBc per Hz at a 200 Hz offset. So now you have your system requirements if
you are trying to get to this performance level. Now from there you can start to choose
what type of signal generator or LO that you are going to need for that system to meet
these requirements. That was a simple radar example. Here we are going to change gears
and now talk about OFDM. In terms of OFDM there is something called pilot tracking channels.
These are usually a very repeated, high lit channels, maybe it is an EPSK with a pre-defined
repeating sequence, and this is used as the system reference on the receiver. And the
beauty about this pilot channel is that you can track out phase error, low-rate phase
error. So close-in phase noise, maybe a kiloHz or less, in this case without pilot tracking,
if you had some phase noise on that transmitter it would rotate the constellation. You would
have a poor EBM, but when you turn that pilot tracking on, now the reference are the pilot
channels, and so they can effectively remove some of that phase error, and now you get
good EBM performance. So a lot of the OFDM standards have that, but typically this is
only good out to about 10 % of the channel spacing. So in an OFDM system you have multiple
carriers. Your channel spacings may be 100 kHz, 300 kHz. Typically it is debatable. Everyone
has a different number, but it is roughly 10 % is a good rule of thumb to start from.
And so the next section of the slides, we are going to do an example calculation. Here
we are looking at an OFDM signal and we are making the following assumptions: we are going
to estimate phase noise effects as it contributes directly to EVM. So we are assuming spurious
and other non-linearities are not very significant. We are moving equalization, and then we are
assuming 10 % pilot tracking is removing some phase noise error. And the example we are
going to show initially is WiMAX, which has 10 kHz subcarrier spacing in a 10 MHz channel.
Here is the quick and dirty. In this case in this phase noise plot we are assuming anything
below 1 kHz, which is 10 % of 10 K is not part of the EVM metric, and in this case we
are showing a 1 MHz channel. This is not the WiMAX example, but in this case we are showing
a 1 MHz channel so anything outside of the 1 MHz channel bandwidth is filtered off. So
the only thing that matters is the phase noise in this region, and you can see right now
the phase noise rolls off at the 100 kHz offset. So we are just doing the integrated noise
over this channel because anything out here it looks to be pretty insignificant. If you
did the math here, we have the example calculation and it ends up being -42 dB. So that is one
example. We have another example here. This is with wireless LAN, and again, following
that same equation, wireless LAN, if you know anything about wireless LAN, it has about
a 312 kHz spacing between the OFDM carriers. So in this case we are going to start 10 % of
that, which is 30 kHz, so we are saying anything less than 30 kHz in the space noise plot is
not significant, but in this case what we are integrating in is over the full – in
this case I believe – 10 MHz span of the signal all the way out to here. Even though
the phase noise out here we think is insignificant, we are just going to go ahead and integrate
over that bandwidth anyway. We do that integration, and just looking at the phase noise plot,
we calculate an EBM to be about -26.35 is what we’d expect. And so what we did is
we went ahead and injected phase noise to this specified amount into our transmitter,
and we went ahead and actually measured it, and lo and behold, it measured to be about
-27 dBm. So the math does work. It is not perfect. There are some differences, and that
has to do with the tracking error of the equalizer, and its ability did not remove the phase noise.
So questions and answers. In this case is it good to generate phase noise that is representative?
Is it a good idea to inject phase noise to find out where your system is sensitive and
where it is not? As a recommendation to you as a designer, I would say yes; it is a good
idea. It is always a good idea to figure out where your system boundaries are and where
your margins of error are. So if you are in a production line, and you want to find out
is my yield going to be good enough? Do I have enough margin to have a 100 percent pass
rate? If you characterize it early enough in R&D, you willbe able to answer that question
more effectively. So we would recommend, yes, try both. Understand your ultimate performance;
find out what your device can actually do, and how tolerant is it to degraded phase noise
in your VCOs and your LOs. And should you turn on pilot tracking or not? Again, it is
kind of the same concept. You want to characterize in your design and understand where it is
going to degrade, and so in this case you do want to characterize both with tracking
on and off. How are we doing on time? Good? I am done. All right. So in a nutshell, basically
there are a lot of things you can do to optimize phase noise, whether you want to substitute
the reference section, use external dividers. There are different techniques, if you use
a divider, what happens is you will improve your phase noise, but you might degrade the
FM deviations, those sorts of things, but there are ways of improving your phase noise
if you are not happy with what you currently have. So that is it in a nutshell. I went
very fast. There is a bunch of references in this presentation if you want to look into
this more. Thank you very much.