hey everybody and welcome to another
exciting episode of EEs Talk Tech my
name is Mike Hoffman
I'm Daniel Bogdanoff and today we're
gonna look at pam-4 technologies and we
have two experts with us in house we
have alex alex communities yourself I'm
Alex Bailes some product manager here
at the oscilloscopes division and
working with Pam4 as well so mainly
focusing on Ethernet awesome and then we
have Steve who is out in our Santa Clara
site Steve can you tell us a little bit
about yourself yeah I am Steve Reinhold
I'm a business development manager with
our internet infrastructure group and I
concentrate mainly on receiver test for
M for awesome so Steve is more of a
receiver guy and Alex you're more of a
transmitter guy so basically we're gonna
let Alex talk to Steve and that will be
the whole podcast and C we'll listen you
don't know Steve very well okay you guys
should have switched specialties anyway
so what is PAM4 what's what's going
on with that for wait for someone you're
at a party and you're like, yeah I do PAM4 or
they're like oh cooking spray cool yeah
that's what most people think of
initially is is cooking spray but no
it's actually pulse amplitude modulation
and it's it's really become very popular
right now because we're moving away from
nrz which is non-return-to-zero which is
really just PAM4 if you think of or
PAM to if you think about it it's just
one zero so there's two levels to energy
basically the way I understand it is we
need more data so the way to get more
data is you either send bits faster or
you come up with a new scheme for
exactly and we're kind of reaching the
limit if you will of nrz at that's at
higher speeds interesting because I
everyone knows pulse width modulation
but this is kind of just modulating the
amplitude to multiple levels as opposed
to the wealth yeah that's right and
people think that that pam has been
around and I get this is a new
but PAM has been around a long time it's
actually been you know  1000BASE-T
which is at your desktop you know it's
been around with  10GBASE-T 10 gig
base T okay which has PAM-16 believe it
or not if you can imagine you know that
many eyes
so basically 16 different levels of yep
since like an analog to digital
converter her vice-versa creating
different output levels you can almost
think of it that way because it's it's
very when you look at it it you're just
it kind of blows your mind a little bit
because it's it's ridiculous amount of
sounds terrifying what's the voltage
swing from like the lowest to the
highest oh well for for PAM4 it kind
of depends on the specification but I
think it's typically between six
hundred millivolts to eight hundred
millivolts she's around that area yes
and they're talking very tight eyes okay
sickly and then Steve on the receiver
side is it like how does how do you set
that up because for for just a one or a
zero you set up a comparator and you're
good to go with multiple levels how do
you overall have what as a receiver look
like well depends on the particular
receiver you're talking about but you
can put multiple slicers and then
interpret the output of those slicers
later on do you suppose processing or
you can like you said put put an A to D
on there and interpret it all at once so
you know was it a zero level one level
two level three level you know the
fundamental contribution of PAM-4 is for
these digital baseband signals nrz was
one bit per clock cycle and with PAM-4
you can communicate two bits per clock
cycles so it's more efficient modulation
and coming coming from an RF background
you know the radio guys had been you
know trained for more efficient
modulation for a long time
but you know the computer communications
standards that we're talking about just
went over copper so they didn't have
like you know spectral allocations that
they had been live with but now with
bandwidth limitations you know with the
communications going so fast you know
you're the the channel bandwidth is what
what limits the ability to transmit data
through the channel and so with PAM-4
you get an effective doubling of kind of
the digital bandwidth through a channel
now it seems too good to be true what's
the downside of shoving more data
through the same clock cycle well
naturally with with four levels in the
same voltage swing that you had two
levels in you take a signal to noise
ratio hit and the ideal signal to noise
ratio hit is 9.6 DB that's just you know
simply factoring the factoring in the
fact that you have you know four levels
versus two levels but in reality that's
even worse than that because not it's
the eyes might not be equal height there
could be some compression effects like a
laser turn-on effect or you know
amplifier compression effect that would
make one of the eyes you know smaller
than the other eyes and in that case the
ultimate signal to noise ratio hit would
be even worse than that 9.60 be so noise
becomes a really critical factor in
PAM-4 communication systems has there
been any sort of analysis done if you
have the exact same channel with similar
electrical characteristics what is the
bit error ratio of an nrz link versus a
PAM-4 link is it twice as bad or 10%
worse or well yes so nrz you know it's a
lot it's kind of a simpler concept you
know you put one slicer in the middle
and it's a one or a zero
but with the additional complexity and
the fact that the channels have a lot of
of loss it's not like you know you're
you have
essentially clean channel and you're
going to just double the bandwidth you
know the channels were pretty impaired
even from an nrz standpoint so when you
add the additional kind of noise hit of
a PAM-4 signalling architecture then
there is there is a hit on BER so a lot
of the PAM4 standards actually are
designed to operate at high BER s where
in nrz days you know you'd operate 1 to the
minus 12th or one a to the minus
fifteenth BER, with PAM4 some of the
some of the standards are targeting one
e to the minus fourth or one a to the
minus fifth and now they clean it up
later on with forward error correction
or FEC now when you're talking about to
the negative 12th versus the negative
5th that's a 10 to the seventh increase
in error rate so a million times more
errors just as my math correct on that
it's it's significant wow that's that's
huge yeah I can see how error correction
algorithms would be huge nowadays
insurance you know if they think about
even in a awful medium like a wireless
transmission to a cellphone or something
I'm sure it's even worse right yeah yeah
the companies you know they're they're
having to invest a lot of money in in
designing more complex receivers and
more complex you know post-processing
kind of compute power to get these PAM-4
signaling systems to work but it's worth
it because they don't have to you know
jack up their existing infrastructure
and replace the channels the channels
are like the computer back planes you
know or you know some of the other kind
of network infrastructure that have
these bandwidth limited channels so if
you can cram twice the throughput
through those expensive kind of fixed
assets then it's definitely worth the
extra effort that you have to go through
to to get a more complex signaling
scheme to work yeah I mean this is all
really driven by Ethernet you know the
idea eventually is to get to one
terabyte
information we're not there yet but
terabytes per for what
/ yeah get you it's the way you think of
it Giga bits per second yeah terabit per
second so we're really at the point now
where we can get you know there's
specifications out there now that are
being developed that are working on 400
for example Giga bits per second and if
you start multiplying lanes you can
actually push that out you know 800 and
so forth so I believe they will get
there but it's it's definitely a stretch
goal that's just internet data demand or
what's really it's just the huge amount
of data demand you know when you go to
Google and you start you know searching
on you know funny cats or something like
that or whatever that's that demand
that's that's getting pulled in there
and it's you know it's mainly in the
data centers they're seeing that kind of
traffic a server side right so people
who are watching or listening to this we
can blame them for exactly we love them
it drives it drives a lot of business
for us you know having the the demand
for bandwidth go up so we we appreciate
that I hope you guys don't compress this
video when you post it it reminds me of
when I first started with it was HP at
the time and the the guys that worked at
disk drive companies would email me huge
like one megabit pictures how can you
clog my email system with a picture that
big and they said simple we want your
disk to get full well it's the same idea
here that's a really clever idea like oh
that can't get all our pictures from our
Mart I mean eat spam because I wonder
why I file probably shouldn't say that
but wonder why certain certain fits
phone brands don't allow expandable
memory but also for clouds we know you
know that that's an Apple thing that
they've been doing forever is like I
don't want to go into it
right cool stuff all right so what
witness what is kind of ...
There's PAM2 PAM4, is there a PAM6 or PAM8 or is there some sort of
diminishing gains to boosting those
levels higher and higher well there's
there's PAM3 actually for
automotive Ethernet believe it or not so
when you PAM3 yeah so when you're
when you're running around in your car
you actually have automotive Ethernet
that's that's doing PAM3 you have
like I mentioned PAM16 PAM5
4050 M three sounds like something that
not a digital guy came up with well
actually it was leveraged by Broadcom
Broadcom actually helped develop that
technology there's some limitations that
ran with that which is they wanted to
have one pair go through the car one
pair of cable that's it so there's a
huge amount of cost savings by doing
just a single pair where as thermal you
know Ethernet technology has four pairs
typically you know you're okay huh
bastey has four pairs going to your
desktop for example you have multiple
lanes when you're dealing with you know
200 and 400 gig you know Ethernet but
when you get into a car every every
little pair adds that much more weight
and also they want to try to keep the
cost down so every cable that's ran
through their it saves them and that's
the major by the way that's that was the
major driver no pun intended
yeah ain't no bit intended to actually
reduce it down to one pair so that's
that's why I came back
now it's PAM3 it just has three
levels so it can only represent three
symbols per is that was I just give it
better signal-to-noise ratio I know cars
are generally a very noisy electrical
environment yes they were there's quite
a few challenges around signal to noise
as you can imagine engine noise and air
conditioner noise and when you plug in
to infotainment system
then you've got rock-and-roll noise that
that's going across the so there's a
there's a lot of implications around
automotive Ethernet yeah I never really
thought of Ethernet as an automotive
thing yeah can buses and then just like
we had Rick on he talked about some PCIe
cabling fryer for like backup cameras
which they're now mandatory I understand
yeah well yeah can scans running out of
it not sidebar I guess before we bore
everybody with car stuff but yeah I
think Ethernet is basically replacing
flex raise at the idea since no one was
going that direction well there was
there's a battle if you will between
different technologies in the car and
and certainly you know can flex ray
there's certain things you can do with
Ethernet like you wouldn't want your abs
braking system be to be dependent on on
Ethernet you know you want something
very quick response so but for
infotainment you know if it takes you an
extra split second to switch to DVD or
or play you know an mp3 whatever to us
as podcasts on the road folks yeah so
other higher levels of PAM though higher
than for I haven't heard anything higher
than PAM16 how about you Steve? Well in
the in the you know radio or the
coherent optical communication space
they do have much more complex
modulation schemes but they're not
limited by the the same noise
environment that some of the you know
the computer channels are that we're
talking about here at least that I'm
familiar with so well when I described
the the noise head of PAM for for a lot
of these computer communication
standards there's not gonna be a PAM8 there's no there's no noise margin
and the FBC algorithms you know aren't
going to be able to correct for more
levels than just four so PAM4 is you
know the the main new coding scheme
when people need more bandwidth through
a digital communications channel and you
know digital meaning this is the
baseband digital data being transmitted
not it's not on a carrier like in some
of the other systems or you know some
you know different concerns apply but
the noise hit trying to put a signal
through a channel with loss you know one
of the things about baseband digital
data is basically it occupies from from
DC if you think about it you know the
the long string of ones or the long
string of zeros has a very low frequency
content you know up up to the you know
the one zero one zero which basically
has kind of an F baud over the
fundamental frequency that's a lot of
frequency content and and you know going
through one of these lossy channels
leads to what's called inter symbol
interference which is you know one of
the things that you have to correct for
both with you know transmit equalization
and and receive equalization to try and
overcome the channel loss
characteristics and and be able to get
any kind of a signal through and so it
one of the additional complicating
factors of PAM-4 design is the the
equalization schemes that they need to
run and then you know another one that
Alex is an expert in is clock recovery
so most of these in fact I don't know of
a BAM poor standard that runs with a
synchronous clock some a lower speed
computer and RZ standards have a
synchronous clock run right alongside
the signal but in all of that PAM-4
schemes that I'm aware of you need to
recover a clock and having such a
diverse array of signal transitions
makes the job of recovering a clock you
know to to to sample that data that much
harder yeah that makes sense now because
of course the idea of any PAM-4 scheme
is to put more data through the same
line so adding a clock would be the
opposite of what you want to do right
but to bring that back a second a little
bit you mentioned inter symbol
interference to be honest with you it's
something I don't know anything about
but now that I'm
thinking about how you described it I
want to roll it back because it's kind
of interesting so if you have say four
levels on a on a PAM4 link and
there's la cena sana cable and by the
time it reaches the receiver it's say
drooped 25% of its voltage a four could
be seen as a three is that basically
what ISI is yep yeah if you roll it all
the way back to nrz it's a little bit
easier to think about on an NRZ signal
so think of the low-speed bits you know
they're coming through your channel and
the zero level let's say you have 10
zeros in a row and it's nice and low and
then it transitions up and then you have
ten ones in a row and it's nice and high
unless swing of that is you know a volt
well then then you put the highest
frequency signal through a lossy channel
and let's say that the loss is you know
just a couple DB at that at that
low-frequency level then you put the
highest frequency series of ones and
zeros through and there's you know
there's like 20 dB loss it's it's one
tenth the amplitude of the you know of
the the low-frequency part of the signal
when you overlay all of those on an eye
diagram you see that the eye tends to
close down very dramatically and and and
you can see it you know it's it's a
little bit easier to understand on an
NRZ signal but the exact same phenomenon
occurs on PAM-4 when you do to overcome
that you do what's called D emphasis or
pre emphasis on the transmit side and
equalization on the receive side and
what that is I'll date myself here it's
kind of like Dolby what you do is you
boost the high frequencies at the
expense of the low frequencies because
the channel has the opposite
characteristic it has loss at higher
frequencies versus less at low
frequencies so if you boost the high
frequencies they will get through
they'll have a better chance of getting
through that channel
so if you have a pre distorted signal at
the near end of the channel you'll have
a better chance of you know receiving it
at the far end but with today's
standards that's not enough you also
have to do similar techniques at the far
end of the channel with receive
equalization so basically you're trying
to pick the signal out of this you know
very impaired I you know what and
reconstruct whether what the original
intent of the transmitter was at the
receive end through these two techniques
transmitted to you know pre-emphasis or
D emphasis and equalization at the
receive end so this may seem like a
silly question how do you boost only the
high frequencies that's probably a
longer discussion but yeah there are
there are circuits there are circuits
that you can design that will react
based on you know what the the history
of the you know the prior history of the
bit stream was and so if it's a non
transition bit it'll it'll come out at a
certain amplitude and then a transition
bit will come out at a different amp low
so if you have a transition bit that
that implies some high frequency
component you'll send that out at a
higher amplitude so that's a that's a
very quick and dirty explanation of how
you pre distort the signal but but it's
a it's a longer discussion and we have
time for today but transmittable is like
just boosting yeah it's not just like
boosting the treble and my equalizer at
home laughter that's why I equated it to
Dolby so if anybody is as old as I am
you can
remember when you had cassette tapes and
if your if your deck had Dolby they
would they would boost the the high
frequencies before they laid it down on
the tape so that you'd have a chance of
recovering you know the high frequencies
when you played it back so that so you
would boost them lay it down in the tape
and then you'd go ahead and attenuate
him when you know when you played it
back so because there was you know a lot
more loss and the signal-to-noise ratio
at the high frequency end of the sound
spectrum was worse so it's a similar
content of that yeah pre just pre
distorting the signal to have a chance
of getting it through a channel with
nonlinear frequency response okay yeah
typically you know Steve mentioned clock
recovery is a big challenge if you can
imagine you know the three eyes and all
of a sudden they're collapsed so much
that you can't even distinguish an eye
well I mean on a scope you still have to
be able to recover that clock to make
the eye at all right so to make the eye
at all or to even know what to begin to
start to understand what the clock seed
or the the clock signal and the data
look like so we have special techniques
in scopes that allow you to do this and
a lot of them are proprietary but but I
guess the idea is that it's very
powerful that you could do this the
second thing is is you can actually
within different tools you can actually
have an impulse response just as Steve
was saying you can look at this impulse
response just the impulse and you can
detect basically whether you need to
de-emphasize okay or you know add some
characteristics to the signal to
compensate for this really ugly looking
so getting the transfer function of the
link basically is that what you're
trying to do
yeah there's basically you know for
Ethernet there's typically three tap
settings for Equalization and what
customers can do that have access to
their chips can actually change those
equalization settings and they know you
know if I if I change this coefficient
I'm gonna get this result if I basically
as their prototyping their boards and
testing their boards they can send a
pulse through see how it looks go back
tweak some settings again so how much of
it is like math you know pre design
based and how much of it is kind of a
guess and check sort of well you hold
don't name companies here but yeah you
hope you design you put in enough
coefficient settings to where you can
compensate for any sort of channel
because these are Asics that they're
using or what is could be an ASIC could
be you know it could be it's a number
different
yeah it could even be an FPGA everybody
so yeah interesting so I think I kind of
in your retrospectively answered my own
question I was asking about higher
levels of PAM because I've remember
already when I was doing some light
research I was seeing qualm and they had
qualms 16 which is basically two  PAM
four-by-fours creating like a little
mental of square basically and then I
saw something about like cram 4,000 or
16,000 or something like that is that
outside of the digital arena or does
clam become is that relevant at all in
the in the digital space or optical
space as well that's probably a better
question for Steve but I know in the
optical space I know in the optical
space they do use more complex
modulations for coherent transmission
but that's not something that I'm an
expert at I I suspect that there will be
some kind of clam in the digital
communication space here in the in the
future wait you know where I I don't
think it's gonna be simply you know PAM8
just knowing the noise
characteristics of these signals I think
the you know PAM4 is probably the
end of the line for the digital signal
time like this but there may be some
different modulation schemes that
compensate differently for the channel
loss
you know and the other impairments
coming in the future I don't know
exactly what direction PAM-4 is yeah
kind of enough to keep my plate full
right now but I know that the company is
looking you know the Fab Four is mostly
applicable to the 200 gigabit foreign or
Gigabit Ethernet space right now but you
know companies are looking towards you
know 800 or terabit kinds of
communications and and something's gonna
have to happen I just don't know what it
is okay well what's interesting is we're
seeing a lot of other technologies adapt
um PAM for now so you've got InfiniBand
possibly Thunderbolt I don't know we'll
have to see PCI Express may do something
with with PAM for yeah cuz we've been we
talked about PCIe on a previous podcast
and they were talking about like 10
millivolt eyes and yeah yeah you can't
get any smaller than that right now yeah
current no silicon I guess right so
it'll be PAM for I think we'll hang on
for a while and be adopted to different
you know serial transmission
technologies digital transmission
technologies okay for a while here it'll
be interesting interesting right yeah
we're actually out of time so told you
to go quick those does so at the end of
every podcast we like to we started
asking what we call a stupid question to
our guests so I stupid question for you
is what is your favorite flavor of
cooking spray wow that is interesting of
the PAM discussion how many are there
I mean is there at least four based on
discussion - - bacon flavored banned for
flavored PAM alright the sweet working
well but I see the bacon we're taking
okay sounds good that's perfect Mike the
other parrot cooking spray what's my
favorite yeah
mmm well these I already stole bacon so
I'll go for chocolate I'm all about
brownies plain okay anyway we're at in
time there's been another episode of EE
stock tech thank you for watching and or
listening you can always check out our
other podcasts on the keysight
oscilloscopes youtube channel or @ ee
stock tech comm that's EES taco tech
comm thank you so much Alex and Steve
for being here we've been great having
you I'm sure we'll hopefully see your
faces again at some point in a future
podcast if there's anything you want us
to cover topically let us know put them
in the comments either on the EE stock
tech page or in the YouTube channel make
sure you give us a rating on your
podcast engine so if you're doing iTunes
itunes stitcher google play whatever it
is give us a rating that really helps
out the show especially if you like it
if you're not going to give us a 5 star
rating and go rate some other podcasters
so that's all for today thanks for
tuning in we'll see you next time
you
you
