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okay so I want to talk a little bit about probing we have several people
that work in in probing they have there's a whole group that's devoted to
probing because it doesn't do any good to have highfalutin performance here if
you can't get that highfalutin performance off the board and into the
oscilloscope one of the hardest things to do is to figure out how to probe that signal
without disturbing it too much and also maintaining its performance all the way
through a system that a human can hold a human can soldier a human needs to get to
from the oscilloscope to the to the the pc board and so there's a number of probe
technologies that kind of up that game in order to give you more bandwidth and
more signal fidelity getting to getting from the board to the scope
considerations for picking a probe one obviously the number one the number one
is a signal bandwidth another one would be the source impedance what is the
impedance I'm trying to probe if I'm trying to probe 25 ohms you know it it
it doesn't do me any good if I put a probe that's 10 ohms there ultimately if
that probe is 10 ohms then that's going to load a huge factor if I'm probing
something that's a megaohm node it doesn't really do me any good to put a
50 ohm input impedance probe on there because that's loading that megaohm
down to the point where you're not going to get any signal into the probe so what
you're trying to probe is one of the fundamental things to you know in
selecting what kind of probe you're going to use the signal size you know
some of our probes that are designed to measure you know multi gigahertz they
really can't accept very very large signals whereas a megaohm or 10 mega
ohm passive probe you can put quite a bit of voltage on those guys before you
destroy it but you're not going to get the bandwidth through the probe
single-ended and differential measurements you know a lot of the
signaling done at very high speeds is differential this guy goes up there's
two traces one goes up and one goes down and the information is carried between
those two so it doesn't it's it's harder to just probe one of them if you really
want to see what the what's going on on the other one so this particular probe
right here is basically can measure both of those
simultaneously and then send those you know convert them from differential to
single them and send it into the single at an input of the scope probe output
impedance is another one what are we driving are we driving the 50 ohm input
or we driving the 1 mega ohm input for driving the 50 ohm input we need a low
output impedance on the probe if we're driving the 1 mega ohm input we can have
a lot larger output impedance on that probe and then of course cost and then
you know ruggedness and all those things play into it so one of the the first
type of probe that yeah you might come across or that and the cheapest one it's
used for the for driving the one mega of one mega ohm input this is called high
impedance passive probes they're more rugged they're pretty cheap they handle
larger voltages and they have the highest input resistance to probe
the highest resistances on a board the cons obviously are lower bandwidth we
can't get more than about 500 megahertz through that guy and they have a lot
heavier capacitive loading which does contribute to the lower bandwidth to the
next set that we came up with was a low impedance resistor divider probe these
are good to maybe 2 gigahertz or so they have lower capacitive loading than a
high z probe does they're cheaper than an active probe but they do have lower
input resistance than this guy does you know this guy might be 10 mega ohms
input this one might be a kOhm or so, so that does limit the the the amount of
impedance that you can probe and the scope because it's a 50 ohm system the
scope needs to have a 50 ohm input if it doesn't then its output impedance well
it doesn't necessarily have to have a 50 ohm put but the the what goes on between
that cable and the and the 1 megohm at the input is that there's a big
discontinuity there and it affects the signal integrity so you'd really want to
drive into a 50 ohm input and then the next class which is where most of Mike
and Ned and the probes team focused a lot of their energy are our highest
performance differential active probes that we have you know
the highest one we have now goes up to about 30 gigahertz really low capacitive
lower loading it does have higher input impedance in the divider probe and lower
capacitance I believe and then it's good for both differential and single ended
measurements you have the ability you could maybe put one of these on ground
and then one on the signal to measure you know single ended signals or you
could put them across a input to measure the differential signals and we even
have one now that that can take and and and depending on how you process the
signals in the probe you can do single ended or differential or make a common
mode measurement you can you can do instead of this minus that for
differential signals you can do this Plus this inside the amp for common mode
signals and you can do that with an electronically switched chip here that
you don't need to change your probing configuration so if you're going to go
to the time to actually get these things soldered in and all that it's really
nice to be at a switch between the measurement classes
well that happen to resolder in them the the leads so what's inside these guys if
this is the debt that we're trying to measure there's some signal we're trying
to measure and it's got some output impedance basically whatever probe we
hook onto that is going to affect the signal we're trying to measure so if
this is really large we want this resistance here to look large in other
words so we don't get a lot of voltage division across there the problem with
it is that you know this type of topology does have a significant amount
of capacitance so you know this is the the high z input passive probe that we have
and you know it's got like I think it's a rated bandwidth of about 500 megahertz
or so that's when we're probing FF of about 25 ohms if this voltage or if this
impedance goes up that capacitance you know is in parallel with that ultimately
and it's going to bring down the bandwidth capability we can measure
based on how big that capacitor or how big this source resistance is so you
know don't think just because you grabbed a 500 megahertz probe you're
gonna be in a measure 500 megahertz on anything you measure it really
depends you know you got to kind of think about what you're trying to probe
and what the loading is of that the next one is that one I talked about so it's a
resistive divider probe this guy basically has a 450 or 950 ohm resistor
and then that works with the 50 ohms of the scope to give you a resistive
divider here if it's 450 it's a 10 to 1 divider to the scope if it's 950 it's a
2:1 divider why would you want to use one over the other
obviously you want more signal maybe but maybe what you're trying to measure has
a Thevenin resistance you don't want to load down as much so you might pick 950
ohms at the expense of the divider again it has lower capacitive loading but you
still have to kind of you know think about what you're trying to measure
before you hook this thing up and figure out what it's doing to your signal
differential active probes there's actually a an IC inside a probe body
this this particular circuit is I think I don't know if it's this exact one I
don't think it is but there's something like this in this little probe body here
so this thing connects to the scope and going down this thing there's DC lines
control lines and everything to actually buy us up the amplifier that's in there
and then ultimately then that guy has two inputs that these little different
accessories plug into to make you know to give you different types of
performance you know the longest accessories and depending on the
probe tip they might be better for more all-around probing at these
frequencies but they don't give you quite the bandwidth of some of the finer
accessories so there's a I mean we have numerous accessories to allow you to
probe various things and part of what Mike and his team do is they educate
customers on what accessory you actually need to probe and these are very
sophisticated customers that still kind of need to be educated because you know
we spend more time thinking about this than they do on what ultimately is the
best set of accessories to probe the signals that they want to measure
basically the way this thing works is you know it's a it's a
a tip resistor that might be large together with a capacitance here and
that forms a what we call a tip zero so at high frequencies you let more signal
in and then on the other side of this guy there's a load that has a resistor
in a capacitor in series with it the multiple the the the resistance times
the capacitance here is equal to the resistance times the capacitance there
and what that will give you is a very flat response versus frequency so it
allows us to do you know do probing you know up into the tens a couple tens of
gigahertz range and then various types of accessories and you have a picture of
some of the other things you know depending and this is a this is one that
looks kind of like this one but this one has a something you can actually hold
and browse around I think this one's not rated quite for the bandwidth of some of
these others but it does give you the convenience to be able to kind of probe
around and see very fast signals in something that you can actually hold in
your hand and then there are some things here these little these little breakaway
components that you can either solder down onto your board and then plug in
this probe head to to get more set or you know better performance and then we
have we have some other applications that if you don't want to probe
something you know with the with that input Network if you're probing like a
coax if there's like a year your test board has coaxial cables coming out or
whatever you can plug something in you can you can screw this guy on to that
and then ultimately you're in a coaxial environment so you can still have the
functionality of some of the probe functions but now you're not soldering
something down onto the board so that's pretty much all I had there's obviously
numerous places that you can go to learn more about all this stuff you know we
have keysight.com there's App notes and data sheets you could probably spend the
rest of your life reading those there's this thing called the oscilloscope blog
that we have and I'd forgotten to put the the YouTube channel on there now
Are there any questions?
ultimately you have to put enough protection on there so the thing is
somewhat robust but not so much that it causes a lot of capacitive loading so
you know ultimately if you're gonna the larger the more energy that you're going
to try to protect against that usually involves bigger structures bigger diodes
you know if they're ESD diodes across your front-end and then you're going to
take a capacitive penalty for that now we do do things where we try to
compensate out that some of that capacitance by judicious placement of
inductances for instance between them so like I've had a capacitance here and I
have two inductors that looks almost like you can configure the L of the
inductors you put there with the capacitance to kind of make it look like
the square root of L over C almost looks like 50 ohms it now it's a lumped
approach so it has a limited bandwidth but ultimately that's the kind of things
that we do so like on that on the eight gigahertz preamp I could go back if I
could find it see this little this little spirally thingy here that's
actually a little spiral inductor on the chip and that's used to compensate out
like I told you that the input capacitance of the amplifier because
there's capacitance there too and also the capacitance of the little ESD diodes
that are there now then a lot of times what we'll do is we'll do some off chip
compensation two more ESD protection there might be limiting circuits here
sometimes we use a thing called a spark gap which is basically a two traces
close close together that if the signal gets large enough it will break down the
dielectric in between them and a lot of that energy will go across that spark
gap and not get into the amp stuff like that I think we've used that improves
before I don't know if there's one on the front end of storm
Ryan do it we don't have spark gaps on that one do it now yeah there's a
limiter though for the 50 ohms yeah and that limiter one of his functions is
to protect that solid-state attenuator in the 50 ohm path so the limiter is
basically the first thing that is find the BNC I didn't have that on the
diagram but that basically protects the the attenuator first and then
secondarily the front end of the amp but I think I think if something's going to
get damaged that's probably the attenuator in this particular guy first
yeah yeah yeah so that wrist wraps important yeah and
then ultimately you know the you know if you're gonna pay hundreds or tens of
thousands of dollars for an instrument you do have to be aware that it
can be damaged if you're reckless in handling it.
One of the ways to determine it is if the signal is too big it's going to be clipped on the
screen so you know if you if you if you basically see it's clipped and you don't
want it you know then you'll change the volts per division but in that in that
auto scale function one of the things it does is it measures the ADC codes that
it's acquiring and if it does side I'm speculating here because I didn't design the
auto scale function obviously but if it sees that the ADC is clipped it will one
of the things that we'll do is reduce or you know it will go in and change the
the front end hardware to put in more more attenuation to get that get that
thing centered on screen better yeah
yeah "over voltage" will detect it, too. over voltage is meant more as part of our
protection strategy you know if you have very slow like maybe somebody puts their
probe down on a 10 volt signal or a 10 volt DC that one it's it's it's
something that's not moving because our over voltage has a limited bandwidth but
ultimately hopefully before it does much damage I mean heating up our termination
resistor or things like that it will it will send a flag there's actually over
voltage circuitry on here to detect that coming in to this guy and then that
sends a flag to the CPU system to basically go and throw that past select
relay so what it does is it throws the relay to the high impedance side so if
you ever if you're ever probe in something and you start moving voltages
up you know like to say you're probing to DC and you're a power
apply and you move the power supply voltages up at some point you'll hear a
real a click and on the screen it'll say over voltage limit reached check your
system or whatever you know and that's part of our protection strategy - yeah
I'm not an expert on calibration but I can talk
about a little let's go to a block diagram so one of the things that
calibration has to do well what calibration is used for is figuring out
you know we get out we get a chip for instance you know nominally if I click
in a gain of two I'd like to have a gain of two but because of process variation
you know variation in the attenuator it might not be a gain of two so that's one
of the simpler calibrations to think about in terms of how do we get the gain
of this to be what we think it is or what we want it to be given variations
in the process and that's where calibration comes in so one of the
things that we will do is we will send from a resource on the on the board or
whatever we will hook in a DC signal here that we for instance will ramp you
know so it's basically one of the ways that we do it is I mentioned that that
offset DAC the voltage DAC that's a DAC that of course that has to be calibrated
too but once it's calibrated and I'm not really sure about the chicken or the egg
thing you know fast somebody has a bit more of an expert in that in right in
those routines and everything but once that thing is calibrated we can
configure that to put out a ramp and if we know what we're putting out into the
front end and the ADC is used to ultimately measure that then we can go
in and fine-tune what the gain gain coefficients are in here and that's
another thing that that gain vernier is used for you know the continuously
variable gain you can set that to to have an offset or whatever and offset in
gain to scale against the errors in the stepped gain for instance but yeah
basically it's a it's kind of a you need to have observability and you need to
a way to stimulate to sit with with stimulate the circuit with something
that's known and once you have that then there's a whole bunch of code you know I
mean it takes several months a lot of times or you know to get a really
sophisticated oscilloscopes to get that first cal to ultimately work and there's all
kinds of calibrations. There's timing cals there's trigger cals there's interleave
cals for high-performance stuff you know so I don't claim to know how all
that stuff works but in general it's providing known things that you can
measure and then figure out what the error in your measurement are and then
coming up with coefficients to minimize that error
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