[HN Gopher] Signal reflections in electronic circuits
___________________________________________________________________
Signal reflections in electronic circuits
Author : zdw
Score : 157 points
Date : 2023-11-26 03:18 UTC (1 days ago)
(HTM) web link (lcamtuf.substack.com)
(TXT) w3m dump (lcamtuf.substack.com)
| mordae wrote:
| > When PCB reflections start getting in the way of digital
| signaling, the usual culprit is a low-impedance source driving a
| comparatively high-impedance load (e.g., a MOSFET gate).
|
| Depends on the MOSFET. Most discrete MOSFETs have pretty hefty
| capacitance at their gate, looking like a short to high-frequency
| inputs.
|
| > The simplest remedy may be adding a "sink" resistor on the
| receiving end, connected to the signal's return path. This is
| usually paired with a series resistor on the driving side, both
| to limit peak current and to at least roughly match the specific
| impedance of the trace.
|
| The simplest remedy would be to use series resistor matched to
| the trace impedance to prevent the reflection bouncing multiple
| times, probe at the gate and only add parallel termination
| resistor if it is actually needed. Because then you need to work
| around the voltage drop.
| dfox wrote:
| Even the significantly smaller capacitance of some kind of CMOS
| input will appear as short for some amount of time. In
| combination with the series termination this will low-pass
| filter the signal, which is (maybe somewhat counterintuitively)
| often desirable for high-speed logic (it reduces EMC issues and
| the effect of excessive power consumption due to reduced slew
| rate is mostly limited to the first input buffer on chip, which
| cleans the edges for subsequent circuitry. Within reason,
| obviously).
|
| When driving some kind of power MOSFET you want it to switch as
| fast as possible because of the power loss in the linear
| region.
| robviren wrote:
| Bless you hardware people in your dealings with high voltage
| wizardry. I have worked with hardware engineers in both high
| wattage POE++ and high voltage pace maker applications and feel
| like dealing with these effects are the nearest thing I have ever
| seen to a dark art that no one truly understands. They appear to
| have the most confidence in a board design one spin, and despair
| filled agony the next when some resonance builds and blows away
| our EMF testing. I'm not sure I've ever witnessed such
| engineering frustration. At least to some degree software issues
| are based on something you can grasp and see. I feel like these
| types of hardware issues are like staring into a crystal ball. My
| opinion as an outsider of course.
| jacquesm wrote:
| Computer simulation has been an absolute game changer for these
| things.
| robviren wrote:
| Does that handle the enclosure the board goes into at all? I
| haven't been in the game for awhile and am curious. We always
| had different behavior with our enclosure and of course when
| the sometimes poorly designed enclosure caused strain on the
| board that added a whole new layer.
| jacquesm wrote:
| That's a very interesting question. There are a ton of
| specialized tools for EMC/EMI simulation, I think they all
| work on the basic principle of assuming that a portion of
| your board will be exposed and that that part will be
| causing some radiation and will receive some from outside.
| After that it's decisions that will affect cost that will
| likely drive how far you want to strive for perfection, you
| could encase everything in grounded copper and it would be
| as close to perfection as you can manage or you can stick
| your board in plastic and live with the consequences.
| Usually, depending on how critical the application is some
| interference is acceptable as long as the device continues
| to operate. But on radiated power there are some very
| strict limits and for say a motor controller any
| malfunction could cause serious trouble so you will want to
| be extra careful there. The hardest to control for me:
| wiring, connectors, density of carbon sprayed inside
| plastic, anything using inductors (including what
| orientation to put them in, you want them at right angles
| to the surface of the board for mechanical stability and
| lack of coupling between inductors on the same board, but
| you want them parallel to the board to minimize radiated
| power and susceptibility to EMI).
|
| Apart from shielding and carefully modeling your board
| (especially the ground plane, supply and any connections
| that carry a significant fraction of inbound power) you
| will always end up testing for compliance and that's the
| gold standard. I think simulation is very useful and can
| cut down on the number of physical test runs significantly
| but I've yet to see a design that did _exactly_ what was
| predicted. Wiring, what happens just outside of the board
| enclosure, environmental factors, it all adds up. I see
| simulation as a way to be more efficient, not as a silver
| bullet to be able to guarantee certification is a one-shot,
| but possibly others have better experience.
|
| There are a couple of very simple tricks to test for EMI
| sensitivity (a handful of coins, an old fashioned piezo
| based stove lighter and the oldest cell phone you can get
| in close proximity to the board), as well as a simple field
| strength meter. Between those you can probably identify and
| eliminate the worst and after that it's trial run time at
| the certification authorities if you are making a device
| that is to be used in a regulated market.
|
| Especially the piezo lighter is interesting, I've had
| circuits that required substantial redesign just to get
| them to the point that they would not lock up hard.
|
| What's interesting about this stuff is how un-intuitive
| some of it is. Note that this field is continuously in
| development and that new tools and techniques are brought
| to market all the time. Look the other way for a few years
| and you feel like a dinosaur.
| krasin wrote:
| > Does that handle the enclosure the board goes into at
| all?
|
| It might. For example, Lukas Henkel is designing an open
| source laptop and heavily relying on simulation for that.
| In the post ([1]), he provides visualization of antenna
| performance for different placements inside the laptop
| case.
|
| I quote, in case, if LinkedIn wants people to log in to
| view the post:
|
| >I want to optimize the antenna positioning in my laptop
| design using open-source tools.
|
| >The shown simulation is a 3D electromagnetic field
| simulation performed with the open-source tool Elmer FEM.
| The aluminum laptop case will have a large impact on the
| antenna gain and directionality. For correctly iterating on
| a good antenna positioning, it is necessary to integrate
| the complex 3D geometry of the laptop case into the
| simulation.
|
| >I'm currently exploring three possible locations for the
| antenna. There may also be one option to make the laptop
| case itself as a part of a cavity antenna.
|
| >The tools used for the shown simulation are all free and
| open source:
|
| >Mesh generation: Salome_Meca
|
| >Solver: Elmer FEM
|
| >Visualization: Paraview
|
| 1. https://www.linkedin.com/posts/lukas-henkel-
| ovt_opensource-d...
| cushychicken wrote:
| It seems a little less magical when you do it for a long time,
| but yes, it is kind of magic.
|
| I wish, as a practicing EE, that it was easier to transmit the
| knowledge in a faster way than doing it for a long time.
| Unfortunately, nobody seems to have figured out how to teach
| it.
|
| It bums me out a little bit. I really love what I do, and I
| think it's magic too. I wish I could share that with more
| people.
| elcritch wrote:
| Well cheaper / better simulators would be better. There's
| software that can simulate these EMF but they're very
| expensive.
| cushychicken wrote:
| Anyone who knows enough to write an EMF simulator probably
| knows there's only a handful of people in the world who
| need it.
|
| Sadly, but justifiably, this means that they go get paying
| jobs.
| lolc wrote:
| When DDR SDRAM came out a magazine illustrated their article with
| light pulses traveling between the bricks and the core. From my
| electronics classes it was clear to me that the line would be
| either up or down, and these pulses were just for illustration.
| What would an illustrator know about electronics, right?
|
| The article then calculated how many bits must be in-flight on
| the board to achieve the higher data rate given the bus width and
| the distance :-)
| mordae wrote:
| I did that with the electronics hobby group kids couple weeks
| ago. Hahaha.
| readingnews wrote:
| It is just a pet peeve of mine (disclosure, I am a terminally
| degreed engineer), but it is something a lot of non-EE people
| forget: circuits do not "push" current, your wall outlet does not
| "push amps" and this applies to micro-electronics (as well as
| pico, nano, fempto, etc)... the circuit is always a combination
| of the source and load.
|
| So, if the load is not equal to the source (or vice versa, does
| not matter), by conservation of energy, we have some
| "leftover"... in signals, this leftover is going to travel back
| from whence it came, causing (possibly) attenuation or
| amplification depending on timing, which some might call
| distortion.
|
| It can be tricky to keep this in mind when you are working with
| complex circuits, and it gets more difficult as speeds increase
| and signal sizes go down.
| jacquesm wrote:
| A good real world example is fluid flow such as a gas
| compressed through a pipe, say a trumpet or an exhaust. If the
| standing wave in the pipe isn't optimal at the exit of the pipe
| a part of the energy will end up reflected which causes a very
| real obstruction and higher pressure. In a trumpet this
| manifests as a resonant filter that will leave all but a couple
| of frequencies without significant damping and _that_ is the
| note (+ overtones) that you hear. In an exhaust such tuning
| will result in the system performing optimally at certain RPMs,
| and if you match that to when you want the engine to produce
| maximum power you can create a peak by get rid of the exhaust
| gases at that peak. Hence resonant exhaust systems tuned to the
| theoretical peak of an engine.
| alfiedotwtf wrote:
| That's a cool analogy!
|
| I wonder if the exhaust could by dynamic i.e change length
| depending on the RPMs
| teeray wrote:
| Many antennas do this (screwdriver antennas come to mind),
| so I'd expect some similar ability to do this with exhaust.
| The only issue I can think of is turbulence from the seams
| of segments that extend.
| jacquesm wrote:
| The answer is: yes. The results: 6% gain or thereabouts
| with a fairly impressive amount of extra complexity. But it
| can be done.
|
| https://www.researchgate.net/publication/322390752_Continuo
| u...
| function_seven wrote:
| Way back when Jet Skis had 2-stroke engines, you could buy
| an aftermarket exhaust pipe with water injection in a
| couple key spots. This allowed the pipe to be dynamically
| tuned to different RPMs by changing the speed of sound
| within the pipe.
|
| The stock pipe was tuned to a specific RPM, allowing the
| reflected exhaust pulse to return to the cylinder just as
| the port was closing and "slam the door" on the fresh
| air/fuel charge.
|
| With variable water injection, you could have more than one
| optimal RPM.
|
| (All this is from memory as I read the ads in Splash
| magazine.)
| jacquesm wrote:
| Oh that's so clever, to use the water that is there
| anyway as a means of affecting the dynamics of the pipe.
| Whoever came up with that was on another level.
| jiveturkey wrote:
| > real world example
|
| electric/electronic circuits operate in the real world.
| mechanical example is what you're after.
| pests wrote:
| Here is a YouTuber who is making a physically based audio
| engine using fluid dynamics (demos so far for various
| engines, a trumpet, a steam whistle, dev videos)
|
| https://www.youtube.com/channel/UCV0t1y4h_6-2SqEpXBXgwFQ
|
| A lot of what you wrote reminded me of what he is working on
| - the trumpet sim is not entirely accurate yet but watching
| those reflections bounce and forth like you describe is very
| interesting.
|
| Trumpet Video:
| https://www.youtube.com/watch?v=rGNUHigqUBM&t=87s
| bee_rider wrote:
| I didn't get a terminal EE degree (it is more fun to tell the
| magic smoke what to do, than try and keep it in the chip), but
| you can of course design a circuit that will adjust voltage to
| keep current constant (within some bounds, etc etc). Could that
| not reasonably be said to be "pushing current"?
| dragontamer wrote:
| If there's no inductor (and inductors are dangerous: see my
| other comment), then nothing on that PCB is pushing any
| current. Its all illusions created by "pulling" current.
|
| > but you can of course design a circuit that will adjust
| voltage to keep current constant
|
| So there's two designs and they're different in important
| ways. But first: the common part of _both_ designs is that
| the transistor is working as a "controlled resistor". The
| question is where you place this special resistor. The other
| commonality is that "negative-feedback" can configure this
| transistor to reach the appropriate resistance very easily.
|
| So with the common stuff out of the way: we have two designs.
| "Series Regulator" and "Shunt Regulators" (traditionally
| voltage-regulators, but they could be current in practice.
| I'll discuss as if they're current regulators).
|
| 1. Series Regulator -- The transistor is treated as an
| adjustable resistor "in series" with the rest of the circuit.
| This "pinches down" the voltage/current to the level deemed
| acceptable to the engineer. Ex: If "downstream", you sense a
| 100-Ohm load and you have a target-current of 10mA, and your
| source voltage is 5V, you set the transistor so that its
| equivalent to 400-Ohms (total a 500-ohm system, so 10mA goes
| through).
|
| But if the downstream circuit changes (a button was pressed
| and a motor is now being driven), and the downstream circuit
| now looks like a 10-Ohm load, to keep the constant 10mA
| current your Series-Regulator will automatically set the
| transistor to act like a 490-Ohm resistor (keeping the
| 500-ohm system, so 10mA remains constant).
|
| 2. Shunt Regulator -- The transistor is treated as an
| adjustable resistor "in parallel" with the rest of the
| circuit. This "diverts" excess energy to ground, causing the
| rest of the circuit to effectively function within its
| specifications. Ex: If "downstream", you sense a 100-Ohm load
| and you have a target-current of 10mA and your source current
| is 50mA, you set the transistor so that it is equivalent to
| 25-Ohms. This shunts 40mA to ground, and the remaining 10mA
| goes to the 100-Ohm load.
|
| But if the downstream circuit changes (a button was pressed
| and a motor is now driven), and the downstream circuit now
| looks like a 10-Ohm load... to keep the constant 10mA current
| your Shunt-regulator will automatically set the transistor to
| act like a 2.5-Ohm load. This shunts 40mA to ground and the
| remaining 10mA goes to the 100-Ohm load.
|
| -----------
|
| Traditionally, series and shunt regulators sense voltage (not
| current), but its not very difficult to turn a voltage-
| regulator into a current-regulator instead.
|
| Series regulators are your typical 7905 or whatever. They are
| more efficient (as you can tell by their obvious operation)
| and simpler to use.
|
| Shunt regulators are traditionally Zener Diodes, or other
| circuits that are based "like" a Zener Diode. They can
| generate constant voltage offsets reliably (ex: if you have a
| 9V line from a series regulator, and you need a 7V reference,
| you can use a shunt-regulator to very accurately create -2V).
|
| --------------
|
| As you can see, its all "pulling tricks".
|
| Of course, the switching regulator (Ex: MC34063. Don't use,
| this is an old chip lol. But maybe TI's Simple Switcher
| series, or similar), truly "push" current thanks to an
| externally supplied inductor... and as a result lead to far
| superior efficiency specs.
|
| Another "pushing" trick is a charge-pump. You can turn on
| capacitors in such a way that they double the voltage. That's
| the thing about "pushing", you need an ability to increase
| voltage until the "downstream" circuit acts the way you like.
|
| Inductors (and capacitors, to a lesser extent) _can_ push.
| But its dangerous and somewhat difficult to design well.
| (Fortunately, we have pre-made modules like TI's Simple
| Switcher or Microchip's MCP1640, etc. etc. that do the job
| for us automatically... as well as pre-made power supplies).
| readingnews wrote:
| I like your explanation and examples.
| dragontamer wrote:
| > circuits do not "push" current
|
| Except for inductors. See: Tesla coils.
|
| If you forget a snubbing diode on a large motor (which is very
| inductive), you'll likely see some fires on your PCB. Inductors
| (and inductive loads) will push current even if the other side
| doesn't want it (ex: even if the other side is a 10 Gig-ohm
| resistance, the current will continue and possibly spike the
| voltage to millions-of-volts and shoot lightning out to allow
| the inductor to keep pushing the current)
| hatsunearu wrote:
| it's easier to think everything is a transmission line and
| everything is propagation with finite impedances rather than
| thinking everything is an idealized element and figuring out
| what the exceptions are.
| mlichvar wrote:
| Here is a nice animation of reflections I found useful:
| https://www.youtube.com/watch?v=ozeYaikI11g
| zelon88 wrote:
| Correct me if I'm wrong, but this sounds like the electrical
| version of "water hammer" in pipes. The electrons have velocity.
| When you suddenly change the impedance of the medium, the extra
| electrons already on the medium that can no longer fit into the
| transition will bounce back.
| fsociety wrote:
| Thinking of an electron as a thing that travels from the start
| of the circuit to the end results in a bad mental model.
|
| Think of it as a wave, and then watch this video
| https://youtu.be/DovunOxlY1k. Then you will have a strong
| mental model.
| FirmwareBurner wrote:
| _> the electrical version of "water hammer" in pipes_
|
| God that's so annoying to hear in your apartment at night. I
| wish building developers wouldn't be so stingy and install more
| insulation or a water ahmmer damper.
| mikewarot wrote:
| Fun fact, the average speed of an electron in a circuit is
| typically that of a snail, unless you're really, REALLY heating
| up the wire.
| AdamH12113 wrote:
| I think a better analogy for water hammer would be inductance.
| Trying to suddenly open an inductive circuit (close a valve)
| produces a voltage spike (pressure spike).
| cushychicken wrote:
| This StackExchange answer is a really great explanation of
| characteristic impedance:
|
| https://electronics.stackexchange.com/questions/59208/transm...
|
| This one is OK too:
|
| https://electronics.stackexchange.com/questions/150222/why-i...
|
| The Art of Electronics 3rd Ed has a great explanation of series
| and parallel circuit terminations that I also highly recommend
| for understanding this, I think in chapter 15.
| tzs wrote:
| This video from Bell Labs showing various behaviors of waves
| using a nifty mechanical wave machine to make everything
| visible might be enlightening.
|
| The whole thing is useful, but those who are just interested in
| impedance and reflections can skip to chapters 4 and 5.
|
| https://www.youtube.com/watch?v=DovunOxlY1k
| dylan604 wrote:
| I just started watching this, and I'm immediately impressed
| with the production values when compared to today's "creator"
| content. All of the edits are matched. There are no jump
| cuts. Well thought out material vs random thoughts attempted
| to be cut together into something coherent is totally lost,
| and yet we reward those that smash cut stuff together equally
| for those that do it well. Generational differences in yet
| another example of I'm getting old (er, I am old)
| dist-epoch wrote:
| Some computer BIOSes had this feature where they would analyze
| the plugged in Ethernet cable using this signal reflection to
| determine if there is a break in the cable, and would tell you
| approximately where (in meters).
| cricalix wrote:
| Still common in the networking world; as an example, Ciena
| optical gear can do Optical Time Domain Reflectometer(try)
| (OTDR) periodically to measure the length of the physical
| circuit, and the nature of the test also reveals where cables
| are joined based on reflections. It's also how you measure
| where a break is, at which point you dig out the geo-coords for
| the path the cable takes to work out where someone just ran an
| optical cable finder (backhoe/JCB digger/whatever you call it
| where you are).
| dboreham wrote:
| The feature is in the Ethernet NIC, not the BIOS. It's been
| present in pretty much all hardware implementations since the
| 1980s. There's a fun story of TDR on "thick-wire" Ethernet
| being used to characterize nuclear detonations[1]
|
| [1]
| https://web.archive.org/web/20160507110751/http://www.csd.uw...
| kjs3 wrote:
| In the thin-coax ethernet days, pretty standard tool in the bag
| was a Fluke[1] portable Time Domain Reflexomter to do this.
| Much more precise than the NIC based measurements. Very useful
| for finding exactly where someones 'cable management' ended up
| nailing through the coax, or where someone ignored bend radius
| limits when installing.
|
| [1] Other vendors made them...I just always had a Fluke
| qiqitori wrote:
| The Commodore 128 had a bodge wire that was added very late in
| the development cycle, and it was there to get rid of an instance
| of signal reflection. Bil Herd mentions it in his book, and I
| think this video of his goes into more depth:
| https://youtube.com/watch?v=kQXdEdsT5qw
| tuetuopay wrote:
| > actually, these had nowhere to go, so here's your energy back
|
| Best way to describe reflection of signals in a wire I've ever
| seen. It's not accurate in the sense that the source does not
| push energy down the wire, but still a good way to see it. Maybe
| I'll finally remember if you get a positive or negative
| reflection when the termination resistor is too low or too high.
| fransje26 wrote:
| Interesting how similar the reaction is to an equivalent gas
| dynamics problem.
| nudgeee wrote:
| In RF electronics, wires end up looking like transmission lines.
|
| Here's one of the classic lectures on wave propagation and
| reflections, where it looks at similarities in wave behaviour
| across mechanical, electrical, acoustic and optical systems.
|
| AT&T Similarities of Wave Behaviour (1959):
| https://youtu.be/DovunOxlY1k
| p_j_w wrote:
| >In RF electronics, wires end up looking like transmission
| lines.
|
| This might be needlessly pedantic, but you've got this a bit
| wrong from a philosophical standpoint. Wires don't end up
| looking like transmission lines, they always ARE transmission
| lines. At low frequencies, transmission lines end up looking
| like the idealized cartoon wires that we use in circuit
| analysis.
| mikewarot wrote:
| What's amazing to me is that you can buy a device called a
| NanoVNA for $300[1], which will test transmission lines from 50
| Khz to 4 Ghz, with a remarkable degree of precision. This used to
| be the domain on instruments costing $20,000 and up.
|
| I myself have a knock-off that goes to about 1 Ghz and was even
| cheaper, and it was quite instructive in helping me get a feel
| for reflections, etc... and came in handy installing some SMD
| capacitors in the right places on a 100 Watt UHF amplifier for
| the 440 Mhz band. A few mm of movement made all the difference in
| the world in terms of matching.
|
| [1] https://nanorfe.com/nanovna-v2.html
| rini17 wrote:
| Can it test USB cables (with some rig of course)? There seems
| to be quite some demand for that.
| opwieurposiu wrote:
| One time we got a new batch of assembled boards that would
| randomly quit working. This was for an older product that we had
| been selling for years without issue. Turns out an IC supplier
| had moved their chip to a smaller process. This smaller chip had
| less gate capacitance and was making much sharper corners on it's
| square waves.
|
| It was slow data, like 10mhz or something. We were safely under
| the 1/10 wavelength rule with the old, softer corners. With the
| new sharp corners we were over the 1/10 wavelength rule and the
| circuit was ringing so hard it would cause nearby chips to latch
| up. We added some termination resistors and it was fixed.
|
| Also note that this kind of thing can be hard to diagnose,
| because when you put the scope probes on, some of the ringing
| energy goes into the scope and it tends to improve the situation
| as long as the probe is there.
| dylan604 wrote:
| >Also note that this kind of thing can be hard to diagnose,
| because when you put the scope probes on, some of the ringing
| energy goes into the scope and it tends to improve the
| situation as long as the probe is there.
|
| When you're in the thick of it trying to debug, these
| absolutely make you want to scream. But once you figure it out
| and have a minute to catch your breath, they are oh so
| satisfying that you _did_ figure it out.
| a1369209993 wrote:
| > when you put the scope probes on, some of the ringing energy
| goes into the scope and it tends to improve the situation as
| long as the probe is there.
|
| The software equivalent is so-called "load-bearing printf"s,
| where for example `printf("broken_var: %i\n",broken_var);`
| causes broken_var to not get optimized out and so start working
| correctly. But in hardware that happens even when you
| (effectively) inspect broken_var in a attached debugger,
| because the closest thing you _have_ to a debugger (eg,
| oscilloscope) effectively just _is_ a load-bearing printf.
| dekhn wrote:
| I wonder if the quantum physics equivalent is heisenberg's
| uncertainty principle. Probably not, but I have observed
| "observing a detail of a system often interferes with the
| running of the system"
| mackman wrote:
| I usually refer to these as Heisenbugs.
| dylan604 wrote:
| In a past life, I worked in a video post house with a very
| competent engingering department. Without fail, if we had a
| problem that required an engineer, they would come in, see
| the problem, and fix it. Unless, if the only engineer
| available was the director of the group. The problems were
| never repeatable when he was present, and everything worked
| fine. This is why there was a constant request to have a
| life sized cutout of him to leave in the room to ensure the
| equipment behaved properly
| dekhn wrote:
| I learned the hard way recently that my attempt at visualizing
| pin states on prototype boards using LEDs without current
| limiting resistors was a bad idea: it caused the LED to light
| up when the output pin was turned HIGH, but the downstream
| receiver of the pin didn't register the HIGH because the LED
| stole too many volts
|
| I only figured this out after building and tearing down my
| prototype and reassembling each of the modular bits, verifying
| they worked, and then noticing it didn't work after integration
| until I removed the LEDs. I have since learned I also could
| have used a buffer driver or LED driver, alkthough that adds
| more complexity to my simple prototype.
|
| (in case you're wondering, yes, I know you're supposed to use a
| current limiting resistor, but I've also observed that my 5mm
| LEDs work just fine when given regulated 5V, they end up
| dropping 4.6V and consuming 40mA, which is about double the
| current they are rated for.)
| picture wrote:
| How bright do you need the LEDs to be? Even 20 mA is a huge
| amount for modern electronics. On my boards I have 100 kohm
| resistors with green LEDs and they are very visible even at
| some 20 uA of current
| kurthr wrote:
| Ahhh yes, the scope probe loading "Heisen Bug"!
| KWxIUElW8Xt0tD9 wrote:
| An easy way to understand what is going on with signal
| reflections is to consider a very long wire -- say, to the moon.
| When you put a voltage on the wire current starts flowing, but
| that current has nothing to do with what is on the other end of
| the wire, because it is far away and there is this thing called
| the speed of light. So the current has to do not with the load
| but with characteristics of the _wire_. Eventually the voltage
| /current will get to the other end of the wire -- the reflection
| is what happens when the voltage/current in the wire does not
| match what is required by the load connected to the end of the
| wire. The problem, especially at high frequencies, is that this
| doesn't require a wire to the moon to see, it can happen in
| _inches_. Try debugging a 400G Ethernet link composed of 16
| traces about 12 inches long on a PC board -- can be very hairy.
| dekhn wrote:
| All I know about this is that during grad school, I had to crawl
| under a multi-ton supercooled superconducting magnet (an NMR
| machine) and turn two knobs to make a number get as small as
| possible- this was adjusting the impedance of the RF (NMR works
| by putting a sample in a huge magnetic field and then bombarding
| it with RF, then switching from transmitter to receiver and
| detecting the faint echos of energy leaving your system).
|
| The magnetic field is so strong you can't have any digital
| electronics within meters of the NMR- but you can put some
| impedance matching analog network along with a couple
| potentiometers and 7-segment displays so a human brain and hands
| (which are much less affected by magnetism) can reduce radio
| reflections.
|
| RF (and all AC) still seems like spooky magic to me, fortunately
| I don't deal with signals above 100khz.
| kjs3 wrote:
| There's a reason one of the go-to books[1] in this area is
| subtitled "A Handbook of Black Magic".
|
| [1]https://www.amazon.com/High-Speed-Digital-Design-
| Handbook/dp...
| brunohaid wrote:
| For anyone who cares enough to watch a 2 hour video on it and
| hasn't seen it yet, this Rick Hartley video is the recent gold
| standard of explaining the issues at play:
|
| https://www.youtube.com/live/ySuUZEjARPY
|
| The mindblowing thing to internalize is that you don't route
| energy as electrons through copper traces (they move incredibly
| slow) but merely use the copper to guide a bunch of waves
| traveling _outside_ of them, where they are up to all kinds of
| shenanigans like coupling, reflecting etc.
| AdamH12113 wrote:
| These are some nice measurements (note that the first scope
| capture is measuring something totally different from the other
| scope captures), but there are a couple errors. First, the
| correct term for the apparent impedance of a long transmission
| line is "characteristic impedance", not "specific impedance".
| "Specific" means "per unit of mass". Second, the usual equivalent
| circuit for a transmission line is parallel capacitors _separated
| by series inductors_. The article tries a hybrid explanation with
| a propagating wave charging capacitors, which doesn 't really
| work.
|
| I've been getting into microwave circuit design this year for
| work. One "fun" thing I've learned is that even at low gigahertz
| frequencies, if you want to make accurate impedance measurements
| with a network analyzer you have to adjust for signal propagation
| delays with _picosecond_ precision. On the boards I 've made,
| every millimeter of transmission line is about 5.56 picoseconds
| of delay, which is about 10 degrees of phase shift at 5
| gigahertz.
|
| Parasitic inductance and capacitance also makes those nice,
| simple terminating resistors look at lot less simple, but that's
| another story...
| haltist wrote:
| I think EE folks should learn about sheaves. It would help make
| better sense of how electricity works. There is obviously
| topological elements they are ignoring.
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