Why 50 Ohms?(microwaves101.com)
microwaves101.com
Why 50 Ohms?
https://www.microwaves101.com/encyclopedias/why-fifty-ohms
11 comments
I'll tell you a neat story about coaxial cables. I worked with quite a few high power transmitters. If you go topside and check the cables to figure out why transmitter power is lost it isn't rare at all to see the cables brittle every half wave. The high frequency does mechanical damage to the cables to the point where the outer mantle will become porous enough to allow water to enter. After that it's only a matter of time, presumably a bit of water will not help with reflected power and speed up the process of desintegration. You can easily tell roughly what band the transmitter is running at just by measuring the distance between two such porous points. That's exactly a half wave length.
> The high frequency does mechanical damage to the cables
How does it do this? Like, is it physical vibration driven by the EMF or what?
How does it do this? Like, is it physical vibration driven by the EMF or what?
The current and voltage in coax feedlines varies with length - he is probably looking at spots where current peaks and thus ohmic heating causes physical damage.
The only way there should be significant nodes and antinodes along a transmission line is if the load match is (very) bad. Not sure why this phenomenon would ever happen in the real world, unless there's a mismatch problem.
This is basically the definition of VSWR. No mismatch = no standing waves. If you're seeing a pattern of degradation due to uneven heating in a piece of coax, check the antenna.
This is basically the definition of VSWR. No mismatch = no standing waves. If you're seeing a pattern of degradation due to uneven heating in a piece of coax, check the antenna.
If the coax is operated at max power handling capability with a good match, then it wouldn't take much reflection (anywhere in the system) to cause internal arcs or heating. No real component has an infinitely low reflection coefficient.
My guess would be ohmic heating.
A few tens of degrees hotter makes PVC plastic insulation go brittle much much faster.
https://www.allaboutcircuits.com/textbook/alternating-curren...
Has this bit in it:
"Transmission lines may be damaged by the high maximum amplitudes of standing waves. Voltage antinodes may break down insulation between conductors, and current antinodes may overheat conductors."
I've never seen any proof of the first but the damage I've seen is consistent with overheating.
Has this bit in it:
"Transmission lines may be damaged by the high maximum amplitudes of standing waves. Voltage antinodes may break down insulation between conductors, and current antinodes may overheat conductors."
I've never seen any proof of the first but the damage I've seen is consistent with overheating.
That half wavelength damage is deviously pleasing somehow.
> 75Ohm gave the lowest loss
The lowest loss of a selection of cable diameters I presume. I'd think lower loss can be achieved with greater diameters (lower specific capacitance), but costs will be prohibitive.
The lowest loss of a selection of cable diameters I presume. I'd think lower loss can be achieved with greater diameters (lower specific capacitance), but costs will be prohibitive.
There is also another story [1]. That standard air-core coax lines were built using regular plumbing pipes in US. As the impedance of a coax is proportional to log(inner diameter of the shield/outer diameter of the core) this generated standard impedance around 50 ohms for many different tube configurations.
(10 minutes later)
However, I just calculated this for standard copper tubing size [2] and this probably is just a myth, except for few specific tube sizes like 1/4+5/8 or 1.5+3.5 inches.
For some reason, tubing is still mostly given in inches and neither its OD nor ID correspond to tube size. OD is always 0.125in larger than tube size.
[1] http://www.rfcafe.com/references/electrical/history-of-50-oh...
[2] https://www.petersenproducts.com/Copper-Tubing-Sizes-s/1979....
(10 minutes later)
However, I just calculated this for standard copper tubing size [2] and this probably is just a myth, except for few specific tube sizes like 1/4+5/8 or 1.5+3.5 inches.
For some reason, tubing is still mostly given in inches and neither its OD nor ID correspond to tube size. OD is always 0.125in larger than tube size.
[1] http://www.rfcafe.com/references/electrical/history-of-50-oh...
[2] https://www.petersenproducts.com/Copper-Tubing-Sizes-s/1979....
Trying to figure out what pipe to ask for when I have the physical measurements in hand is one of the ongoing mysteries of the world for me. "Oh that pipe is <some number you will not get with any type of measuring tool>" is one of the most bizarre conventions out there.
That said, this isn't even the only place it turns up - there are timber sizes which are specified as one dimension and just "commonly known" to actually be a different one: https://www.thesprucecrafts.com/why-isnt-a-2x4-a-2x4-3970461
That said, this isn't even the only place it turns up - there are timber sizes which are specified as one dimension and just "commonly known" to actually be a different one: https://www.thesprucecrafts.com/why-isnt-a-2x4-a-2x4-3970461
Pipe was originally measured by inside diameter.
Anyway, over the past 100 years, metallurgy got better and they could make the same strength pipe with thinner walls.
So now, because of the thickness change, the old pipes that measured 1/2” inside and the fittings for those pipes were the right size for new pipes that measured about 3/5” inside. They had the same outside diameter, but a larger inside diameter.
Instead of confusing everyone and requiring them to remember compatibility charts for every size pipe (and guessing about the age of the pipe), they just “solved” the problem by selling the new pipes under the name of the old size they were compatible with.
Tl;dr: Plumbers have technical debt that goes way back.
Anyway, over the past 100 years, metallurgy got better and they could make the same strength pipe with thinner walls.
So now, because of the thickness change, the old pipes that measured 1/2” inside and the fittings for those pipes were the right size for new pipes that measured about 3/5” inside. They had the same outside diameter, but a larger inside diameter.
Instead of confusing everyone and requiring them to remember compatibility charts for every size pipe (and guessing about the age of the pipe), they just “solved” the problem by selling the new pipes under the name of the old size they were compatible with.
Tl;dr: Plumbers have technical debt that goes way back.
Having seen the lead pipes Romans used, yes, plumbers have had some rough times.
The United States was still installing lead pipes until 1930-1986:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2509614/#__sec4...
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2509614/#__sec4...
They are called plumbers for a reason.
I know of at least one place which still had the original lead Roman piping quite late in the 20th century.
The timber sizes are different because you are buying planed timer, rough timber would have the exact dimensions you are looking for. So a 2x4 rough is really 2x4"
The timber sizes are different because you're buying planed dried timber. For some reason it was decided that measuring the timber wet and unplaned was a good idea.
On the other hand over here in metric-land if you buy a 44-by-88 piece of wood it'll be real close to 44x88mm, Whether you're buying it planed or not.
On the other hand over here in metric-land if you buy a 44-by-88 piece of wood it'll be real close to 44x88mm, Whether you're buying it planed or not.
That's true, but they had to set their saws to something and a typical sawmill will process the logs as they're brought in, not after drying them because it would take forever to dry the whole log and it would likely split or be infested with creatures. So the logs are sawn 'green' to a rough size, dried (sometimes kiln dried to speed up the drying process) and then planed. Both drying and planing affect the dimensions.
It's quite amazing how the speed of the drying process can affect the later tendency of the wood to warp, as well as the degree to which it will warp. Counteracting this is quite tricky and requires all kinds of techniques, the most effective of which is laminating odd/even stacks of wood.
It's quite amazing how the speed of the drying process can affect the later tendency of the wood to warp, as well as the degree to which it will warp. Counteracting this is quite tricky and requires all kinds of techniques, the most effective of which is laminating odd/even stacks of wood.
The same limitations hold in metric-land, and they still manage to label wood the size it actually is when you buy it. I'm sure it's a difficult problem but as far as I can tell it's also a solved problem. I imagine that the odd piece of lumber they have that dries with a severe warp or size deformity simply gets resawn.
Well, in 'metric-land' they don't have a history of naming their wood for the thickness of some guys thumb in integer multiples.
The USA could switch from 2x6 to 1.5x5.5 (planing takes 1/4" off each surface) tomorrow if they wanted to but I guess there would be quite a bit of confusion. The building industry is super traditional.
And when you do that you might as well go metric (for reference: 38x140mm).
The USA could switch from 2x6 to 1.5x5.5 (planing takes 1/4" off each surface) tomorrow if they wanted to but I guess there would be quite a bit of confusion. The building industry is super traditional.
And when you do that you might as well go metric (for reference: 38x140mm).
> in 'metric-land' they don't have a history of naming their wood for the thickness of some guys thumb in integer multiples.
Yes we do. The metric system didn't fall out of a clear sky one day, it had precursors.
That's hardly the point though, even when using metric you could make the decision to measure wood wet. Even when using the imperial system you can make the decision to measure wood dry.
Yes we do. The metric system didn't fall out of a clear sky one day, it had precursors.
That's hardly the point though, even when using metric you could make the decision to measure wood wet. Even when using the imperial system you can make the decision to measure wood dry.
That's because historically it was cut then seasoned. What you ended up with a year later was somewhat smaller.
Everybody thinks that, but it isn't true. historically some sawmills where producing 2x4s that were 2x4, while others were using different sizes (I can't find evidence but I'm guessing they were trying to cheat their customers for profit and get away with it).
What matters isn't the actual size though, what matters is that when you are short a piece of wood you can get one that fits.
What matters isn't the actual size though, what matters is that when you are short a piece of wood you can get one that fits.
I’m in the throes of renovating an old house at the moment and dealing with replacing rotting sections of floor has proved a little fiddly due to this. The slightly different joist heights aren’t too bad - you just pack them out before strapping to the existing joists. Worse has been the existing chipboard floor, which is 20mm and I can only buy 18mm or 22mm. You can hack your way around it, but that one subtle difference adds quite a bit of time to the job - something I don think have much spare of at the moment!
You actually can get 20mm plywood, it will be a bit more expensive and likely much harder to find. Try a cabinet maker and ask if they can order it for you, cabinet makers have access to a vastly superior source of materials compared to your normal building store. Another option is that they route some grooves in thicker plywood at the spot of the joists (I guess that would qualify as a hack).
Thanks for the tip.
It’s irrelevant now because the flooring people have just walked out saying they won’t lay on to the chipboard anyway. Need to run a thin layer of ply over the whole thing. Live and learn I guess!
It’s irrelevant now because the flooring people have just walked out saying they won’t lay on to the chipboard anyway. Need to run a thin layer of ply over the whole thing. Live and learn I guess!
The sizes for timber come the actual physical size early in the production process as your link nicely explains. Where to the "commonly known as" sizes for pipes come from?
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Technically HDD manufacturers are doing exactly the same.
I see this repeated time and time again, and every time it isn't true.
This is what my console shows when it sees a 1 TB drive:
[ 1.629496] sd 2:0:0:0: [sda] 1953525168 512-byte logical blocks: (1.00 TB/932 GiB)
If you multiply those 2 numbers (1953525168 x 512 bytes), you arrive at 1000204886000 bytes, or 1000.204886 GB. You can write to every single one of those bytes/sectors, none of them are reserved.
Your choice of filesystem dictates the maximum amount of space you can use on the drive, but the drive itself is giving you all of the capacity you asked for, and if you don't use any filesystem at all, you can write 1TB to it.
The confusion comes from Microsoft Windows being involved. Windows measures in IEC units, but displays the value as an SI unit.
So, for example, it will tell you that a 16000000-byte file is 15.26 MB. It isn't. It's 16 MB; it is also 15.26 MiB. Likewise, Windows will tell you that a 1 TB harddrive is 931 GB. It isn't, it's 1000 GB; it is also 931 GiB.
If I'm a cable vendor, and you ask me for a reel of 8 kilometres of cable, and I give you a 5-mile reel, I gave you what you asked for. 5 is less than 8, but 5 miles is not less than 8 kilometres. 931 is less than 1000, but 931 GiB is not less than 1000 GB.
This is what my console shows when it sees a 1 TB drive:
[ 1.629496] sd 2:0:0:0: [sda] 1953525168 512-byte logical blocks: (1.00 TB/932 GiB)
If you multiply those 2 numbers (1953525168 x 512 bytes), you arrive at 1000204886000 bytes, or 1000.204886 GB. You can write to every single one of those bytes/sectors, none of them are reserved.
Your choice of filesystem dictates the maximum amount of space you can use on the drive, but the drive itself is giving you all of the capacity you asked for, and if you don't use any filesystem at all, you can write 1TB to it.
The confusion comes from Microsoft Windows being involved. Windows measures in IEC units, but displays the value as an SI unit.
So, for example, it will tell you that a 16000000-byte file is 15.26 MB. It isn't. It's 16 MB; it is also 15.26 MiB. Likewise, Windows will tell you that a 1 TB harddrive is 931 GB. It isn't, it's 1000 GB; it is also 931 GiB.
If I'm a cable vendor, and you ask me for a reel of 8 kilometres of cable, and I give you a 5-mile reel, I gave you what you asked for. 5 is less than 8, but 5 miles is not less than 8 kilometres. 931 is less than 1000, but 931 GiB is not less than 1000 GB.
> I see this repeated time and time again, and every time it isn't true.
What isn't true? That they're using a definition that's technically correct but doesn't align with the lay definitions of the units?
I don't think you can call that untrue, no matter how prescriptivist you get.
And it's worse than that, because other technical users also mean powers of 2^10.
And it goes far beyond Windows, and has nothing to do with file systems.
What isn't true? That they're using a definition that's technically correct but doesn't align with the lay definitions of the units?
I don't think you can call that untrue, no matter how prescriptivist you get.
And it's worse than that, because other technical users also mean powers of 2^10.
And it goes far beyond Windows, and has nothing to do with file systems.
Gibibyte is not a different definition of gigabyte. They’re different units.
I though it was SI retroactively standardizing the Ki Mi Gi prefixes in an attempt to fix things, and before that the common usage of GB was just 2^30 Bytes.
The SI system can give a definition of GB, but that doesn't mean they have given the definition of GB.
The SI system can give a definition of GB, but that doesn't mean they have given the definition of GB.
Neither. G is the SI suffix Giga, and always means 1,000,000,000. GiB is an IEC unit, not an SI unit. They're from entirely different bodies.
When they shorten it to “G” they are absolutely trying to deceive.
On reading this on iOS I selected the word and pressed “look up”. It seems that iOS strips out formatting in its dictionary, make superscript full size. Not ideal.
Gigabyte “a unit of information equal to one thousand million (109) or, strictly, 230 bytes.”
That's exactly what he meant.
I never understood why specifically HDD manufacturers are criticized when in fact all vendors and all types of hardware (expect RAM) follow the same convention of using decimal prefixes: http://blog.zorinaq.com/decimal-prefixes-are-more-common-tha...
Technically, everyone in computing BUT the HDD manufacturers are doing this. That's why we now have MiB, GiB etc. units as well as MB, GB etc.
Can you explain what you mean?
Some people use decimal prefixes but they mean the binary values, for example kilo is 1000 but in computers is most often used as 1024. HDD manufacturers express the size in decimal (1 TB = 10^12 bytes), people that think in 1024 multiples will calculate this is less Gigabytes than expected.
HDD manufactures often report a storage capacity that is larger than the effective capacity, since some capacity is used for formatting information, or error correction. This is the same situation with protocols (like gigabit Ethernet).
There's also "punning" between the SI préfixes, where e.g. "kilo" can be used to mean 1000 or 1024. See https://en.m.wikipedia.org/wiki/Kibibyte
There's also "punning" between the SI préfixes, where e.g. "kilo" can be used to mean 1000 or 1024. See https://en.m.wikipedia.org/wiki/Kibibyte
I think modern HDD and SSD manufacturers have been pretty honest about the actual capacity of their drives. They have no way of knowing what kind of filesystem you will be putting on the drive and how much overhead it has, or whether you might be putting it in a RAID 1 array (which cuts the effective capacity per drive according to the number of drives in the array).
The number of bytes they specify is the actual number of bytes available to your operating system. It seems like a pretty fair measurement.
The 1000 vs. 1024 thing is an unfortunate point of confusion. Terms like kibibyte and such were an attempt to solve it, but they never took off. (If you were to ask me which number a kilobyte or kibibyte was, I would have to go look it up!)
Even if we did use the kilo vs. kibi terms, HDD and SSD manufacturers are the ones who are getting it right. As the article you linked notes:
> 1 kibibyte (KiB) = 2^10 bytes = 1024 bytes
> The kibibyte is closely related to the kilobyte. The latter term is often used in some contexts as a synonym for kibibyte, but formally refers to 10^3 bytes = 1000 bytes, as the prefix kilo is defined in the International System of Units.
The same applies as you go up in the units. One mega-anything is 1,000,000 of those things, giga- is 1,000,000,000, and tera- is 1,000,000,000,000 things.
So a one-terabyte HDD or SSD should have a true capacity of 1,000,000,000,000 bytes, before any operating system or RAID overhead. Of course its actual physical capacity has to be higher, to support remapping of failing sectors or flash blocks and such. But that's all hidden by the drive controller.
I think it's the memory people who got this wrong, by co-opting a "kilobyte" to mean 1024 bytes, contrary to the standard definition. It was a handy coincidence of terminology at the time, but the error was amplified as we got into multiples of that size.
And then the operating system and utility people (or many of them) completely messed up by using the power-of-two definitions for disk/flash storage instead of the correct power-of-ten definitions.
This is why, for example, every single Amazon listing of a 1TB drive (HDD, SSD, flash card) which honestly provides the correct 1,000,000,000,000 bytes of storage to the OS will have at least one review complaining:
> Claims to have 1TB but only has 931GB as reported by my operating system.
The "missing" 69GB isn't due to formatting or any misdoing on the part of the drive manufacturer, it's because the OS is using the wrong units.
The number of bytes they specify is the actual number of bytes available to your operating system. It seems like a pretty fair measurement.
The 1000 vs. 1024 thing is an unfortunate point of confusion. Terms like kibibyte and such were an attempt to solve it, but they never took off. (If you were to ask me which number a kilobyte or kibibyte was, I would have to go look it up!)
Even if we did use the kilo vs. kibi terms, HDD and SSD manufacturers are the ones who are getting it right. As the article you linked notes:
> 1 kibibyte (KiB) = 2^10 bytes = 1024 bytes
> The kibibyte is closely related to the kilobyte. The latter term is often used in some contexts as a synonym for kibibyte, but formally refers to 10^3 bytes = 1000 bytes, as the prefix kilo is defined in the International System of Units.
The same applies as you go up in the units. One mega-anything is 1,000,000 of those things, giga- is 1,000,000,000, and tera- is 1,000,000,000,000 things.
So a one-terabyte HDD or SSD should have a true capacity of 1,000,000,000,000 bytes, before any operating system or RAID overhead. Of course its actual physical capacity has to be higher, to support remapping of failing sectors or flash blocks and such. But that's all hidden by the drive controller.
I think it's the memory people who got this wrong, by co-opting a "kilobyte" to mean 1024 bytes, contrary to the standard definition. It was a handy coincidence of terminology at the time, but the error was amplified as we got into multiples of that size.
And then the operating system and utility people (or many of them) completely messed up by using the power-of-two definitions for disk/flash storage instead of the correct power-of-ten definitions.
This is why, for example, every single Amazon listing of a 1TB drive (HDD, SSD, flash card) which honestly provides the correct 1,000,000,000,000 bytes of storage to the OS will have at least one review complaining:
> Claims to have 1TB but only has 931GB as reported by my operating system.
The "missing" 69GB isn't due to formatting or any misdoing on the part of the drive manufacturer, it's because the OS is using the wrong units.
> Even if we did use the kilo vs. kibi terms, HDD and SSD manufacturers are the ones who are getting it right.
I'd remove that "even".
If we used both terms, then they'd be getting it right.
But since we're not, they're being misleading compared to the typical use.
> I think it's the memory people who got this wrong
The memory people? It's pretty much everyone that uses "gigabyte" and isn't selling you a storage device.
I'd remove that "even".
If we used both terms, then they'd be getting it right.
But since we're not, they're being misleading compared to the typical use.
> I think it's the memory people who got this wrong
The memory people? It's pretty much everyone that uses "gigabyte" and isn't selling you a storage device.
Many Linux utilities do things with the kibi terms, using the MiB, GiB, TiB units.
There have been a few lawsuits about this: https://en.wikipedia.org/wiki/Binary_prefix#Consumer_confusi...
The outcome is that hard drive manufacturers need to explicitly print their definition of a "gigabyte". IMHO, it's not a right or wrong discussion, just a difference.
There have been a few lawsuits about this: https://en.wikipedia.org/wiki/Binary_prefix#Consumer_confusi...
The outcome is that hard drive manufacturers need to explicitly print their definition of a "gigabyte". IMHO, it's not a right or wrong discussion, just a difference.
Your second line is right. Your first isn't really. Formatting only reserves about 100MB. Error correction bits are completely hidden and not marketed at all. It's the 10% difference between "TB" and "TB" that really matters.
The age old usable capacity complaint. (i.e. a 1TB drive will never actually give you 1TB of usable space)
1GB RAM > 1GB HDD
Some of the first experiments with coaxial lines were indeed made by pulling a center-conductor into standard copper pipes.
There are several articles about that in BSTJ, with pictures.
There are several articles about that in BSTJ, with pictures.
The coolest coax cable is the YK-217 cable. 14.8 ohm impedance. 25kv. The center conductor was a 6mm plastic rod. Then came the braided inner conductor, covered with some very thin black plastic shit. Then some serious HV insulation, followed by the outer conductor and a thick outer protective jacket. The cable was about 16mm in outside diameter. I had to strip 100 of these cables, with the inner conductor extending about 35mm from the outer conductor. The only way to do this accurately was on a lathe. In my youth, I spent all afternoon and evening doing this, then went to a concert at Winterland in San Francisco.
From these cables, and a ton of other machining and one large 50nF capacitor, I built a nitrogen laser reliable enough to use in my thesis experiment. Every once in a while, a cable would short, always where my lathe knife had gone a tad too deep. As the laser pulsed at 17Hz, it sounded like a machine gun when this happened. I stopped the laser, used my grounding hook to discharge everything, and calmly unclamped the offending cable and threw it away. The laser ran perfectly well with 99, 98,97… cables. That’s why I used cables.
I'd really like to understand impedence some day. I understand the text book definition is resistance to A/C, but I fail to understand impedence matching, coax cable ratings, high impedance inputs, and pretty much anywhere else the term is used.
The way I rationalize impedance matching is like the Index of refraction.
The analogue to two pieces of wire with different impedance is two pieces of glass with different index of refraction. What happens when you shine in light is a partial reflection at the boundary of the two pieces of glass, which also means less light gets through.
Impedance matching is just trying to match these indices of refraction to me. Sadly, I have no good analogue for a balun or impedance matching circuit.
I have no idea whether this is mathematically correct, but it gives me some intuition for why it matters.
The analogue to two pieces of wire with different impedance is two pieces of glass with different index of refraction. What happens when you shine in light is a partial reflection at the boundary of the two pieces of glass, which also means less light gets through.
Impedance matching is just trying to match these indices of refraction to me. Sadly, I have no good analogue for a balun or impedance matching circuit.
I have no idea whether this is mathematically correct, but it gives me some intuition for why it matters.
It is technically correct, but might not be the easiest way to understand it intuitively (this will be different for everyone, but I didn't quite get the explanations in the thread).
I'll try to make it simple: for now, let's admit impedance is resistance. It is just a way to express how much the medium will slow a propagation down.
This works with multiple fields: thermal resistance, soundwaves, lightwaves (refraction index is c/v), or you can just imagine yourself going trough different mediums: when diving, you encounter some resistance where the medium starts slowing you more. You lose energy there. If instead the resistance was the same all the way (if you were already swimming), there would be no interface to lose energy at. Diving championships use bubblers to soften the water, that's adapting impedance, in a sense.
Of course, this analogy is a bit extreme at our scale, due to the high absorption and terminal velocity differences between the two mediums. But you have the same with soundwaves: a wall will reflect your speech because sound has a lot higher speed in concrete. Some surfaces will do it better than others. And to avoid that kind of parasite reflection during ultrasound imaging, a gel is employed to match impedance between the emitter and the body [1] (avoiding a thin air layer in between).
OK, now for real (actually complex) impedance, you have to take into account that the speed inside the medium varies based on frequency. That's called dispersion in optics and acoustics, and is sometimes compensated by employing mediums with an inverted dispersion relationship [2].
OK, I'll admit that propagation speed isn't strictly what impedance is, but it is pretty close, and they probably have an intimate relationship (which I can't reflect on right now, but feel free to elaborate below, if you feel like it).
This actually sent me down a small rabbit hole in wikipedia: https://en.wikipedia.org/wiki/Impedance_analogy
[1]: https://en.wikipedia.org/wiki/Medical_ultrasound [2]: https://en.wikipedia.org/wiki/Dispersion_(optics)#Dispersion... [2]: http://www.m2optics.com/blog/bid/61431/Chromatic-Dispersion-...
I'll try to make it simple: for now, let's admit impedance is resistance. It is just a way to express how much the medium will slow a propagation down.
This works with multiple fields: thermal resistance, soundwaves, lightwaves (refraction index is c/v), or you can just imagine yourself going trough different mediums: when diving, you encounter some resistance where the medium starts slowing you more. You lose energy there. If instead the resistance was the same all the way (if you were already swimming), there would be no interface to lose energy at. Diving championships use bubblers to soften the water, that's adapting impedance, in a sense.
Of course, this analogy is a bit extreme at our scale, due to the high absorption and terminal velocity differences between the two mediums. But you have the same with soundwaves: a wall will reflect your speech because sound has a lot higher speed in concrete. Some surfaces will do it better than others. And to avoid that kind of parasite reflection during ultrasound imaging, a gel is employed to match impedance between the emitter and the body [1] (avoiding a thin air layer in between).
OK, now for real (actually complex) impedance, you have to take into account that the speed inside the medium varies based on frequency. That's called dispersion in optics and acoustics, and is sometimes compensated by employing mediums with an inverted dispersion relationship [2].
OK, I'll admit that propagation speed isn't strictly what impedance is, but it is pretty close, and they probably have an intimate relationship (which I can't reflect on right now, but feel free to elaborate below, if you feel like it).
This actually sent me down a small rabbit hole in wikipedia: https://en.wikipedia.org/wiki/Impedance_analogy
[1]: https://en.wikipedia.org/wiki/Medical_ultrasound [2]: https://en.wikipedia.org/wiki/Dispersion_(optics)#Dispersion... [2]: http://www.m2optics.com/blog/bid/61431/Chromatic-Dispersion-...
Impedance matching is a technique required because equivalent amounts of energy can take many different forms, such as low power for a long time, or high power for a short time. In electronics, the term "impedance" is most often understood in terms of an ohms law tradeoff (higher current at lower potential or lower current at higher potential), and the related formulas are easy to find. But the requirement to match impedance is easier to grasp if you understand how energy could be transformed from one form to another and why that would be desirable.
Consider a system in which we have raised 100 baseballs to a height of 10 centimeters. The amount of potential energy stored in this system is equivalent to the energy in a system with a single baseball raised to a height of 10 meters. We could say that these systems, while storing equivalent potential energy, would have a different kinetic impedance when the energy is released. Imagine how different it would feel to be laying underneath a blanket of baseballs dropped from a few centimeters versus standing underneath a baseball dropped from several stories! One would be uncomfortable, the other, barely survivable. This is the impact that mismatched impedance can have on an electrical device.
Like baseballs, electrons in their orbitals have a certain potential energy "voltage" relative to another nucleus. This energy is released (electrical current flows) when there is a conductive path for electrons at a higher potential to move to a lower potential. A system with 10 billion electrons at 1 volt potential has the same energy as a system with 10 million electrons at potential of 1000 volts, but the one with the higher voltage would have proportionally less current (fewer electrons) than the one with the lower voltage. We would describe these two systems as having different electrical impedance.
In practice, electrical impedance is more complicated than a simple ohms law exchange because of the wonderfully useful property that impedance varies with frequency in all natural materials. This makes analysis less straightforward, but allows us to build filters.
If "electrons with potential" seems abstract, it can be conceptually worthwhile to examine the many analogs to impedance matching in the mechanical realm which are easiest to see when a natural or convenient energy source is transformed into a more useful form. No energy is created (indeed, energy is lost to heat due to inefficiencies); only the impedance is transformed, as we see in the following examples:
An automotive transmission is just an impedance transformer, taking the engine's optimal energy output at 1500-2000 RPM at low force and delivering it to the wheels at a lower RPM with higher force. The transmission matches the output impedance of the engine to the impedance of the car on the road so the engine doesn't stall under too high a load or burn too much gas under too low a load.
A butter knife is an impedance transformer (transforming the low pressure in your hand across the large area of the knife handle into 100X the pressure across the tiny 1/100th area of the blade.) The knife matches the pressing impedance of your hand to the slicing impedance of the butter.
A nut-cracker is an impedance transformer that transforms your hand's low force over a long distance into a very high force over the very short distance required to crack the nut. It matches the impedance of your grip strength to the cracking impedance of the nut.
An electrical utility transformer is an impedance transformer, transforming high-voltage low current into low voltage, high current. It matches the impedance of the utility line to the impedance of your toaster.
A hydro electric dam usually takes the high cross sectional area and low speed water flow of a river and chokes it into a single point with a much lower cross sectional area and a much higher speed where it can drive a turbine. The dam transforms the impedance of the river to the impedance of the turbine.
Even an air conditioner compressor could be said to be an impedance transformer; the coolant starts at room temperature and at a regular volume. Compressing the coolant increases its thermal potential while decreasing its volume. When the higher potential energy is radiated out through the exchanger coils, the coolant is cycled back inside and decompressed, but since it has lost energy to radiation, it is cooler. The air conditioner matches the impedance of the coolant to the thermal radiation impedance of the exchanger coil.
With the exception of the compressor example, all of these devices are simple and passive: some gears, a knife, a lever, some coils, a funnel... impedance transformers are everywhere, doing the simple and passive task of matching the form of energy you have into the form that you need.
Consider a system in which we have raised 100 baseballs to a height of 10 centimeters. The amount of potential energy stored in this system is equivalent to the energy in a system with a single baseball raised to a height of 10 meters. We could say that these systems, while storing equivalent potential energy, would have a different kinetic impedance when the energy is released. Imagine how different it would feel to be laying underneath a blanket of baseballs dropped from a few centimeters versus standing underneath a baseball dropped from several stories! One would be uncomfortable, the other, barely survivable. This is the impact that mismatched impedance can have on an electrical device.
Like baseballs, electrons in their orbitals have a certain potential energy "voltage" relative to another nucleus. This energy is released (electrical current flows) when there is a conductive path for electrons at a higher potential to move to a lower potential. A system with 10 billion electrons at 1 volt potential has the same energy as a system with 10 million electrons at potential of 1000 volts, but the one with the higher voltage would have proportionally less current (fewer electrons) than the one with the lower voltage. We would describe these two systems as having different electrical impedance.
In practice, electrical impedance is more complicated than a simple ohms law exchange because of the wonderfully useful property that impedance varies with frequency in all natural materials. This makes analysis less straightforward, but allows us to build filters.
If "electrons with potential" seems abstract, it can be conceptually worthwhile to examine the many analogs to impedance matching in the mechanical realm which are easiest to see when a natural or convenient energy source is transformed into a more useful form. No energy is created (indeed, energy is lost to heat due to inefficiencies); only the impedance is transformed, as we see in the following examples:
An automotive transmission is just an impedance transformer, taking the engine's optimal energy output at 1500-2000 RPM at low force and delivering it to the wheels at a lower RPM with higher force. The transmission matches the output impedance of the engine to the impedance of the car on the road so the engine doesn't stall under too high a load or burn too much gas under too low a load.
A butter knife is an impedance transformer (transforming the low pressure in your hand across the large area of the knife handle into 100X the pressure across the tiny 1/100th area of the blade.) The knife matches the pressing impedance of your hand to the slicing impedance of the butter.
A nut-cracker is an impedance transformer that transforms your hand's low force over a long distance into a very high force over the very short distance required to crack the nut. It matches the impedance of your grip strength to the cracking impedance of the nut.
An electrical utility transformer is an impedance transformer, transforming high-voltage low current into low voltage, high current. It matches the impedance of the utility line to the impedance of your toaster.
A hydro electric dam usually takes the high cross sectional area and low speed water flow of a river and chokes it into a single point with a much lower cross sectional area and a much higher speed where it can drive a turbine. The dam transforms the impedance of the river to the impedance of the turbine.
Even an air conditioner compressor could be said to be an impedance transformer; the coolant starts at room temperature and at a regular volume. Compressing the coolant increases its thermal potential while decreasing its volume. When the higher potential energy is radiated out through the exchanger coils, the coolant is cycled back inside and decompressed, but since it has lost energy to radiation, it is cooler. The air conditioner matches the impedance of the coolant to the thermal radiation impedance of the exchanger coil.
With the exception of the compressor example, all of these devices are simple and passive: some gears, a knife, a lever, some coils, a funnel... impedance transformers are everywhere, doing the simple and passive task of matching the form of energy you have into the form that you need.
Really nice description.
One way that I think about impedance matching is with the idea of resonant coupling. If one wants to record a heartbeat, the microphone needs to resonate and couple with the heart. But since sound waves don't transfer well across gaps of materials with different impedance, to enhance resonant coupling, the microphone is embedded within another device that can better couple with the skin -- e.g., through increased surface area or with material that is acoustically similar to the skin. The use of acoustically similar materials is also called impedance matching.
This article is nice, especially the illustration of how air ducts can be used to create low/high-pass and bandpass filters. https://www.britannica.com/science/sound-physics/Impedance
One way that I think about impedance matching is with the idea of resonant coupling. If one wants to record a heartbeat, the microphone needs to resonate and couple with the heart. But since sound waves don't transfer well across gaps of materials with different impedance, to enhance resonant coupling, the microphone is embedded within another device that can better couple with the skin -- e.g., through increased surface area or with material that is acoustically similar to the skin. The use of acoustically similar materials is also called impedance matching.
This article is nice, especially the illustration of how air ducts can be used to create low/high-pass and bandpass filters. https://www.britannica.com/science/sound-physics/Impedance
That was a beautiful explanation! Thank you for sharing those examples.
> I understand the text book definition is resistance to A/C, but I fail to understand impedence matching, coax cable ratings, high impedance inputs, and pretty much anywhere else the term is used.
Actually, it's easy to understand the basic concepts in pure DC. Let's see...
high impedance inputs - this is the easiest one to explain. A device with high-impedance input only allows a small, often negligible amount of current to flow into it, as if nothing is connected.
Imagine a 5-volt power source followed by an 10k ohm output resistor in series, and we put it in a blackbox. If you connected another 10k ohm resistor across the blackbox, you create a voltage divider, and the voltage across your resistor would be 5 x (10k / (10k + 10k)) = 2.5 volts, and a current of 0.25 mA flows.
But if you increase the value of your inserted resistor to, e.g. 10M ohm, now the 99.9% of the voltage is now across your resistor, and you get 4.995 V, and there is almost no current flows across the power source, as if nothing is connected, minimizing the "observation effect".
This allows you to measure the voltage without disturb it by drawing power from a circuit. Example:
ideal voltmeter - gives a voltage measurement.
ideal oscilloscope probe - just an ideal voltmeter that takes many measurements and display a line on a chart.
ideal ADC - just an electronic voltmeter.
ideal transistor switch and logic gate - output only depends on the voltage reading of the input, it doesn't absorb power.
ideal buffer and operational amplifier - measures the voltage at the input without affecting it, and creates a replica of this voltage by using its own power source.
impedence matching - This one is difficult if we're talking about reflections, S-parameters, and standing waves, but it's easy understand if we only talk about impedence matching for maximum power transfer. People often say that, to get the maximum power transfer, the impedance of the source and the load should be matched. But it's misleading. Theorem of Maximum Power Transfer is actually telling us how to choose our load, given a known source, not vice versa.
Imagine an ideal 5-volt DC voltage source. The maximum available power from such a power source is infinite, and only depends on the impedance of your load. By connecting an infinitesimal resistor, infinite power will be absorbed by the resistor.
Now imagine an ideal 5-volt DC voltage source followed by 10 ohm of output resistor in series. This is closer to a real power supply, or a signal generator, or a radio antenna, whose internal impedance sets an upper limit of available power.
What the Theorem of Maximum Power Transfer is telling is that, if we want to absorb the maximum available power out of this power source, our load must be equal to the source impedance, in this case, 10 ohm.
If our load has the same impedance as our source, the voltage across our load will be Vcc x R2 / (R1 + R2), or 5 x 10/(10+10) = 2.5 V, and the power is absorbed by our load is V^2 / R, or 0.625 W.
If our load is much smaller than the source, there will be more current but less voltage. A 1-ohm load will get a voltage of 5 x 1/(1+11) = 0.416 V, and absorb only 0.173 W.
If our load is much bigger than the source, there will be more voltage but less current. A 100-ohm load will get a voltage of 4.54 V, and absorb only 0.206 W.
It's simple if the following misconceptions are cleared.
* Misconception 1: If I have a 300-ohm load, the output impedance of the power supply should be matched to 300-ohm for maximum efficiency.
False. Maximum Power Transfer does not imply Maximum Efficiency. Efficiency is the ratio, Useful Power / (Useful Power + Wasted Power), and it has nothing to do with maximum power transfer.
Recall the example above, when a 100-ohm load is connected to a 10-ohm power source, the voltage across the load is 4.54 V, drawing 0.206 W. Meanwhile, the voltage across the internal impedance of the power is 0.46 V, drawing 0.021W. The efficiency is 91%. But when impedance is matched, 100-ohm load is connected to a 100-ohm power source, the efficiency is "only" 50%.
Often, the efficiency is the best when there's low power transfer. You don't waste a lot of power if you don't take away a lot of power.
* Misconception 2: If I have a 300-ohm load, (e.g. a headphone), the output impedance of the power supply (e.g. a headphone amplifier) should be 300-ohm for maximum power transfer.
False. Maximum Power Transfer is only about how to select our load to absorb maximum power when we have no control over a given source, not vice versa. In real-life, for non-RF circuit, if we have control over the source, the solution is to make the output impedance of the power source close to zero.
For a 5-volt DC source with a 300-ohm impedance, connected by a 300-ohm load, only 0.02W of power comes out because there's only 2.5 V across the load. But if we have the output impedance to be 0.1-ohm, 4.998 V is now across our load, and drawing 0.083 W of power.
This is called impedance bridging, and it's how power supplies and modern headphone amplifiers are designed - they are designed for maximum efficiency by minimizing their output impedance, while the headphones themselves only draw a little bit of power.
On the other hand, a radio receiver is a good example when we have no control over the source impedance, given the same antenna, it cannot be changed. In this case, the better our receiver is matched to the impedance of the antenna, the higher its sensitivity.
BTW, in RF-circuit, even if we have control over a signal source, we use matched impedance, in this case, the purpose is not to get the maximum power transfer, but for avoiding reflections, a completely separate issue.
* Misconception 3: 50-ohm coax cable has 50-ohm of electrical impedance.
False. The "50 ohm" and "75 ohm" for coax cable is its characteristic impedance, not electrical impedance.
This is when things get complicated and when we must think electrical signals as waves. But the other comment has a good explanation, so it won't be repeated. In short, when a wave hits an impedance discontinuity in its medium of propagation, some of the energy is reflected, so in an RF system, 50-ohm everywhere is desirable.
A small piece of lossless coax cable can be modeled as an inductor in series and a capacitor in parallel, it's the radio of the inductor and the capacitor that determines its characteristic impedance that we can match to avoid a discontinuity in the circuit, thus avoiding reflection. Or we can say that the characteristic impedance is the input impedance when the length of the lossless coax is infinite.
It's not an actual electrical impedance that we can measure by an ohmmeter. A 10-meter coax has an electrical impedance of less than 1 ohm.
Actually, it's easy to understand the basic concepts in pure DC. Let's see...
high impedance inputs - this is the easiest one to explain. A device with high-impedance input only allows a small, often negligible amount of current to flow into it, as if nothing is connected.
Imagine a 5-volt power source followed by an 10k ohm output resistor in series, and we put it in a blackbox. If you connected another 10k ohm resistor across the blackbox, you create a voltage divider, and the voltage across your resistor would be 5 x (10k / (10k + 10k)) = 2.5 volts, and a current of 0.25 mA flows.
But if you increase the value of your inserted resistor to, e.g. 10M ohm, now the 99.9% of the voltage is now across your resistor, and you get 4.995 V, and there is almost no current flows across the power source, as if nothing is connected, minimizing the "observation effect".
This allows you to measure the voltage without disturb it by drawing power from a circuit. Example:
ideal voltmeter - gives a voltage measurement.
ideal oscilloscope probe - just an ideal voltmeter that takes many measurements and display a line on a chart.
ideal ADC - just an electronic voltmeter.
ideal transistor switch and logic gate - output only depends on the voltage reading of the input, it doesn't absorb power.
ideal buffer and operational amplifier - measures the voltage at the input without affecting it, and creates a replica of this voltage by using its own power source.
impedence matching - This one is difficult if we're talking about reflections, S-parameters, and standing waves, but it's easy understand if we only talk about impedence matching for maximum power transfer. People often say that, to get the maximum power transfer, the impedance of the source and the load should be matched. But it's misleading. Theorem of Maximum Power Transfer is actually telling us how to choose our load, given a known source, not vice versa.
Imagine an ideal 5-volt DC voltage source. The maximum available power from such a power source is infinite, and only depends on the impedance of your load. By connecting an infinitesimal resistor, infinite power will be absorbed by the resistor.
Now imagine an ideal 5-volt DC voltage source followed by 10 ohm of output resistor in series. This is closer to a real power supply, or a signal generator, or a radio antenna, whose internal impedance sets an upper limit of available power.
What the Theorem of Maximum Power Transfer is telling is that, if we want to absorb the maximum available power out of this power source, our load must be equal to the source impedance, in this case, 10 ohm.
If our load has the same impedance as our source, the voltage across our load will be Vcc x R2 / (R1 + R2), or 5 x 10/(10+10) = 2.5 V, and the power is absorbed by our load is V^2 / R, or 0.625 W.
If our load is much smaller than the source, there will be more current but less voltage. A 1-ohm load will get a voltage of 5 x 1/(1+11) = 0.416 V, and absorb only 0.173 W.
If our load is much bigger than the source, there will be more voltage but less current. A 100-ohm load will get a voltage of 4.54 V, and absorb only 0.206 W.
It's simple if the following misconceptions are cleared.
* Misconception 1: If I have a 300-ohm load, the output impedance of the power supply should be matched to 300-ohm for maximum efficiency.
False. Maximum Power Transfer does not imply Maximum Efficiency. Efficiency is the ratio, Useful Power / (Useful Power + Wasted Power), and it has nothing to do with maximum power transfer.
Recall the example above, when a 100-ohm load is connected to a 10-ohm power source, the voltage across the load is 4.54 V, drawing 0.206 W. Meanwhile, the voltage across the internal impedance of the power is 0.46 V, drawing 0.021W. The efficiency is 91%. But when impedance is matched, 100-ohm load is connected to a 100-ohm power source, the efficiency is "only" 50%.
Often, the efficiency is the best when there's low power transfer. You don't waste a lot of power if you don't take away a lot of power.
* Misconception 2: If I have a 300-ohm load, (e.g. a headphone), the output impedance of the power supply (e.g. a headphone amplifier) should be 300-ohm for maximum power transfer.
False. Maximum Power Transfer is only about how to select our load to absorb maximum power when we have no control over a given source, not vice versa. In real-life, for non-RF circuit, if we have control over the source, the solution is to make the output impedance of the power source close to zero.
For a 5-volt DC source with a 300-ohm impedance, connected by a 300-ohm load, only 0.02W of power comes out because there's only 2.5 V across the load. But if we have the output impedance to be 0.1-ohm, 4.998 V is now across our load, and drawing 0.083 W of power.
This is called impedance bridging, and it's how power supplies and modern headphone amplifiers are designed - they are designed for maximum efficiency by minimizing their output impedance, while the headphones themselves only draw a little bit of power.
On the other hand, a radio receiver is a good example when we have no control over the source impedance, given the same antenna, it cannot be changed. In this case, the better our receiver is matched to the impedance of the antenna, the higher its sensitivity.
BTW, in RF-circuit, even if we have control over a signal source, we use matched impedance, in this case, the purpose is not to get the maximum power transfer, but for avoiding reflections, a completely separate issue.
* Misconception 3: 50-ohm coax cable has 50-ohm of electrical impedance.
False. The "50 ohm" and "75 ohm" for coax cable is its characteristic impedance, not electrical impedance.
This is when things get complicated and when we must think electrical signals as waves. But the other comment has a good explanation, so it won't be repeated. In short, when a wave hits an impedance discontinuity in its medium of propagation, some of the energy is reflected, so in an RF system, 50-ohm everywhere is desirable.
A small piece of lossless coax cable can be modeled as an inductor in series and a capacitor in parallel, it's the radio of the inductor and the capacitor that determines its characteristic impedance that we can match to avoid a discontinuity in the circuit, thus avoiding reflection. Or we can say that the characteristic impedance is the input impedance when the length of the lossless coax is infinite.
It's not an actual electrical impedance that we can measure by an ohmmeter. A 10-meter coax has an electrical impedance of less than 1 ohm.
Audio frequencies are a much more limited band than RF and it can be intuitive to look at a simple hardware consideration.
Such as your speaker's voice coil, which is a fairly low-resistance length of thin copper wire commonly wrapped around a hollow paper core an inch or two in diameter and enameled in place. The paper form is cemented to the center of the suspended speaker cone, with the coil itself free to move back & forth within the matched gap of the speaker magnet.
When the audio signal (which is a series of waves, therefore an AC voltage) comes in on the wires, the coil moves the cone in response to the signal.
DC voltage is not allowed to pass to this kind of speaker since DC would just displace the cone in a single direction for the duration of DC voltage application.
But when you measure the resistance in Ohms of an individual _8-Ohm_ speaker, it is always less than 8 Ohms, sometimes between 6 & 7 Ohms because regular Ohmmeters are measuring DC resistance using a DC battery inside the Ohmmeter as a reference.
The DC resistance of the coil is no different than it would be if the whole coil was unwound, it basically depends on the length of the wire and the diameter of the wire, similar to the way precision wirewound resistors are available for lower Ohm service. Thinner wire and longer wire has higher resistance to current flow.
But when audio is passing through the coil alone in free air, IOW no magnetic or ferritic core is present, there will be slight additional resistance to the audio AC (over and above the fundamental DC resistance measurable with the regular Ohmmeter), simply because of the wire being shaped into a coil. This total working resistance to AC is what is referred to as impedance. When a ferritic or magnetic core is in place, there will be that much more of a challenge for the audio signal since it needs to then reverse polarity against the inertia of the magnetic material at whatever frequency it happens to be operating at. This magnetic bump in AC resistance is significant and causes a nominal 6-Ohm-resistance speaker to operate as a characteristic 8-Ohm-impedance audio load.
Speakers do work really well with much higher than 8-Ohm voice coils, provided a matching amplifier is available. Practically this is not done since the speaker wires from the power amplifier to the speakers in a high-power system would then be required to carry audio AC voltages as dangerously high as the line voltage coming in to the amplifier.
So using the everyday low-impedance speakers we are stuck with, a slightly poor connection having an extra 1-Ohm of unwanted resistance, due to corrosion for instance, will result in that connection dissipating a significant percentage of the power passing through it, as heat, in many cases accelerating a corrosive source of power loss.
Such as your speaker's voice coil, which is a fairly low-resistance length of thin copper wire commonly wrapped around a hollow paper core an inch or two in diameter and enameled in place. The paper form is cemented to the center of the suspended speaker cone, with the coil itself free to move back & forth within the matched gap of the speaker magnet.
When the audio signal (which is a series of waves, therefore an AC voltage) comes in on the wires, the coil moves the cone in response to the signal.
DC voltage is not allowed to pass to this kind of speaker since DC would just displace the cone in a single direction for the duration of DC voltage application.
But when you measure the resistance in Ohms of an individual _8-Ohm_ speaker, it is always less than 8 Ohms, sometimes between 6 & 7 Ohms because regular Ohmmeters are measuring DC resistance using a DC battery inside the Ohmmeter as a reference.
The DC resistance of the coil is no different than it would be if the whole coil was unwound, it basically depends on the length of the wire and the diameter of the wire, similar to the way precision wirewound resistors are available for lower Ohm service. Thinner wire and longer wire has higher resistance to current flow.
But when audio is passing through the coil alone in free air, IOW no magnetic or ferritic core is present, there will be slight additional resistance to the audio AC (over and above the fundamental DC resistance measurable with the regular Ohmmeter), simply because of the wire being shaped into a coil. This total working resistance to AC is what is referred to as impedance. When a ferritic or magnetic core is in place, there will be that much more of a challenge for the audio signal since it needs to then reverse polarity against the inertia of the magnetic material at whatever frequency it happens to be operating at. This magnetic bump in AC resistance is significant and causes a nominal 6-Ohm-resistance speaker to operate as a characteristic 8-Ohm-impedance audio load.
Speakers do work really well with much higher than 8-Ohm voice coils, provided a matching amplifier is available. Practically this is not done since the speaker wires from the power amplifier to the speakers in a high-power system would then be required to carry audio AC voltages as dangerously high as the line voltage coming in to the amplifier.
So using the everyday low-impedance speakers we are stuck with, a slightly poor connection having an extra 1-Ohm of unwanted resistance, due to corrosion for instance, will result in that connection dissipating a significant percentage of the power passing through it, as heat, in many cases accelerating a corrosive source of power loss.
What a great article. I would love to see more hardware/electrical articles to balance out all the software ones.
Thanks, this is my intention.
Hardware and electronics was an important area in computer engineering, and many early hackers were hardware hackers. Today, the field has shifted as software development at large, but hardware and electronics is still an indivisible part of the hacking community. It can be seen from the fact that EECS is still taught at schools, the fact that embedded electronics is having increased popularity due to IoT, and the renewed interests within free and open source community.
I'll keep submitting more hardware/electrical articles which I find interesting.
Hardware and electronics was an important area in computer engineering, and many early hackers were hardware hackers. Today, the field has shifted as software development at large, but hardware and electronics is still an indivisible part of the hacking community. It can be seen from the fact that EECS is still taught at schools, the fact that embedded electronics is having increased popularity due to IoT, and the renewed interests within free and open source community.
I'll keep submitting more hardware/electrical articles which I find interesting.
For those nostalgic over classic 90's websites, the RF community has you covered:
[1] rfcafe.com
[2] antenna-theory.com
Yeah, it's nice to see a website that loads instantly, is accessible for those with disabilities, doesn't require executing code, and who's design is based around content rather then the other way. The amateur radio community is generally pretty good at this.
[Remember to add the "http" part to trigger the magic autolink http://rfcafe.com and http://antenna-theory.com ]
I guess the follow-on might be "Why 50 ohms for 10b2 Ethernet, which isn't microwave band?"
Ethernet transmission is broadband not tuned like a radio but the same equations as presented in the article hold across most of the bandwidth (where the skin effect holds). Actually, I am not sure whether the article is referring to tuned or broadband communication but in a sense it is irrelevant because practically, a large proportion of the energy transmitted in a modern broadband system is in the skin effect domain. If this wasn't the case, the system would be highly inefficient in the use of available bandwidth.
The carrier frequency for 10b2 is 10Mhz.
It's not a modulated signal encoding. There's no modulation, and so there's no carrier.
The bandwidth is probably 20MHz, due to the Manchester encoding.
The bandwidth is probably 20MHz, due to the Manchester encoding.
It is Manchester encoding on a 10MHz clock signal. Perhaps carrier isn't quite the right word.
I take back by correction. Thanks, and sorry. Manchester encoding really is a form of phase modulation (BPSK). It makes sense to think of it as a 10MHz baseband signal modulated by a 10MHz carrier, and therefore having a 20Mhz bandwidth.
Because 50 ohms is an universal standard for most radio frequency equipment, not just microwave. It (often) doesn't make sense to invent your own coax, connectors, transformers, and chips. At the end of the day, the selection had its rational back in the days, but it eventually became an agreed convention that everyone just follows.
The convenience of existing cables and connectors seems to be the motivation.
> The first Ethernet used 9.5-mm coaxial cable, also called ThickNet, or as we used to curse it as we tried to lay out the cables, Frozen Yellow Snake.
> To attach a device to this 10Base5 physical media, you had to drill a small hole in the cable itself to place a "vampire tap."
> So-called Thinnet (10Base2) uses cable TV-style cable, RG-58A/U. This made it much easier to lay out network cable.
https://www.hpe.com/us/en/insights/articles/the-birth-and-ri...
> The first Ethernet used 9.5-mm coaxial cable, also called ThickNet, or as we used to curse it as we tried to lay out the cables, Frozen Yellow Snake.
> To attach a device to this 10Base5 physical media, you had to drill a small hole in the cable itself to place a "vampire tap."
> So-called Thinnet (10Base2) uses cable TV-style cable, RG-58A/U. This made it much easier to lay out network cable.
https://www.hpe.com/us/en/insights/articles/the-birth-and-ri...
The neat thing about vampire taps is that you don't need to disconnect the cable so it could be done on a running bus topology network without interruption. If you did it right...
" So-called Thinnet (10Base2) uses cable TV-style cable, RG-58A/U. This made it much easier to lay out network cable."
That's not quite right. Cable TV type cable is 75 ohm RG59. I guess maybe they just mean both were coaxial and relatively flexible.
That's not quite right. Cable TV type cable is 75 ohm RG59. I guess maybe they just mean both were coaxial and relatively flexible.
Well, they didn't say 'cable TV cable'. They said 'cable TV-style cable'.
Because they were using existing BNC connectors and cables.
With very modern point to point microwave systems there are zero 50 ohm coaxial connectors at all. The radio unit has a direct waveguide flange interface and mounts directly on the rear of a dish.
What where the old pigtail connectors to TVs?
Was that 300 ohm to 75 ohm?
Was that 300 ohm to 75 ohm?
Short answer, yes. Techie answer is that 75 ohms is best for receiving signals. For air dielectric coax, 77 ohms provides the lowest loss of signal. '...the standard coax impedances is 75 ohms because that is the impedance you end up with after you run a 300 ohm 1/2 wave folded dipole impedance through a classic 4:1 hairpin balun. You've got folded dipole, you gotta have a balun, and baluns don't come any simpler than the hairpin balun (and there isn't any easy way to get to 50 ohms from 300 ohms)'. Modern (US) coax cable is foam filled PTFE which works best at 50 ohms.
Yes -- the ones with two flat spade connectors on one side of a box, and a coaxial screw-in connector on the other.
Interestingly for coax, as the frequency approaches DC (say a few 100 kHz), the Z0 jumps slightly.
Here is a 1934 article as an example, long before anybody had even named microwaves:
https://archive.org/details/bstj13-4-532
75Ohm gave the lowest loss and thus the longest distance between repeater-amplifiers on the trans-continental carrier-telephony coax-cables.
These same cables (and their brethern: microwave links) also carried television, thus enabling the first "television networks" as opposed to "television stations".
In many cases even the stand-alone television stations paid AT&T to connect their down-town studios to the hill-top transmitter.
So television uses 75Ohm because AT&T did.
And AT&T did to minimize the number of repeater amplifiers across USA, because 75Ohm has the lowest loss.