12 ms·
Why 50 Ohms?
- tyingq 7y agoI guess the follow-on might be "Why 50 ohms for 10b2 Ethernet, which isn't microwave band?"
- fsh 7y agoBecause they were using existing BNC connectors and cables.
- bcaa7f3a8bbc 7y agoBecause 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.
- whoopdedo 7y agoThe 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-rise-of-ethernet-a-history-1706.html https://www.hpe.com/us/en/insights/articles/the-birth-and-ri...
- tyingq 7y ago" 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.
- godbyk 7y agoWell, they didn't say 'cable TV cable'. They said 'cable TV-style cable'.
- jacquesm 7y agoThe 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...
- k0stas 7y agoEthernet 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.
- tyingq 7y agoThe carrier frequency for 10b2 is 10Mhz.
- gugagore 7y agoIt'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.
- tyingq 7y agoIt is Manchester encoding on a 10MHz clock signal. Perhaps carrier isn't quite the right word.
- gugagore 7y agoI 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.
- IndrekR 7y agoThere 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-ohms.htm http://www.rfcafe.com/references/electrical/history-of-50-oh... [2] https://www.petersenproducts.com/Copper-Tubing-Sizes-s/1979.htm https://www.petersenproducts.com/Copper-Tubing-Sizes-s/1979....
- XorNot 7y agoTrying 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 https://www.thesprucecrafts.com/why-isnt-a-2x4-a-2x4-3970461
- heavenlyblue 7y agoTechnically HDD manufacturers are doing exactly the same.
- Stratoscope 7y agoCan you explain what you mean?
- gugagore 7y agoHDD 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 https://en.m.wikipedia.org/wiki/Kibibyte
- eebynight 7y agoWhat a great article. I would love to see more hardware/electrical articles to balance out all the software ones.
- segfaultbuserr 7y agoThanks, 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.
- dillonmckay 7y agoWhat where the old pigtail connectors to TVs? Was that 300 ohm to 75 ohm?
- dsr_ 7y agoYes -- the ones with two flat spade connectors on one side of a box, and a coaxial screw-in connector on the other.
- Pick-A-Hill2019 7y agoShort 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.
- jojobas 7y agoCoincidentally, 50 Ohms is also impedance of a typical whip antenna near ground.
- mNovak 7y agoFor those nostalgic over classic 90's websites, the RF community has you covered: [1] rfcafe.com [2] antenna-theory.com
- gus_massa 7y ago[Remember to add the "http" part to trigger the magic autolink http://rfcafe.com http://rfcafe.com and http://antenna-theory.com http://antenna-theory.com ]
- superkuh 7y agoYeah, 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.
- madengr 7y agoInterestingly for coax, as the frequency approaches DC (say a few 100 kHz), the Z0 jumps slightly.
- phkamp 7y agoThe choice of 75 Ohm is documented in ultimate detail in Bell Systems Technical Journal. Here is a 1934 article as an example, long before anybody had even named microwaves: https://archive.org/details/bstj13-4-532 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.
- jacquesm 7y agoI'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.
- walrus01 7y agoWith 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.
- exabrial 7y agoI'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.
- rocqua 7y agoThe 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.
- MayeulC 7y agoIt 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 https://en.wikipedia.org/wiki/Impedance_analogy [1]: https://en.wikipedia.org/wiki/Medical_ultrasound https://en.wikipedia.org/wiki/Medical_ultrasound [2]: https://en.wikipedia.org/wiki/Dispersion_(optics)#Dispersion_control https://en.wikipedia.org/wiki/Dispersion_(optics)#Dispersion... [2]: http://www.m2optics.com/blog/bid/61431/Chromatic-Dispersion-in-Optical-Fibers http://www.m2optics.com/blog/bid/61431/Chromatic-Dispersion-...
- aj7 7y agoThe 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.