Why doesn't a 75 Ohm cable measure 75 Ohms?
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- čas přidán 6. 05. 2023
- We investigate a 75 Ohm TV cable to see where the 75 Ohm measurement comes from. We also look at reflections in the cable and why you cant just connect more than one device like a TV to a signal source without loosing signal power.
- Věda a technologie
Thanks for this. I've never really understood, why termination resisters were required. You've explained in a way that is both objective and easy to process.
Thanks!
I must say, after the (incredibly excellent) explanation of the reflections when open and shorted, and after the WONDERFUL direct practical demonstration of "Look, if we put a resistor at the other end that 'matches the rated impedance', the reflection disappears..."
When you then brought back the longer pulse and explained "The first bit is before the reflection, so we first only see the characteristic impedance as if the line were infinitely long"... I thought to myself, "This video is genius!"
Really love the super practical aspect of it! I've learned about the theory of transmission lines before, but it's amazing to see it so concretely demonstrated with real, simple, physical electric equipment like this.
Great work! Thank you for your clear explanations! ❤
I so appreciate your comment - thanks! I'm quite thrilled at how well this approach to explaining it has worked for so many viewers. I did a very similar experiment in a lab as a student and its one of the very few that I was so taken with I have always remebered. It was great fun to do it again after so many years.
More things like this to come - mirrors, reflections from transparent objects like glass, speed of light ... . All with as simple equipment as possible.
Yeah, I loved that bit.
I am a retired EE. I really like the way you explained the reflection polarity at the end of the line. I never thought about it that way. Smith charts don't give you an intuitive understanding of what is going on. Thanks!
Thanks - I really appreciate that! I was so lucky to have a number of great EM Profs when I was a student and thats where I got that intuitive approach from.
I actually do not understand that explanation. Is not that the free wire acts like a crude capacitor and the shorted end makes a loop for an inductance? Inductance makes a counter voltage. Capacitor just charges up and send back the charge when voltage is zero.
Excellent video! Been in the field for 40+ years and that is the BEST explanation I've seen for a neophyte.
Thank you so much! I was really trying to do it without the math to make it understandable to almost anyone.
That is brilliant: I now understand what I was doing for all those years cabling up the college where I worked with thinnet (50 ohm) coax cables! Thank you!
Your welcome! For what its worth I did my share of thinnet way back as well. Seemed so fast back then when files were small :)
Absolutely awesome demonstration on the importance of impedance matching! Not what I was expecting when I clicked on this video but I'm glad I did. Thank you and subbed :)
Thank you so much! Welcome abord to EM Videos!
Still new to ham radio and working with 50 Ohm coax; this really helps me understand what's going on with the signal. Thank you so much! 😎👍
What cool hobby - something I always think I should get into when I have time. I have done long range digital HF communications at work in the past - quite amazing the distance that can be achieved even just a few hundred watts if the conditions are right.
Just so that anyone reading this understands, while my example was 75 Ohms, what I did also applies to 50 Ohm cable or any other characteristic impedance cable.
At some point I will cover SWR and quarter wave transformers to match different impedance's which is also pretty neat and related to this. Look it up if you haven't already! Thanks for the comment!
I was a metrologist in the Air Force for 6 years. I always wondered why the cables were 50ohm or 75ohm. It’s been about 13 years since and I finally understand. Thank you.
Your welcome! Glad I solved that mystery!
@@ElectromagneticVideos you're*
:(
@@NoNameAtAll2 Do you have some videos?
@@NoNameAtAll2 god not the grammer not zee. Math and physics is his thing. We are not here for the spelling content!
@@rwood1995 I can’t imagine how you native english speakers can mix up your and you’re. I know it’s pronounced the same, but with a bit of logic you can differentiate them.
Finally, now i understand reflection in even other types of cable like RS485 networks. But also the whole 75 Ohm cable spec was unclear before. Thanks, this was a clear and complete explanation! Edit: Just fixed the Afri-English 😆
Thanks! I'm so pleased the video helped provide an explanation of reelections and characteristic impedance.
Thank you for this detailed video. It's always useful to know why you should use splitters and what they actually do.
Your welcome! Yes! Its a lot harder to justify the nuisance of a splitter without the "why".
"That's one of the greatest videos I've ever seen. Anyone who wants to deep dive into High-Speed PCB design should watch this video and the one about measuring light speed. I truly admire the video creator's efforts to expand our knowledge."
Wow - thank you so much! High-Speed PCB design - I should actually dig out one particular circuit board from years ago where I designed a couple of long impedance controlled transmission line traces for 100Mbps twisted pair Ethernet which is a great example of transmission lines on circuit boards.
I know this is an obscure topic, but great job explaining it in simple terms without fancy equipment!!!!!
Thanks! Keeping things simple was hat I was trying to do!
@@ElectromagneticVideos I think your 2ch HP scope is quite fancy 😊
@@sophya5796 Actually it is even for a 25 year old scope! I specifically bought it (used) a few years ago troubleshoot some glitches on an i2c bus - it did a wonderful job on that and many other things.
Something like a 100MHz analong scope would be fine for this demo. Or slower with a longer cable.
Such a good demonstration of transmission line theory and conservation of energy. They should use this example in EE undergrad courses!
Thanks! Actually the video was inspired by a lab a million years ago whne I was an undergrad. I always thought that lab was an eye opener!
@@ElectromagneticVideos come to think of it, I just took a class where we did a very similar demo. Since the class was online though, the voltage waves were simulated in Matlab’s Simulink. If you have any more demos from a million years ago, I’d love to see them!
@@kurtttttttt More million year old demos to come :) It get having to use simulations for online courses, but I always feel you get a better sense of things with real experiemnts. And they are more fun!
For some reasons this bit of TDR part of my electronic course at uni always stuck with me. Good to "reflect back" the explanation of this again.
And also I better put on the the to do list, get upto the loft and replace the two bits of cables tied together 😅 and put a splitter in. 👍 Thanks for a informative video.
Glad you found it useful! What I remember about the university lab we almost always did the lab before the theory which was too bad in terms of understanding it.
You know, if your setup works, I wouldn't go to the effort to change the connection to a splitter... "dont fix it if it aint broke" :)
I've used 75 Ohm BNC cables for networking and then digital audio cumulatively for 40 years and never got an explanation of it. Thanks for the clear and well thought out video! I do feel its better to say that 'cable capacitence rounds off the square wave signal' rather than saying it 'messes things up'. Square wave is every harmonic of a sine wave so it makes sense that the tiny capacitence will filter the upper harmonics away. Great video though - thanks! :D
Well I'm glad you liked it and found it informative.
In terms of loosing the higher frequency harmonics, for a well matched/terminated system, the cause is the changing characteristics of the components of the cable. That includes the insulation plastic between the center and outer conductor becoming more RF absorbing at higher frequencies, , and the increasing effects of skin depth making the conductors more and more restive as the frequencies go up. That being said, many cables today commonly used in our or wireless devices work remarkably well in the GHz region although the spec sheets generally show the loss/distance increasing with frequency.
This video has answered several questions I've had as a HAM radio enthusiast.
Thank you for the wonderful explanation.
Thanks! I'm so glad it was useful and answered a few things.
That’s a very clear explanation, great work ! ❤
Thank you so much! I really appreciate it!
Absolutely fascinating, packed with so many facts that I did not know.
So glad you found it so interesting! It is really stuff!
I'm getting into amateur radio, and setting up my antenna. This is how I got into impedance, and impedance matching, which was a mysterious topic to me. Your excellent video made this so much clearer. Thanks for sharing it.
So glad it helped understand impedance matching. Since you are getting in to amateur radio, consider getting a NanoVNA. For about $100 this amazing devices does the job of what used to be a multi-thousand $ piece of test equipment. It great for measuring impedance of things like antennas, determining SWR at various frequencies etc. It really brings all that stuff to life!
I have never understood reflections either in college or technician explaining to me and now I know what it is because your class were clear, objective and easy to me understand. CONGRATULATIONS, Thank you.
Thanks so much! I'm glad you found my way of explain easy to understand - certainly what I try and do!
Awesome video!! I enjoyed every second of it! Explained well with also a real demonstration!! 👏👏👏
Thanks you so much!
A passive splitter (built with a resistor network) does not increase the output signal amplitude, as was stated in the video. In fact, it results in lower amplitude compared to just connecting the three cables together:
When connecting the three cables without a proper splitter, 1/3 of the signal is going to each of the two outputs, and 1/3 is reflected back.
With the splitter, only 1/4 of the signal is sent to each output, and half the signal is absorbed by the resistors in the splitter.
Even though the splitter reduces the amplitude of the signal, it still increases the signal quality (and in case of TV leads therefore to better reception) by eliminating the reflections.
Very nicely stated. I will add you can get non-resistive splitters generally with limited bandwidth that do a very good job of both impedance matching and transferring close to have the signal power to each of the two outputs.
Great explanation, well organized presentation! Somehow you targeted my level of comprehension to explain something that I've been struggling with for a long time. Thanks!
Well thank you Mike! I'm glad that somehow the way I presented it worked for you. The video was based on a lab experiment I did as student and I have never forgotten it. Hopefully soon something similar with light ...
I've never had the chance to learn some of these fundamentals despite making tons of professional cable/dealing with signals in this world. Thanks for making a video about it
Your welcome! Glad you found the fundamentals behind things like impedance and termination interesting!
This is an awesome demonstration of transmission lines. I'm going to make a lab for my students based on your methods. Such a powerful use of CZcams!
That made my day! This was a reduced version of a lab I did as an undergrad. If you want to make it more challenging, non-resistive loads (capacitor or inductor) can be used at the end of the lab. I think we even had to identify the contents of some mystery loads.
@@ElectromagneticVideos I stress the concept of Max Power Transfer beginning on the first day of the course. These are technology students, so not sure how they would do with the reactive dummy loads, but will consider it. I would also look to have them identify the impedance of other cables (twin lead -300ohm, and come 50ohm coax) to validate the method. This brings home that the concept of line impedance is really and not something made up.
I also like the time-domain measurements that brings home the concept of matching and reflection as it relates to open and shorted lines.
Last thing, I'm curious if given enough of cable run, could the students also identify the velocity factor for a specific cable?
I use an old Anritsu Sitemaster in TDR mode at work to find breaks, although it usually isn't required. The break is usually in the vacinity of a completely oblivious plumber or domestic electrician with a hammer in his hands.
"The break is usually in the vacinity of a completely oblivious plumber or domestic electrician" oh that's so funny! And I don't doubt it for a moment.
Plumbers make sure that electricians have jobs.
@@jannejohansson3383 :)
Got to love those. We had similar problems with crushed flexible waveguide sections on B-52s from crew chiefs (usually) standing or climbing on them. All of them were marked in day-glo yellow paint "NO STEP". You would think that would be easy to read.
I’ve been selling Video cables for 15 years , Finally Some one explain it in such a clear and easy to understand way , Thank you sir
You welcome! So glad I was able to help! This was actually inspired by a lab I did in a course I took as a student in university. I always thought looking at reflections like this really helps understand what is going with matching and reflections.
This is the best explanation of characteristic impedance I've ever heard. Well done!
Thanks! Apreciate that!!!
Very neat, now I understand the importance of terminating open ends of unused outputs with 75 Ohm terminal resistors!
Glad you liked it. A warning thought - different cables have different impedance. 50 Ohms is also very common, so you have to make sure you are using the right value terminator.
Great job explaining this all! It makes it easy to understand how detectors for broken or shorted cable actually work and how network wire analyzer measures the length of the wire.
Thank you so much - really appreciate your comment. Have you ever used it for non-RF cable? I have wondered how well it works with cable not designed or installed with constant impedance in mind.
@@ElectromagneticVideos I haven't but if I remember correctly, ethernet cabling has effective 100 ohms impedance and that's the most commonly used cable for such tests.
I would expect that you can measure any cable type as long as you can tune the signal generator: you only need steps in the signal. The speed of light is always close enough to 1.0c that you get distances roughly correct without knowing the true impedance.
And since the sign of the voltage is enough to separate broken wire from shorted wire, that's good enough, too, without knowing the true impedance.
@@MikkoRantalainen Your right about the Ethernet cable! I should really experiment with some 14/2 house wire - I wonder what its impedance is (should be easy to measure) and how constant it is when it is near other cables since it has no shield or twist.
@@ElectromagneticVideos house wire is intended for very low frequencies (50-60 hz), so no thought is given to impedance.
More interesting would be twisted pair cables used for lighting systems (DMX).
@@TomCee53 Just looked up DMX. Looks like its similar or even repurposed RS-485 so maybe 120 Ohm cable?
Superb practical demonstration of the importance of impedance matching, and the impact of not performing impedance matching correctly. Needed another project to work with, and I've got an idea for making an impedance measuring tool now. Thank you.
Thank you so much! If you want an incredibly cheap but capable tool for that sort of thing, look at the Nano VNA if you havnet already seen it - does smith charts, measures RF impedance etc, all at a cost of about $100 or so. There are numerous variants of it available - I got one that was in $150 range - larger screen and up to 3GHz operation - best RF toy ever!
Great demonstration! Very informative. I have always known, since the days of MFM encoding for computer hard disks, SCSI bus interfaces, coax network cabling, PLC buses, and PROFI instrument bus, that either end of the transmission line needed resistors installed or activated or reflections would result and therefore system function would be unstable and unpredictable; but I never understood the mechanism of the reflection, nor have I ever seen the time delay introduced by the resistor or extended cable length. I just assumed that the added length diminished signal strength accordingly, it’s impact on timing never occurred to me. This fact would/could affect analog systems, but would seem to affect digital transmission even more so.
Thank you so much! MFM and SCSI - I remember those so well, and squeezing 50% space out of a disk with RLL controllers. It actually does have a significant impact on analog and there will be a future video on that topic. In the analog world, consider reflections at different frequencies (=wavelengths) going back and forth on the transmission line if not perfectly terminated. At certain wavelength the reflections may produce constructive interference = greater signal amplitude, at others, destructive and lower amplitudes. So get filtering effects increasing and decreasing the power at certain frequencies. Often a nuisance, but sometimes used to advantage!
@Tiberius Tchaikovsky My first computer featured a 20 Megabyte 1/2 height, which I upgraded to a 30 MB full-height drive. I loved that drive, but like the Seagate, it failed prematurely, in my opinion. Thanks for responding, and congratulations on recalling the model number. 🙂
@Tiberius Tchaikovsky I have several examples of the form factor you reference, with magnificently powerful magnets. I think you can see where I’m heading. Too expensive in that larger format (and then they went to 2.5”, and now just memory chips). I’m like you, holding a full-height, 5.25” disk drive, you felt that you had something substantial. But then I worked with techs who remember the original ‘Winchester’ drives big as washing machines. People have turned their disks into coffee tables, so there is that; progress and all. Again, thanks for the response; enjoy the conversation.
Excellent video. Helps in understanding the importance of the terminating resistors in all the communications in the industry I have worked with for almost 30 years now. From Allen Bradley DH, Remote I/O or even if it was ControlNet using Coax all those reflections from an open line could cause a lot of comm issues for sure.
Thanks! Yes - so important but also something that is really not understood by most except people in communications or ham radio types. Since you have been doing this as long as I have, do you remember when when Ethernet was really Ethernet and used 50 Ohm coax in both thin and thick formats? And 10mbps was so incredibly fast who could imagine needing anything more!
@@ElectromagneticVideos
I really don't know much about the earlier format. I just did not understand much about it then. You are leap years ahead of me in the science of it and the signals. I absolutely thought 10mbps was more than we would ever need since even hard drives back in the 90's were only 250MB and back then I was using RS232 protocol for most systems at 9600 to 57.6Kbps Max.🤯
@@ThriftyToolShed I'll have to try and get some old Ethernet cards and do a video showing the signals. The idea was (in the most primitive form) all the computers were attached to one long coax (the Ether) and to send data they simply wrote data chunks (packets) on the cable. If two computers wrote at the same time it was detected and re-transmitted at a random time later. Cheap, simple, and effective as long as the network wasn't close operating at full load. And originally came from U of Hawaii where they used satellite link operating in a similar way to link the Islands.
I sure remember pushing RS232 it to the higher speeds. And hard drives - paid $1000 for a 20M one for an early PC - yikes!
@@ElectromagneticVideos That would be a great video!
@@ThriftyToolShed I think so - thanks for making me think of it! I'll have to look for some old network cards and motherboards to do something like that. I just did a quick scan - not much of that stuff available anymore.
Awesome! I could never understand fully the signal and reflection and why use termination resistor of a certain ohm value till now. Great work on the explanation!
Thanks so much! Glad my explanation was helpful!
Fanbloomintastic, this was something that had foxed me for decades, excellently explained and demonstrated many thanks!
Thanks you so much! Glad to be able to shed some light on it for you!
I taught myself electronics pre internet. The hours that I spent fretting and poring over equations and diagrams trying to understand this topic. I could have made so much more progress if born 30 years later. Let alone the price of equipment.
p.s. channel w2aew has some videos that will complement this explanation very nicely and he covers many other topics too.
You know I often wonder what things would have been like for me if all the internet resources were available. On the one hand the weath of resources we have today making understanding stuff so much easier. But then the constant distraction students have with messaging etc.
And yes - the price of equipment and parts was such a killer. I used to go out on large item garbage day and pick up things like old TVs and radios and strip them of parts like resistors and capacitors. Restors now cost 100 for $1 or better!.
Just searched w2aew and found it and subscribed. Thanks for pointing it out to me!
Yeah but if you were born 30 years later you wouldn't know what good music sounds like. Or good movies.
@@johnclawed Ha! Yes!!!
Would have been interesting to put a resistance box on the end of the t-line to watch the effect of a range of values around 75 ohms on the scope.
I was so close to doing that with a potentiometer. In the end I decided the video was just getting too long. I will do a future one about Ethernet twisted pair so it may be a neat thing to do in that video.
You have a very interesting chanel, and you explained this better than any of my proffesors once did.
Thanks! I was fortunate to have great profs in the subject who explained it to me!
Thank you very much! You explained characteristic impedance very clearly and I learned so much from your video.
Your welcome! Glad you felt seeing the video was worthwhile!
My learning about these things started, and nearly ended with the math. Luckily there was the lab exercise that returned the issue to a practical level! After that, the understanding has been just like you explain it. But the real "fun" starts when you add some stubs at different distances from one or the other end of the cable. In effect, you can start changing or matching different cable impedances. Just a hint, the antenna impedance is not necessarily the same 75 ohms that your cable is. By the way, in addition to the end termination, it is possible to deduce the cable impedance from the attenuation of the supplied signal by the series resistor. You obviously had 75 ohm source, as the 75 ohm cable received nicely half of the signal generator output pulse amplitude. Good job in any case!
Thanks! Your points are bang on! I purposely did not get into all the nuances of antenna impedance and mismatches to try and keep the video accessible to everyone with the math kept at a minimum. You mentioned stubs - I do intended to do a video on quarter wave transformers in the context of anti-reflection filters on glass sometime, so that sort of thing is not going to be forgotten in terms of future videos.
this adds confusion for me, so the cable resistance as shown in the video is not 75 ohms, and he is simply matching the signal generator impedance? If this is the case (as I understand it), then why are we calling the cable 75 ohm? Is this so we hook this particular cable to a signal generator with 75 ohms of output resistance, in series with the 75-ohm termination resistor? Thanks!
@@nv2134 Sorry that the video further confused things. Let me try and briefly explain:
If you had an infinity long cable and measure the resistance at the end you would get 75 Ohms. Thats the definition of characteristic impedance. That resistance is the cable appearing to absorb power as it carries the energy away along the cable at almost the speed of light.
With a shorter line (in my case 100 feet) the power going down the line hits the open end and gets reflected back. When it gets back to the source end (the meter) it raises the voltage to what we would have seen with an open circuit. In other words it takes a few hundred nanoseconds for before the refection arrives and we find out that the end is open (or closed or anything else). In that brief time before the refection gets back, we can measure the impedance of the line and it is 75 Ohms.
Since an infinite line looks like a 75 Ohm resistor in every way, and being infinite never sends a refection back, we can also say a 75 Ohm resistor looks like an infinite line. So of we put a 75 Ohm at the end of shorter line (ie 100 feet) the line thinks its connected to a further length of line that is infinity long and so no reflection comes back.
Hope that helps!
I came to watch that video telling myself "I know the answer but I want to see how it's explained.", well it turned out I also learned a lot! Very well explained, thank you! It reminded me the video from Veritasium about transmission lines, I also recommend!
Thank you! I'm glad you found my explaining worked - I always find that with each of us havinig a slightly different prespective on things, sometimes one or another explanation works best. The veritassium poynting vector one? Its another neat look at similar stuff - a bit missleading but that may just be my perspective. I am actually thinking of recreating it with an experiment. Course being a small channel I cant afford 300,000 km of transmission line so it will be more like 300 feet :)
Great explanation. In earlier days I used this for fault localization in telephone cables between our office buildings
Thanks! Cool that you actually used it!
This was good. I'll have to watch a few times to absorb it but it's something i've never really dug into but wanted to know as a radio enthusiast
Glad you liked it. Certainly keeping reflections to a minimum will help with both receive and transmit powers!
Very good and clear explanation to this topic. Help me a lot about what’s really going on.Thanks.
Thanks! Glad it helped!
Excellent demonstration. Perfectly clear. Thank you.
Thanks so much!
Sir, thank you! A great contribution to the world!!! It really is - I'm not joking. How many people have wondered about that and have found no decent explanation ... now we can get an excellent explanation. Once again: thanks!
I am so honored to have received so many comments like yours. Its funny - I was rushed doing the video and its certainly not the quality I wanted it to be and never would have guessed it would get the attention that it did. It is based on a 2nd or 3rd year lab experiment I did as a student decades ago. My challenge in this video was to try and show how transmission lines works without the math - I'm thrilled that I seem to have succeeded! Thanks so much for the kind comment!
Excellent explanation. Your are a teacher par excellence, Sir!
Thank you so much! Its always a challenge to explain a mathematically intensive topic in an intuitive way with minimal math - I'm thrilled to apparently have succeeded!
I should have subscribed when I saw your video the other day. Great job once again.
Thank you so much! Just so you know, I try to do a video a week but like right now, work or other things occasonally gets in the way. I do have one more video almost edited - with luck I'll get it up before I get back home. Thanks for subscribing!
Wow. This has been bothering me for YEARS. Only, not intensely enough to wade into div/grad/curl type of explanations (that explain nothing really)
So this very real, practical explanation is a dream come true (I pinched myself - ouch - to verify)
THANKS
Subscribed immediately
"not intensely enough to wade into div/grad/curl" :) Actually the neat thing is you can do the transmission line math with plain old differental equations becuase its really a one dimensional object from the wave standpoint.
Butther eis nothing like seeing a real experiment to understand as you said. Glad you liked it and welcome to my videos!
Wow, thank you very much sir. This was a mystery to me. I was looking for the why's of the signal reflection and characteristics impedance. As this concept forms basis of signal cabling.
Mind-blowing explanation. Thanks a lot for making it easier to understand than what it really is!!
Wel thank you so much! "Easier to understand than what it really is" well maybe easier to understand that with the full blown math? Even with the math I find a good experiment like this really helps me undertand what going on.
Even though I already had a bit of an understanding of what was going on, it's always good to get a new explanation. You never know when something new might click. And, indeed, the need for a splitter was never something I'd actually ever attributed to impedence matching.
"it's always good to get a new explanation. You never know when something new might click." How true!!!!! I know for me often one explanation really give me a sense of how something works, and another (equally good one) may not click with me.
@@ElectromagneticVideos❤
Great quality of explanation. I am missing experiment with two 75 Ohm resistors on the ends of split cable
Thanks! That part was to show that if you just splice cables together, it looks like two resistors in parallel = half the impedance and you get a reflection. So you need an impedance matching device like a splitter sending the signal to two TVs for example.
Good explanation, passed it through to some friends, who still thinks that cables from 50R suffice for 75R cables :)
Thanks! Just threw 50 Ohms and 75 Ohms into the reflection equation and the mismatch results in a 20% voltage reflection. Not a huge amount, but enough to loose signal power and smear things if a mismatch on each end of the cable.
I have been to a 2 day course about TDR, and this video taught me more
Thank you so much! Glad you found it useful. I`m complelty fine with any instructors playing the video to a class or distributing the link so if you would like to suggest it to the instructor, or suggest an in class demo is done based on the video, please feel free to do so. Any number of free online CZcams video downloaders could be used to ge the video file so a class does not get subjected to the ads that are normally inserted.
Amazing explanation, I don't think I've had this explained to me before, thanks.
Thank you so much! Glad you liked it!
Really neat to see an oscilloscope in use after seeing one at restore
I remember the one you probably thinking of, If must have sold and hopefully found a good home. This is a much more modern one - only 25 years old!
You are a passionate. Thank you very much for your explanation and actual testing. 👍
Thank you! I have always loved combining theory with actual real stuff - it really brings it to life!
Excellent! Thanks for spending the time putting this together
Thanks! Really appreciate your comment!
Very nice video, studying for a transmission lines exam for next week!
Glad you enjoyed it! You must be taking a 2nd or 3rd year EM course - those were some of my favorite courses ever. Good luck on the exam!!!!!
I remember when I was in high school I was always told fibre optic cables were better than coaxial because “light is faster”, which I now know is not the true story, and it’s actually because of these TL characteristics that limit the bandwidth of a cable. Your video is so easy to understand, I wish I was shown this when I was younger
Glad you found it easy! Actually fibre is more similar to coax than may be obvious. The speed of light in glass is ususally 2/3 that of light in free space, so similar to the coax and for the same reasons! And, optical fibers do have bandwidth limitations too, but at the much higher frequencies of light, bandwidth is so huge its mush less restricting. And - relfections at the ends and matching impedance to prevent loss of optical power is all the same stuff just done with glass or other transparent materials. I have a few videos planned with all of that stuff - stay tuned!
This is an excellent explanation, well done mate!
Well thank you! Nothing better than the explanation being called excellent!
I used to use a time domain reflectometer (TDR) on network cables back in the day. Length, kinks, shorts, and/or other issues showed up as well as the location. Nice tool.
So how obvious were things like kinks? Did the cable really have to be bent to show up?
@@ElectromagneticVideos Depends on the sensitivity of the instrument. We used a USM-430 TDR in the military that when properly calibrated could be used to located impedance changes as low as 10% of the nominal value. Larger discontinuities generated larger return pulses on the CRT display. It was also capable of measuring phase shift within .1 degree if memory serves, and we used that function to check the phase matched RF cables between the receivers and processor on the F-4G.
@@ElectromagneticVideos
Yes, bends would show up as changes in impedance.
It was pretty usable, it had a scope/screen, but also an option to printout a ribbon of thermal paper with length graduations along it. Every 2 1/2 meters, if a vampire tap/connector was present, there'd be a little blip. A sloppily performed tap may short, effectively ending the cable as for as the TDR would sense (as I recall, any signal traveling beyond the short or it's echo would be eliminated by the short).
Distance to a node does have limited value. A story I can share is of a Sun OS "pizza box" (pre Slowaris) was recycled as a licence manager for a large distributed CAD site. IT lost track of were the little box was and when it was time to rebuild the Kernel, they tried to find it to no avail. Fearing bringing the LMS down and not being able to bring I back up the machine ran for well over 400 days before a custodian found "an old computer" hidden above a closet on a drop ceiling panel.
Of course, a TDR doesn't tell you the MAC or IP of anything on a cable, just reflected signals/reactance.
@@RobertLBarnard So print a measing tape - that something I have never heard of. Interesting about taps going bad. I didnt come across that with thin ethernet but sometimes a cable connector would come off - either from the cable or the BNC not being attached properly. I have a bunch of BNC Ts from the old networking days - will have to see what type of reflections they produce whne attached to an old network card.
A Sun hidden in a drop ceiling - thats a new one! I remeber those pizza boxes well - whne I was a grad student Sun was the standard workstation in our lab. How great they seemed back then and years ahead of PCs in terms of performance and a window system - wasnt the window sustem SunView. And and pre solaris their unix was SunOS.
Many thanks... this was a very insightful explanation. I always had a general understanding about the concept of impedance, but now I'm understanding it much better.
Well thank you! So glad I help you understand it a bit better!
@@ElectromagneticVideos What is the resistance of the resistor (-network) you used on the input side? Is that 75Ω?
@@FrankBitterlich So most signal generators are 50 Ohm outputs. The TV coax I used was 75 Ohms. To make them both see the right load, I used a 10 Ohm resistor to ground. The Signal generator was connected to that resistor though a 39 Ohm resistor so it would see about 49 Ohms (close enough). The coax was connected to the 10 Ohm resistor with a 68 Ohm resistor so it would see 78 Ohms, close enough to the 75 Ohm Characteristic impedance. Note that I am assuming the 10 Ohm resistor is so low compared to the others that any effects of the resistors on the other legs can be ignored. If you use some 50 Ohm cable - almost any cable other than TV - and a 50 Ohm signal generator - you don't need the matching network.
I've always wondered about this since my first thinnet LAN. Thank you so much! Are cable splitters designed in a way that one or both TVs can be removed without passing the reflection back upstream?
I believe most splitters do need a good load to work properly. I'm not sure how much of a reflection happens when unconnected as you describe. That might be a neat experiment to test that and perhaps open up a splitter. Maybe a future video ....
Nicely done explanation and demonstration. Was very curious how you were going to pull this topic off and I was impressed. I have tried to explain this to people but without much success. Your video is going to be in my tool box from now on. Thank you.
Thank you so much! I was actually wondering if I could do it with minimal math too - pleased tio have been largely sucessful!
This is a great explanation! Fantastic video!!!
Thanks you so much! I just subscribed to your channel - looks wonderful - too bad I don't speak Italian but hopefully the CZcams captions translation function will work! Greetings from Canada!
I really like how you explain things
Well thank you! Glad my way of explaining works for you!
One of the best arguments for fiber optic internet service I have seen.
I'm sure old cable wiring with unterminated splices drives cable company installers nuts when they try and get cable modems going!
cool video! well explained and cool examples. Thanks and keep it up! looking forward to watching more of what you gotta say :)
Thanks you and I will keep doing them. I do put out videos in spurts depening on how much free time I have. A number of other ones already in the planning stage.
Wow, that is the finest explanation of TDR on youtube. Thank you.
What a wonderful complement! Thank you so much!
@@ElectromagneticVideos :)
Thank you! You are an excellent teacher. Things always make MUCH more sense with a demonstration.
I always wondered if there was a cheap/quick/easy way to verify if the test cables that I got are really 50 ohm. I've got a Rigol and the same AWG. Time to do some testing!
BTW, thanks for the tip about the AWG software. I'd never looked in to it.
It is neat experiment. If you want to try someting intersting, try a variabe resistor at the end of the line. As you turn it you can see the refelction go from positive to none to negative. One thing I didnt try was watching thereflections as really sharp bend is made or the line is crunched with pliers - might be interresting to look at.
@@ElectromagneticVideos Yeah! I was thinking of trying a ganged 10-turn linear pot. So that you have two variable resistors that are pretty well matched on each end of the cable.
Definitely, or what it'd look like if you have a 50-ohm coupler between two 75 ohm cables. Would something as tiny as a mismatched coupler show on a scope? Fun stuff!
@@button-puncher The other thing to try is capacitors inductors or some RLC combination at the end. That was part of the undergrad lab where I first came across the experiment as a student. We had mystery loads that we had figure out by looking at the reflections. Probably really tough for those who did the lab before the theory was covered in class (There were a bunch of different experiments. Each lab group did a differnt ine each week, so they werent always timed well with the lectures).
Great explanation. You mention reflections in glass.. I have always wondered how individual photons reflecting from different position of a flat surface are able to reflect in the same direction.
Thanks. So the glass reflection video will be from the classic EM standpoint: how the only solution to an EM wave at a surface dufferent impedance is some being reflected and some going though. Your photon question goes into quantum physis: Here is the very short explanation: Each photon has a wave function with highest value were it is most likly to be any point in time and space. The highest values are wher we expect it would go with classic physiscs. The wave funtion includes a complex terms (e to the iwy + theta) which carries the wave properties in terms of wavelength and phase. These probability wave funtions interact with the sheet of electrons in the metal mirror. Your could think of everry point on the mirror reradiating a probability wave with a phase releated to the phase of the impinging probability wave. And whne you sum it all, you end up with the highest probabilities along where we would expect the relfect ray/phototn to be. Put a gazillion photons together and all the tiny controbutions from each photon add to a very stable average to become the EM fields we are used to. So thats the verys quick handwaving answer that is intented to give a sense of whats happening as opposed to rigerous physics. Hpe that helps!
@@ElectromagneticVideos Thank you very much. I appreciate the clarity of your explanation.
What a great explanation and demonstration of transmission line impedance, thanks so much! I'm still confused as to why the need for a 75ohm resistor at signal input. This seems to be detrimental in that it cuts the signal amplitude in half. If you're performing transmission only, would this 75 ohm resistor be omitted?
Thanks! Your absolutely right about the transmitting side! No need for the impedance of the signal source to be matched - in many situations having a 75 Ohm resistor there would be just a waste of power.
However, if you have a situation is where the far end of the transmission line is poorly matched to its load and reflects some signal back, it can be useful to have the source side matched so that the reflected signal gets absorbed when it gets back to the transmitter, rather than get reflected a second time and a cause all sorts of weird frequency dependant effects and it reflects back and forth. This can be important for low power signal situations where power is less of a concern.
Another example is bi-directional data transmission where one transmission line carries signal in both directions. The old analog landline phone was an early example of this. These days, gigabit Ethernet sends signals in both directions along each of the 4 sets of twisted pair transmission lines, so both ends should be well terminated to minimize reflections interfering with the signal going in the same direction as the reflection.
What a tremendous explanation! Thank you!
Thanks yo so much!
What a Brilliantly explained vidclip, Thanks a Stack from ChCh, NZ
Your welcome from the other side of the planet - Canada!
@@ElectromagneticVideos Thanks ..... Appreciate your reply .... I am into CB / Ham type Radios and love building Antennas and Your Expanation just 'Hit the Spot' .... Best to You and Yours from ChCh, NZ
@@kiweekeith Thanks again - glad the explanation 'Hit the Spot' . Its really neat stuff - I'm an EE and this sort of stuff is covered in the EM fields and waves courses in university which I always liked. One of these days I'm going to get a Ham license - what a neat hobby!
My EE instruction in electromagnetic was abysmal. Thanks for the clear explanation.
Glad you found it useful! Its funny/sad how different instructors can taint ones understanding and enjoyment of a subject. I had the best profs for EM and ended up loving it. I had the worst prof for control theory - to this day I dont much care for the topic, but at least I have finally gotten a decent understanding of it over the years.
Thanks for great video. Question if I may. If I have a length of metal and I only have access to one, assuming I know it's impedance, is there a way to know it's length via reflection?
So your assuming it forms a transmission line-like system and even a bare wire can (look up Goubau line). You would also need to know its propagation velocity - the speed the signal travels down the line which is usually close to the speed of light but varies based on geometry etc. With that, yes, you can get the length using reflections.
Great practical demonstration! Thank you!
Thank you!
I love these kinds of videos, so many know it all speech makers.
I wish this lesson was taught in my 1974 electronics class!
It is such a nice, simple to to do experiment its too bad it isn't done more often.
Amazing test setup.
Thanks! I hope by amazing you mean amazingly simple. Should be easily duplicated at home or in a classroom with relatively inexpensive equipment.
Great demonstration and explanation of impedance matching. Looking forward to your video on ethernet twisted pairs.
Thanks! The twisted pairs video is delayed - had to go on a suddden business trip. Back in 10 days so should be able to get back to videos!
You mad very good video ,very clear and really instructive.
Thanks.
Thanks! I appreciate that!
Beautiful explanation of an often difficult to understand concept.
Thanks!
Thank you this was very clear visualized explanation. Although it is not my field but I find it interesting. Bravo 👏🏻
Thanks. So glad you found it interesting!
I really enjoyed this video, the content was valuable and easy to understand. Double 👍👍up.
So glad you liked it! It was inspired by a time domain reflectometry lab in an EM course I took as an undergrad in university. I never forgot that lab experiment and am thrilled so many people enjoyed this video.
Good demonstration and explanation. I do believe the most common velocity factor for consumer grade TV coax is .66
Thanks! You could be right. I looked up the cable I used and it was 0.8 . There were others listed at 0.66 and some as high as 0.92 (that may have have been 50 Ohm).
I feel like you opened my eyes. It's kinda amazing how electricity works.
Your comment made my day! Thank you! Yes it is amazing - the thing that I found most amazing when I was studying it as a student was how you can measure static non-moving electric and magentic field characteristics and from that get the speed of light. And how light is just and electric and magnetic field - wounderfully amazing. And I will do a videe on that sometime!
thank you, after decades of messing around with electronics, I finally get it.
Sorry to hear it took that long :) Glad to hear that seeing this video helped!
Excelent video. However, some details are missing. For example, is the output impedance of the signal generator talked into account for sizing the resistor? Or was the "resistor and capacitor" some kind of network matching impedance from 50 to 75 ohms?
Thank you - really appreciate that!
Here is a copy and past of a previous discussion that describes the impedance matching network in more detail than you probably need, but will be useful for anyone reading this:
Most signal generators are 50 Ohm outputs. The TV coax I used was 75 Ohms. To make them both see the right load, I used a 10 Ohm resistor to ground. The Signal generator was connected to that resistor though a 39 Ohm resistor so it would see about 49 Ohms (close enough). The coax was connected to the 10 Ohm resistor with a 68 Ohm resistor so it would see 78 Ohms, close enough to the 75 Ohm Characteristic impedance. Note that the 10 Ohm resistor is so low compared to the others that an effects of the resistors on the other legs can be ignored. If you use some 50 Ohm cable - almost any cable other than TV - you dont need the matching network.
Ham radio operator here, unmatched impedance in antenna systems is what makes power outputs in radio transmitters fry. Reflected signal goes back and heats up amplifier transistor. There is a device measuring reflections in feed lines called SWR Meter (Standing Wave Ratio Meter) to tell us how properly matched is our transceiver - antenna system.
Yes! I didnt go there because I wanted a visual way to show people reflections rather that having to explain standing waves. I will probbaly do something with standing waves or interference patters soon.
Very nice explanation. Thank you!
Thanks!
Kind Sir, I am a retired Electronic Technician and Instructor; with RCA for 33 yrs; and then 12 yrs doing it by "subbing" all around America. So I say this to you kind Sir: You are truly a "Cut-Above"; and you just solved a long waning question; "WHY do we have to put a 75 Ohm resistor at the end of the cable?".
And you showed me why easily! Wow! Thank you kind Sir. Because you are an incredibly, awesome instructor. I shant ever forget this! May Jesus continue to bless you and yours always.
What a wonderful complement! Thanks you so much - I really really appreciate your kind words. I have to say that I owe this intuitive understanding to a wonderful EM Prof I had in university who made EM fields and waves one of my favorite subjects. Have a wonderful day!
Oh my friend, i never see this explained...amazing; "look´like waltz of mathematics and physic"...thanks!!!
Thank you so much for that comment and what a wonderful way of describing it : "waltz of mathematics and physics". Thats such a great way of talking about electromagnetics - the intertwining of electric and magnetic fields to give us everything from light and radio waves to reflections in a transmission line. More of this type of stuff to come!
Very good explanation. Clear, well demonstrated. And I wonder if using a digital oscilloscope the delay to sample the signals would give the same clear results(or, maybe, the delay will be the same for each channel and, in the end, the showed results will be fine, only not in real time). Congratulations and thank you for the very good work.
Thank you so much! The scope is digital - but an old one which uses a green raster crt to make it look like a "real" analog scope. A good digital scope should have equal delay between channels, and signal-to-screen delay so low it is instantaneous. Having said that, a cheap old analog scope with a with maybe 30Mhz bandwidth should also work well for this sort of thing. Might need longer cables to make the delays longer for a slower scope, but cable is often cheap!
I see your from Brazil! Greetings for Canada, and believe it or not, I was born in Rio de Janeiro! (parents worked there)
@@ElectromagneticVideos OMG, I am in Rio de Janeiro and you was born here! So, you are a "Carioca"! Thank you for your kindness and beautiful explanation also about your HP digital scope and how they made it looks like a analog one - it fooled me! Warm regards from Brazil - and I hope your parents liked Brazil and Rio while they were here. One day I will visit Canada: only good mentions about your country and Canadians.
@ Actually they LOVED Brazil and particular Rio. They lived in many countries and Brazil was their favorite. My dad worked at the Canadian Embassy - that was before Brasília. For what its worth, they lived next to the canal that connects the Lagoa to the ocean. Sadly I was baby when they were posted somewhere else so I have no memories of Brazil, but want to visit sometime. And I do remember my parents using the term "Carioca"!
Great work in simple terms..Thank you❤
Thank you!111
Very informative and easy to watch. Thanks!
Thanks! Nice to hear it was "easy to watch"!
Excellent explanation! Thank you very much.
Thanks - really appreciate the complement!
Very cool deep dive I never seen in action before! It all makes sense. This is why is may be useful to cap the unused fittings in a house if there's a weak antennae or cable signal to prevent "signal loss", I guess that'd be this noisy reflection you demonstrated that could get quite noisy with multiple potential outlets in a home or business connected to a splitter that may or may not be in use.. This is how the cable guy quickly turns on ur service by connecting a specific resistor cap to the incoming wire or how thieves invented black boxes to unlock certain blocked pay channels.
I was waiting for you to talk about the benefit of using it to find breaks in a wire but it also has real world usage in everything from an alarm system reading the state of wires with an end of line resistor so the panel can know if you have a short, open or closed condition (did the wire break or cut, someone twist the ends together to bypass an alarm or is the window open, etc)
Oh, this is how ur car knows if the taillight bulb is burned out and many other conditions of sensors, too!
Yes! I didnt get into this but the trick is to use connector caps that have 75 Ohm loads built into them prevent reflections. The reflections are actually worse than this video shows when there are multiple improperly matched connections or unterminated ends - when the cable distances between those ends is multiples of the wavelength of a particular frequency. the signal at that frequency can be completely wiped out due to destructive interference or greatly enhanced due to constructive interference. This results in all sorts of weird effects. This is actually used to make some RF filters.
The real cost saving use is of course to find the location of faults in things like buried cables. And of course, if you take away the transmission line and replace it with an antenna, its RADAR.
I had that exact DMM for decades, it was my favorite...until one day a couple of years ago, it just outright died, would no longer turn on anymore. RIP favorite DMM.
Yes - its old and and wonderfully simple and mechanically rugged hardware store multi-meter. Sadly many of the more recent ones are so filled with functions that its a nuisance. I have one recent one that has a wonderful of array of functions but DC voltage is not on the dial switch. Voltage defaults to AC and you have to press another button. Almost like designed by someone who has never used one!