Showing how a square wave signal is distorted by reflections from the unterminated end of a cable. Also shows proper terminating resistance and its effects.
Пікірлер: 76
@AlexsandroRNeto3 жыл бұрын
That was the most simple, understandable and complete explanation I had ever seen... thank you!!!
@drewmutt8 жыл бұрын
Thanks so much for this, I've been looking around for someone to explain reflections / impedance succinctly, and that's exactly what you've done. Hands on ftw!
@GIGLIndia12 жыл бұрын
awesome .finally i did understood what characterstic impedance really means thank you
@netmana11 жыл бұрын
thank you so much, i was studying the electrical concept of a Time Domain Reflect-meter, and i saw exactly what you were explaining on this video on my oscillascope. i did not understand, and did not find the answer anywhere until i saw your video. thanks.
@DaveMueller19803 жыл бұрын
Hats off to you for this excellent video! Great job explaining the concepts and demonstrating some characteristics that may be seen when troubleshooting a signal reflection issue. This video also highlights the need for termination resistors in high-speed digital and RF applications.
@NotSoLiberal11 жыл бұрын
I enjoyed watching this video 2 years ago, and I'm enjoying watching it again now!
@danielnoriega66554 жыл бұрын
Finally a real world explanation of the issue, thank you so much. Now I understand.
@signfang12 жыл бұрын
You saved us almost 5 hours!!! A big thank you from SNU!
@Ytlakku11 жыл бұрын
Thanks a lot . Your practical demonstration about reflecting waves is quite clear and understandable.
@ivorytower713 жыл бұрын
A brilliant and very intuitive explanation. Thank you!
@jorgerive73353 жыл бұрын
great video. Thank you. It would have been good to add a brief explanation to the reflections as a function of wavelength. ie, at what ratio of signal's wavelength to the length of the cable do the reflections begin to be visible/measurable? Why don't we typically see this issue with low frequency signals?
@BTCInstrumentation12 жыл бұрын
You are very welcome. It took me years to finally understand what characteristic impedance means as well!
@jd08065712 жыл бұрын
Awesome demonstration and explanation. I will have to set this up in my lab now to work through the numbers myself now that I understand what I will be looking at. Thanks a lot.
@Orcinus24x512 жыл бұрын
Great video! You should have gone a little bit further and mentioned that the width of the reflection pulse can be used to calculate the length of cable, using the speed of electricity in that particular type of coaxial cable, i.e. 82% or 66% the speed of light
@anthonysandolo100010 жыл бұрын
Best explanation/video I have seen on this subject!! Really enjoyed it !!
@BTCInstrumentation13 жыл бұрын
@hlbpr You do actually get reflected pulses even with DC, but the pulses come and go so fast you will never "see" them with a meter. That's why we only worry about high-frequency signals: when the reflection time of the pulses approaches the period of the waveform we're interested in. Then the reflections corrupt the signals we're trying to put on the cable. This is also why you can avoid a lot of signal reflection problems in a computer network by simply turning down the data rate.
@mustaphaalkhafaaf55127 жыл бұрын
first class explanation,,, Excellent
@NotSoLiberal14 жыл бұрын
Great video, Great explanation. I wish my instructors had shown something like that when we were learning about transmission lines.
@jeremycoryell85493 жыл бұрын
Wow. Perfect explanation. Good teacher. Thank you.
@EdEditz7 жыл бұрын
So that's how you can measure the impedance of a cable with a scope. I guess you can take the potmeter off after leveling the squarewave and measure the resistance and that would be equal to the impedance. That's good to know.
@EfficientEnergyTransformations9 жыл бұрын
Good & enjoyable practical tutorial. Thank you. Also, gratitudes to Oliver Heaviside for, firstly, theorizing these concepts.
@BTCInstrumentation13 жыл бұрын
@humaxf1 Reflections create standing waves at a constant frequency, so yes this is related to SWR. The better the impedance match at the cable's ends, the less reflections, and the better your SWR.
@TheRealMonnie2 жыл бұрын
Beautiful. Thank you for this.
@sergiodjf Жыл бұрын
Impedance & capacitance of the cable affects more the signal the longer the network. It acts as a big capacitor in addition to the wire resistance.
@kibora9 жыл бұрын
Nicely explained. Great job
@bigfist8556 жыл бұрын
Excellent practical explanation
@m9jbhakar3 жыл бұрын
Questions: 1. How does the signal knows difference between human made joints and factory made joints? i means as you said - "signal reflects from the joint you made to red and black wire". so how does signal knows that , that joint is made by you so signal should reflect from there. and not from in between the cable because that is factory made. does signal have brain? 2. what do you mean by signal propogation? do you mean that signal (current or voltage whatever) runs in the wire with speed of light, so it take some time to reach them to the end. and then when that current come back to the other end, their phase difference is called impedence mismatching? 3. How will you explain impedence without complex numbers and any mathamatics(because i dont want to know exact amount, instead i want to know "process". mathamatics only shows amount, and hides actuall process what going on there.) 4. when they say "for maximum power transfer, impdence should be matched", what does it means to match impedence? do they means phase of current and voltage should be same? because bothe capacitance and inductance make phase shift by 90 degree. so you should add right amount of capacitance and inductance to make voltage and current "in phase" (because phase shift by inductor is opposite of phase shift by capacitor). i just want to know what "causes" this "impedence mismatching" "physically". please dont bring complex numbers and mathamatics here.
@GrantsPassTVRepair13 жыл бұрын
Thank you for this demonstration.
@anasal-daghestani637110 жыл бұрын
Thank you... helped me a lot in my presentation on COAX :D
@andrewknight7559 жыл бұрын
I appreciate the attempt to simplify the concept of signal reflection,but i think it can be better explained if we look at your circuit like a LC circuit connected to a square wave source with small internal resistance.The L is of course the loop of wire and the open end of the cable is acting as a capacitor , this model would explain the sudden small step in voltage(voltage across inductor (wire loop)) when square pulse is going positive and also the overshoot as the inductor dissipates its stored energy into the cap. similar analogy can be applied to your other circuits (closed loop and resistive termination) as well. how can you justify that small step is a reflection(voltage divider) and not the voltage across the inductor (transient response)?
@we-are-electric14455 жыл бұрын
The open circuit is like a capacity. That is an unrealistic analogy. The voltage applies an electric pressure on the cable. When it is released (ie the square wave makes the transistion back to zero ) the electric pressure is released and the energy moves back down the cable. When a resistor terminates the cable the energy passes through the terminating resistor. If the resistor is of a low value most energy is dissipated as heat.
@EonTide1111Ай бұрын
Is the signal generator and oscilloscope Both terminated at the entrance or positive lead of the cable ?
@robt77857 жыл бұрын
Can you explain how an oscilloscope can read RF waveform and voltage accurately with its 1 megohm (typical) impedance? In other words, why aren't there standing waves on oscilloscope leads that throw off the measurements by a huge amount?
@TheElectronicDilettante2 жыл бұрын
So, if you measure the resistance of the potentiometer at the position where you have the clean, square wave form , that would indicate the characteristic impedance of the cable or wire. That should work on an antenna , right? Thanks for the video!!
@ABCElevator198911 жыл бұрын
Thanks Tony I appreciate it buddy keep up the good work
@TheArozconpollo6 жыл бұрын
Great visual explanation, I know know what some of the waveforms I've seen mean.
@netmana11 жыл бұрын
oh yeah, i find during my experiment that the reflection time, is approximately 700ns, doing simple math: 983571056 * 0.0000007/2 * 0.7 speed of light, times reflection time, divide by 2 b/c of cable lengh, and multiply by 0.7 because wire is not ideal (does not travel at the speed of light), its propagation characteritic is 0.7. the answer is 240 feet, that's exactly about the size of my spool, this is a NIFFY trick to calculate total length of a wire.
@qzorn44403 жыл бұрын
Gee, i have just rewatched this video great stuff.. CanBuss is on the factory floor with more and more sensors being hooked up to PLC_Can_networks... great when they work as per all things... however, when very short-time errors occur, i need to have a way to catch these problems that may happen once per hour, day, month etc... thanks...:)
@BTCInstrumentation11 жыл бұрын
"Termination" simply means a way to "end" the cable, or better put, how to *properly* "end" the cable so that it won't generate reflected waves that will mess with the integrity of the signal.
@brencostigan5 жыл бұрын
Nice video, thanks for taking the time to prepare and post it. Some of the other commenters asked if the signal generators impedance mattered..... A good way to think of it is this: Instigation (as in the signal generators output impedance) Propagation (the cable’s characteristic impedance) Termination (the terminating load impedance) All of the above three components need to match to ensure efficient energy transfer from one to the other without reflections. So if you are using a 50 ohm signal generator, use a cable with a 50 ohm characteristic impedance and ensure that you terminate it into a 50 ohm impedance load.
@ingDemurtas2 жыл бұрын
very interesting. For a RS485 cable would you always search the perfect resistance with a potentiometer? typically 120 ohm is used but testing the line with scope+function generator + potentiometer would terminate it perfectly.
@BTCInstrumentation13 жыл бұрын
@NotSoLiberal It's a shame this isn't demonstrated more often -- it's not a very difficult thing to demonstrate. All you need is a square-wave source, a long cable, and a 'scope. Feel free to forward the video link to your instructors, maybe giving them some ideas!
@tunicana10 жыл бұрын
Very neat explanation.Thx
@SkylerF10 жыл бұрын
Awesome video! What type of cable did you use? I have a 500ft roll of 75ohm coax, a signal generator, and a 25mhz scope and I want to try this experiment! Does the characteristic impedance of the signal generator matter? I think its 50ohm but I'm not sure
@joseguardado487 жыл бұрын
amazing explanation
@Timson2056 жыл бұрын
It was a really good explanation, thanks a lot. What I'm trying to find out right now is how to use this setup to detect breaks in an XLR-cable, so I can find the exact position of the break, cut the cable before this point and re-solder everything. My problem is that I find a lot of information on coax cables (especially about the velocity factor), but I cannot find any information about the velocity factor for XLR-cables. Which is quite bad if I want to find out the position of the break. Any hints or doesn't it work at all? Thanks a lot!
@marek36504 жыл бұрын
Thank You , great job
@mingchi215 жыл бұрын
Excellent, very appreciate.
@ckhemken11 жыл бұрын
Thanks again for your videos. I am going to try this but I thought it may be interesting to use the 2nd channel of the scope and look for the voltage drop across the cable and if the scope has the capability, use the math features to add or subtract the channels. Was the ohm value of the variable resistor equal to the specified impedance at the point where the wave looked square again?
@ooltimu10 жыл бұрын
Is reflection a result of self induction? I just found out about this phenomenon and I'm trying to understand what happens.
@LuisBrandoIngTec14 жыл бұрын
Excellent!...and cheap to try
@HoaNguyen-dw3hm3 жыл бұрын
How the signal reflect back? Assume we generate pulse signal from device into cable but how extractly the pulse reflect back to device?
@robertgift6 жыл бұрын
Well done! Thank you.
@netmana11 жыл бұрын
This method does not work when the wires are decoupled from each other (do not run in parrallel). let say, if you remove the + and - strand of the wire, make + wire lays in north to south direction, and - wire lays in west to east direction, you would not see the reflection delay. I do not understand, but what what I remember in my electronic class, a while is modeled by a repeating RC circuit, hence it has an impedance characteristic. When two wires are removed, wire charact. becomes R, no C
@netmana11 жыл бұрын
C (or capacitance) exists b/c the two wires are spaced close together separated by air, and so it behaves like a capacitor. B/c the capacitance of the wire is removed (when the wires are separated), signal does not have the energy to reflect back. this is just my theory, please explain if you know the answer. thanks
@xionggor214610 жыл бұрын
HI, i would like to ask a question. I have done this for my experiment. The wave I obtained for the reflected wave is positive and not negative as shown in the video although my incident signal still remain in positive signal. Is there any reason?
@williamkopanchuk7 жыл бұрын
Excellent!
@AbdolazimHasseli7 жыл бұрын
Very amazing! What was the frequency at the end? What was the resistor value when you got the clean square wave?
@we-are-electric14455 жыл бұрын
Probably 50 Ohms. The typicall characteristic impedance of a cable .
@jonjohnson12596 жыл бұрын
I think i understand now what your saying is that a signal goes flying down the cable charging its self capacitance and inductance then at zero cross the inductors and caps release a charge if there is mismatch PLEASE tell me if I'm right. Its a bit like Powerfactor correction in 50 or 60 cycle systems yes? Ok sorry to ask so much but I'm want to learn
@kcheng07047 жыл бұрын
Hi, experts! I am wondering why the reflection always showed at Vpp/2? Any comments or advices are welcome. Thank you!
@leedchristensen7 ай бұрын
Can you calculate the length of the wire from this?
@ABCElevator198911 жыл бұрын
Hey Tony I'm just getting into differential signaling and communications. I was wondering what the meaning behind the terminating resistors was. It's throwing me for a loop . I would greatly appreciate it. Thanks Tony
@louco24 ай бұрын
Impedance matching, when you have it you have full transfer of energy to the resistor and none reflected back. If I understand...
@sarahmohammed62366 жыл бұрын
That's so great
@batmandeltaforce6 жыл бұрын
Maybe not. Maybe it's just capacitance? This has nothing to do with time, that is silly.
@italebismail73386 жыл бұрын
very useful
@humaxf113 жыл бұрын
is this similar to SWR and an antenna?
@bsdiceman7 ай бұрын
Cool
@vasudhavedula97827 жыл бұрын
that was helpful, but if the connections are showed it would be helpful to do the experiment personally.
@espenbgh25405 жыл бұрын
I think you forgot to turn on the sound of the microphone.