Quite possibly the best film ever produced. Twenty-five action-packed minutes of high-energy (pun intended) transmission line science. "I give this video +3dB" - Gary Schwartz
I have worked in the radio and radar industry for over 52 years and never fully understood the operation of transmission lines. This movie taught me more in 23 minutes about transmission lines than i learned in those 52 years. Kind of makes me wonder how many antenna jobs i screwed up!!!!!
Ted Swimerr I agree.. I understand the math but never really understood the real physics.. Imagine how much more we could fully understand electronics if all concepts were explained so well....
@@bradleysmith681 They are explained just as well and better if talking to the right people. I have known this for more than 30 years. Want to see one even worse try chasing down a ground loop sometime...
When MIT tried this sort of thing in the 1990s, the students rose up in arms, and got rid of it. (This was Belcher's E&M course, with animated field-diagrams.) They wanted obscure math only. If it's taught without math, with visual-intuitive animations, then any mere technician can understand all the concepts! They put a stop to it. (The student-revolt was among physics students. It's like having a bunch of Medieval pre-meds who eliminate any professor who would teach, not in proper Latin, but in English which any outsider could understand.)
Some people might think that this video is boring but actually it is exciting if you use imagination to understand the principals. Consider that these are also the principals of acoustics where a "reflection" is called an echo. A string on an musical instrument behaves exactly the same way constituting an acoustic transmission line with a small load, the string is plucked or bowed to create a pulse, this pulse travels to the end of the string and is inverted and reflected, then it is reflected again and re inverted off the opposite end. The pulse moves back and fourth making a tone that gradually decays. The instrument body functions as the load and the players motions as the temporarily connected battery. The pulse contains many harmonic frequencies and the nature of the load(the instrument body) verses the length and structure of the string determines the standing wave properties that filter the harmonics which provides the timbre (the tonal quality) of the instrument. Your breath is a battery, your vocal chords are a switch, your throat and nasal passage are two acoustic transmission lines and the air outside your lips and nose are the load. The transmission lines in this case are very complex with different adjustable impedance's distributed along their length( unlike an electronic transmission line with its consistent impedance) such that the end result is you can sing and speak. Most people think electronics are very abstract and remote form day to day existence but you are using these principals every to time you speak, you are appreciating these principals every time you listen to music.
I was watching this at 2× speed, kind of bored until about the six minute mark... and then I sat up and said, _whoa!_ I had to rewind because I'd never learned what characteristic impedance actually was. I knew the effect of an impedance mismatch was that the signal bounced, but I didn't get _why_ it did. In rapid succession there were three key things this video taught me about high frequency signals: 1. The insulator between conductors, even if it is air, is the "dielectric" of a capacitor. 2. Every wire, even an ideal one with zero resistance, has inductance. 3. To make it easier to conceptualize, it's okay to chop up the line into multiple segments of repeating inductors and capacitors that pass the signal one to the next. Also helpful was the reminder that the characteristic impedance Z = sqrt(L/C). Increasing the length of the cable increases L at the same rate it increases C, so it cancels out when calculating Z. Therefore, 4. Theoretically, a cable will have the same characteristic impedance no matter how long it is.
Good stuff. For those who didn't know, the PC pci bus works like that reflected flattop signal at around 20: min. The driver only makes half the amplitude/current to get to logic level and the bus is sampled at reflection time so things see the full desired level. Fairly slick, but obviously length dependent in the extreme. If anyone worked with ECL back in the day, the only other good reference for this work was from Motorola ECL design handbook...and I still use the knowledge today.
I actually screamed out loud at 13:38 because it offended my senses of what should and should not happen so deeply. I knew why and understood but never was it displayed so clearly to me.
We used tectonic oscilloscopes, when I was employed by Western Electric AT&T. It was to me, like a mechanic is to a wrench, or to a hammer is to a carpenter. In my opinion they made the best oscilloscopes available at that time.
University programs are mostly overpriced trash. More often then not someone has to get over a language barrier before they can start to understand and integrate the basics. I love these videos.
True, I learn a lot more watching these videos than a professor explaining about theories and mathematical calculations, without the most basic foundations, unfortunately universities today only form a bunch of illiterates, sad that.
Thanks for posting this!! Best explanation ever! Now I understand the basics that I've been looking for. These old training videos destroy anything produced today. If anyone wants to see some great explanation of antenna theory, look up the old videos titled "Antenna Fundamentals"
Wow! I stepped in a time machine here! That is some old footage. Best explanation I have heard on what is characteristic impedance. Very good info here!
After watching this kind of videos you're left with a profound sensation of awe and satisfaction about the vibe and delivery of this kind of content, now old style videos. Glad some of them are preserved in time in here...
This video not only opened my eyes, it also open my skull, erased what i thought i knew about transmission lines and put back what i should know about transmission lines. Thanks for sharing it.
Now I finally understand transmission lines, characteristic impedance, and reflections. Now to apply this knowledge in the real world to RF and in helping better understand antenna theory. Though I still wonder why impedance matching and reflections aren't an issue at low frequency, with electrically short conductors (transmission lines). Particularly in the Audio world where power amp output impedance is ideally 0 Ohms, going into a load of a few to a few hundred Ohms. Oh, is it that the characteristic impedance of a transmission line is high enough to be negligible at low frequencies?
Ronnie was a smart guy and knew quite a lot about quite a few things. He may well have known how a transmission line worked. However, I highly doubt he went up to Beaverton to voice this video. :-)
10 лет назад
that was my first experiment with an oscilloscope 6 years ago
For a short circuit, the return signal was said to be reflected. But aren't the short circuit's 2 emitted pulses each just transmitting through the short circuit to cross to and return in the opposite line, which explains the short circuit return signal's opposite polarity?
Well yes, but the major point is that the signal doesn't disappear at the end, it returns. And by convention we call that return a reflection when talking about transmission lines, regardless of the exact mismatch phenomenon causing the return. (If we were talking about radar, the reflection of the energy from the target is called a "return", just to be confusing.)
@@lwilton Thanks. I think that, because there really is no reflection in a short circuit, to avoid further confusion, the convention should be changed. I don't find the radar reflection to be confusing, since it really is a reflection.
