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- เผยแพร่เมื่อ 13 ส.ค. 2014
- Dave explains a big trap in high frequency measurement with your oscilloscope.
Based on a viewer request, Dave demonstrates how to incorrectly and then correctly measure the signal output level over frequency of your function generator using your oscilloscope.
Some whiteboard transmission line theory, reflections, termination, group delay, phase delay and assorted stuff is thrown in as well.
Forum: www.eevblog.com/forum/blog/eev...
Check out Doug Ford's Oscilloscope probe article which explains a lot of this stuff, and why x10 probes are better than x1.
www.dfad.com.au/links/THE%20SE...
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I would like to add something to "reflect" on even though this is 8 years later. This wonderful engineering "trap" was actually used to create the The Tektronix 1502C Metallic Cable Tester which I provided maintenance and calibration for about 30 years ago or so. At the time it was popular mostly to trace down opens and shorts in aircraft wiring harnesses. Man that take me back!
This is why I love EEVblog. I am an electrical engineering student and they never go over things like this in college! I mean we read about it in textbooks, destructive interference blah blah but we never get to actually see it like this!
The magnitude of the reflection seen at the generator isn't changing, it is the phase of the reflected signal that is changing with respect to the source, and the amplitude of the resulting sum of forward and reflected waves is the variation in amplitude that you're seeing with the probe (at the end of the video).
***** Yes, that's what I was getting it. I probably didn't explain that adequately. Ultimately what you get is a change in amplitude as measured and displayed by the oscilloscope. That's what I explained on the whiteboard, and that applies at any measured point in the transmission line system, be it source or destination, or in-between.
Yeah, these are tricky concepts to explain. That's why I've done at least 5 videos related to this topic, and there's still details that could be explained better/differently. Nice job on the video, as always.
***** Yes, countless ways to explain this and scenarios etc. This one was supposed to be a quick 5min job on answering the persons question about why their function gen output voltage went up and down over freq more than expected. Then I waffled on...Would have approached this differently if it was say a deliberate Fundamentals Friday on transmission lines.
Hmm, a topic like transmission lines and reflections would be more like a Fundamentals February, instead of just a Friday.
It would be cool to use a wave tank to show forward and reflected wave trains and the results.
Us hams often use stub filters and impedance transformers made out of quarter wavelength transmission lines. I've done a bunch of videos on these things too - ranging from TDR to coaxial properties characterization, terminations, etc.
Transmission Line Terminations for Digital and RF signals - Intro/Tutorial
How to measure coax velocity factor VF and impedance Z
Cheap and simple TDR using an oscilloscope and 74AC14 Schmitt Trigger Inverter
Use a scope to measure the length and impedance of coax
Basics of Tracking Generators and 1/4 wavelength coaxial stub filters
Yes, I do know this is a 4 years old post.
Nonetheless:
Very informative videos, I recall when we used terminations for 50-Ohm coax LAN connections. Thanks for sharing.
Do you mind also sharing how to embed links in plain-text as you did above? That's pretty cool.
Cracking explanation, condensed a tedious 2 hour lecture into 30 mins with practical demo of effects, superb, as always, thanks Dave.
Tony Bell Thanks!
EEVblog No, thank you for inputting your time and knowledge.
Yes very well prese
I wish i had a teacher like you at school. I watch loads of your videos and learning so much. Thank you
In 1970 we used a 32 feet long open ended cable joined with a T-piece to a terminated video lead to remove the colour subcarrier of 4.43 Mhz, to make the picture black and white.
We were using a colour telecine machine to nconvert B/W Films to video.
the subcarrier is 4,43361875MHz - I'm asking me long time why is it on the end 4 433 618,75Hz
I've never watched a video of yours and walked away empty handed. Fantastic
Try something closer to your level like changing the batteries in a flashlight.
I vote for having Alan from w2aew join you in the lab for a day or two for an RF primer similar to what you did with Doug Ford and microphones!
Thanks for the lesson Dave
I always knew that a line needed to be terminated, but never really understood the details of the issue. Now I do!
Fantastic video, thank you for all the effort you put in to these very, very helpful videos. Nobody else comes close, marvelous.
I guessed it! I must be finally learning this stuff. Excellent video, the demonstration by adding the length of coax for 180 degree cancellation was an especially cool physics demo.
Hey Dave,
I really appreciate your expert opinion in electronics. I play other electronics channels on youtube to drown out the silence when I work on electronics, but I can't do that with your videos! Having studied in electronics engineering, and having spent hours working on my own engineering projects, I always take the time to listen to your advice.
Your use of electronics/engineering terminology is fantastic, and truly sets you apart from other channels. Keep up the good work!
Great video Dave. Some of the navigational systems I worked on we used this delay to great effect. Two antennas feed 180 degrees apart will produce a null between them. Before network analyzers we had what was called a vector voltmeter which measured that phase delay in degrees. If the delay was off we calculated how much cable to chop off to get the correct delay.
Some of the older cabling we had to age the cables to account for expansion and contraction of the cabling due to temperature variations. The higher the freq you deal with the weirder things get. Transmission line/Antenna theory always seemed the hardest thing for newcomers to understand.
Again great video showing the practical effects of improper termination of signals.
Steve
Got my first signal generator yesterday and came across this exact issue. Was scratching my head for a while over this. I guessed it must be due to the cables rather than the generator or scope.. Now I understand what's happening perfectly - thanks Dave!
this was spot on, and answered one of my noob questions in the forum perfectly. Thanks Dave! :)
Great video and demonstration on how coax stubs work as frequency traps.
What w2aew said. ie., DIDO. He knows his stuff. He has several videos that deals with this issue. Thank, THUMBS UP.
It always amazes ... I learned all this stuff in some class way back when ... but I never really understood it until I see a Dave video.
Of course, this only skims the surface of high frequency propagation, reflection and interaction with capacitance, impedance and cable types and transmissions. I once took an introductory course on RF and propagation and it was just the tip of the iceberg. There are some very complex physics going on with this and I applaud you for taking this head on in a 24 minute video.
Eric Haney Yeah, it's deep complex stuff if you really want to get into it. Heavy duty math and physics. The goal here was just to answer someones question, not really get into transmission line theory much.
EEVblog Agreed. A great video as always! Keep up the good work!
Indeed, wait until it gets up to 15-40GHz. then it really becomes crazy!
Ahh thankfully this was again one of your videos where I don't feel like a complete doofus, I actually learned this freshman year in the university :-) Great video, Dave!
very good explain how to measure signals with termination. Thanks, David.
Good video, Dave. It was a great intro to why termination is so important in signaling applications. I had seen similar results if sketchy waveforms when i was trying to diagnose where in the signal chain a fault was occurring when troubleshooting theatrical LED fixtures (DMX protocol). It was a bit of a head-scratcher when I had tested from the brand new controller console and was seeing misshaped waveforms. After about one minute I realized that I needed to terminate my signal pair but it was one of those situations where you get relieved to find out that what you just thought was going to be a return on a piece of hardware was really just a red herring and you can get on to the actual test to find a fault.
Reminds me of early 90s networking, and the grief caused when I used to take my PC around to my friends to play deathmatch Doom... "Aww crap, where's the terminator gone?"
Didn't understand a thing about signal reflection, all I knew was you needed t-pieces and terminators to make it all work :-)
I was a mechanic for 30 years, Kinda retired now, so i understand basic crap very well, but i wanted to know more, Dave has helped expand a lot of what i couldn't figure out on my own, I built a mini oscilloscope and learned how to use it, now im getting a real one and cant wait to keep learning from dave, Plus he's funny as fuck, especially when mad.
Great fundamental Friday, Dave.
I am sure glad W2AEW is keeping you in line :)
This is why electronics is cool. You plug a signal generator to a scope and measure. Simple isnt it? Not quite, you can spend hours and hours studying what you see and still not be able to explain it. Thanks Dave for sharing wisdom. I appreciate that!
wooaaahhhh this was really a nice one Dave !... thanks for the lesson
I remember learning about this reflection problem in computer and electronics classes. Old coaxial bus networks were a nightmare for this. You could have 50 computers hooked up to a single bus and if just one got unplugged, the whole network would become unusable. I heard a story from a technician that this happened to, took more than half a day to find it. Also in electronics class, we called terminators 'Arnies'. ^-^
High caliber video, Dave. Thanks.
The step you see at the leading edge of the waveform is due to the coax's characteristic impedance being a load in series with the output impedance of the generator. 50 Ohms in series with 50 Ohms drops the input square wave to 0.5 of its level. This occurs irrespective of the end of the coax being terminated, open or short circuit. As the wave travels down the coax at approx. 0.6 the speed of light (say 5 ns/m) the measured signal at the start (the step) has no knowledge of the termination of the coax at its end hence the step. Try 75 Ohm coax and the step is a different value. Once the reflections have settled down the measured voltage will return to the same level as expected for an open circuited load, that is no change in level! You can work out what to expect by thinking 50 Ohms in series at the start of the wave and then think what would I see if it was just a DC signal level change for the long term part of the signal. The reflection occur because the impulse from the leading edge of the signal has no where else to go except back to the source where it is then absorbed - just like throwing a ball at a wall and catching it on the return bounce. After twice the time delay for the signal to travel to end of the coax all the reflections should have died out and the distributed capacitance of the coax will be fully charged - inductance is not applicable in the long term because no current flows in an open circuit ( think DC again).
Awesome & very interesting vid Dave... applause!!!
Thanks a lot!! now i found a nice way to explain my technical problems to my communications theory teacher, greetings from Mexico! you have my subscription and thumbs up!
Enjoyed the video, it gives the brain cells something to kick around .It opens up a deeper way of thinking about what you (someone) is doing at there bench. Thanks 73 Leo k1zek
Great video - Thanks, Dave!
Great demonstration!
this is a engagement of the video, I'm seriously engaging here
Excellent explanation!
great video as usual, Dave!
You're awesome!!. The video is truly helpful.
Wow! Excellent video! Thanks for sharing (: Greetings from Spain!
Great video, sharing with buddies
Beautiful demo!!!!!
Of course I understood this video : )
w2aew did a video about signals bouncing back down a length of coax to determine length, That I did understand mostly.
I sure am pleased to see that a video from almost 4 years ago helped with exactly the problem I am facing. I am using an old 100 Mhz dual channel analog Hitachi 'Scope and an old GW-Instek 3Mhz function Gen/ 150 Mhz frequency counter. The sig-gen out is labeled 50 ohm impedance and he scope input is labeled 1M. Something didnt feel rite when I read instructions telling me to connect a dual end BNC cable direct to each device. This video helped a lot and I will follow up with the article by Doug Ford. Many thanks to you Dave. I was concerned I might damage the front end of the scope. I feel much better now ! The Sig-Gen has no probes, so I figure I should try a set rated for 200Mhz (to accomodate the 150Mhz freq-counter). Seems I should be looking for a 50 ohm inline terminator too.
Never knew about the probe skew function. I'll have to try that out on my Agilent.
Excellent review for me. Being at one time a cable TV technician trouble shooter. Ancient history. I just got an oscilloscope etc recently.
You just saved me from returning my brand new SDS 1104X-E lol. Thank you!
Great video... Learned lots. Thanks
I would have liked to have seen this video when I was studying for my Extra Class Ham License. I was baffled how a single piece of coax could present itself (characteristic impedance) as an open circuit, short circuit, or anything in between based on frequency. My background is computer engineering, so my little digital based brain was sent reeling. Excellent video Dave!
Thank you a lot, I learnt a lot thanks to this video
Hey Dave, great video.
It would be interesting to watch a demo of some wave guides from that TV station you got that xmission gear from. Great Video as usual! Thanks Dave!
Thanks for good explanation! About twenty years ago I sometimes had to attach terminators to arcnet (and ethernet?) cards to make LAN works, but didn't understand why (and it seems that nobody around did ;)
Thanks! Very good explanation!
Knew that characteristic straight away. Many years ago when working on Novell networks, a particular school used to get regular failures. Students would remove the termination resistor and the system would go down. I also remember using a borrowed Telecom TDR (time domain reflectometer) to diagnose a faulty cable that would go open circuit in windy weather - the fault eventually was identified as a faulty spot weld junction which would go open as the cable flexed. A real tricky fault to locate in hundreds of meters of cable. The TDR would show EVERY junction based on it's reflected signal.
I remember the problems some people had with correct iSCSI termination leading to really funny effects...also BNC-ethernet cabeling was a source of unlimited trouble when folks just moved their pc around not take a good look at termination.
Nice vid, thanks
SCSI not iSCSI....
Great video.
I remember trying to explain Fourier analysis in square waves teaching RF considerations, I had a lot of blank faces so I used the Hammond organ sinewave additive to produce an approximation of a square wave on a CRO and it was a good way of showing the principal even though we were dealing with audio frequencies. Showing standing waves on a open 2 wire transmission line with a transmitter connected at one end and impedance mismatch at the other using a florescent tube passed at right angles to the line along it and watching it glow and fade as it passed voltage highs and lows.
The behavior of any AC in a circuit is a hard thing for most people to grasp, you have to have a good imagination and be able to think laterally. That's why so many people bomb out of electrical and electronics courses, AC theory needs to be thoroughly understood in all aspects.
Great info. Thanks
wow so much info in just one video, I miss content like this. Inline terminators? whaaa
A great lesson, thank you
You can also calculate the cable length from the frequency you get your minimum at the source from: you get the strongest reflection when the reflected signal is exactly 180 degrees out of phase and since your source signal has to go through the cable twice (forward and back), you hit that lowest amplitude when the path is down to quarter-wavelength.
With a minimum at 49MHz, that puts the total path length at around 1m.
Grateful for this video :) thanks.
Mind blowing! Absolutely mind blowing!
Great vid thanks
Very informative.
It was a very informative video as always. Could you please shed some light on how to do the impedence matching for feeding high frequency signal from function generator into a circuit being designed.
Time Domain Reflectometry (TDR) - Feed a 1KHz square wave into a 100 foot long coax cable open or shorted at far end. Watch the square wave wave on an oscilloscope and note the reflection from the far end and other reflections from cable faults. Reflections will be very close to the rise of the square wave and distance to fault can be determined by speed of light and propagation factor of coax cable, or for a known coax cable length the cable propagation factor can be calculated from time out and back relative to speed of light for same distance.
Thank you
Some early high power coaxial transmission lines were made with standard copper pipe sizes, so the impedance was actually 51.5 ohms. Modern types have custom size conductors, so they are exactly 50 ohms.
10:00 this is how you can measure the speed of light, or, to be precise, the propagation speed of signal in the medium of which the signal's conductor is made.
14:00 Time domain reflectometry,used to check long lines and opticfiber underground.
EEVblog Dave, Thanks mate. I install TV Antennas and have been told that I can use 2 antennas of the exact same spec, to produce better gain, *So long as BOTH the cables that I use from the antenna to the mixer, diplexer or masthead Amp are precisely the same length*
Now I have seen why. Thank you!
Or if your scope has the ability to terminate with a 50 Ohm impedance, just do that and then you can use a coax line without any issues. Dave did do this on the Agilent, but because most cheaper scopes do not have this ability the video was necessary. This is good info to have.
I just set my scope to 50 ohm termination and call it a day. As a backyard RF engineer that's something I look for in a scope. And of course I always have a spectrum analyzer at hand.
That explains a lot of signal traces on high speed digital circuit boards (PC motherboards) that looks wavy like snakes instead of straight lines for no apparent reason. They are trying to match the length with some other traces.
Dave, great video!
I have a question, why does the amplitude drop to around 0.6V after double terminating the outputs, but when you leave only one channel connected the reading goes back to the expected 1V (minute 8 of the video)?
Thank you in advance!
A good follow up to this would be to show what happens when you send a very fast pulse (fast rise) down the transmission line IE TDR basics.
Hi Dave: Your video is great as usual (but don't get cocky, stay humble). However, the comments, currently 31, are an additional and informative tutorial. Interesting stuff: . . . used a 32 feet long open ended cable . . . , . . . to remove the colour subcarrier of 4.43 Mhz. Just great. I think Alan (W2AEW) hit the nail on the head that this is a subject worth exploring a bit more. Cheers, Mark
Visual explanation for the need of Impedance Matching! You'll understand everything Dave's talking about here:
AT&T Archives: Similiarities of Wave Behavior (Bonus Edition)
Thanks
Years ago, Tektronix made a great little 23min film called "Transmission Lines" There is a copy on You Tube. It is very clear and accessible with easy, clear maths -though it is not at all math intense. I highly recommend it as an introduction to this topic for those wanting to get into this deeper. I also highly recommend Dr John Shive's "Similarities of Wave Behavior". In this 28min film, Shive explains wave transmission theory, impedience matching, etc. using easily grasped mechanical metaphors. It is also on You Tube, on the AT&T Channel as I recall.
I'm surprised you didn't mention the resonate length of the coax can play with readings on a scope as well. Since the resonate length changes with frequency.
Thanks, just thanks!
Here I thought you might break out some wave guide ;)
Nice video, but I must quibble with the harmonics analysis of the square wave - you get the exact same behaviour with a single edge which has a continuous spectrum - this is a behaviour best explained purely in the time domain looking at reflected edges. The signal hits the t-piece and 50 ohm terminator and sees 25 ohms (50 || 50) until the reflection from the open end comes back and causes another step change (which itself propagates back to the open end and back a few times with decreasing amplitude - causing a stepped waveform dependent only on the length of the open stub).
Basically you've got a TDR there!
Back in school our teacher had a saying: If you are not using a 10x probe you are certainly getting a wrong result, If you use a 10x probe you are likely getting a wrong result.
It loses a little wordplay in translation but the point was we should always think and figure out if there is some shenanigans going on with the results.
Would you give an example of a high frequency x10 probe?
During the Olympic games our radio club had a station running many HF transmitters at once. Several frequencies interfered with each other so we used a open piece of coax tuned to the interference frequency like you showed to act like a filter. It worked great and now after watching this video and watching what happens I understand exactly what was going on to achieve the filtering effect.
Interesting stuff, Thanks for the great explanation!
So the coax has capacitive reactance due to its innate physical construction which has a different reactance at different frequencies which makes the amplitude go down due to the impedance it creates. By adding inductive reactance you counteract the capacitive reactance to reduce the impedance to near zero... correct?
Thanks man....
Another good tip is not to trust some old RG58 BNC Ethernet cables and use them for that sort of measurements. Their flatness even below 20 MHz is designed to just work with Ethernet. Even proper termination won't help. Those cables bit me years ago, and since then they went straight to the bin. I believe this problem is the same for any cheap RG58 cable you can buy online these days.
EEVblog Now this is weird because you start with "they are exact same length" and there is no explanation for the difference of amplitude between both channel until you start to play with phase and mesure the length difference between both cable which now explain the amplitude difference between both cable, right ?
Very cool.
Thanks Dave. I've never seen a BNC connector that your probe sticks into. What do you call it?
It comes with the probes. I first saw them when they came with my Agilent scope.
You can calculate the length of of coax using a square wave by measuring the difference between the first and second rising edge see w2aew video for the calculations. Great video and explanation.
Perhaps it's a good idea to think of the coax as a tunnel? The sig gen is at one end, shouting down the tunnel to the scope at the other end. All kinds of echoes can happen and possibly even resonance. Adding the terminator is like making the tunnel much less echo-y. Not a perfect analogy, I admit, but gets the point across.
Cool. I wonder, is it possible for a company to make the oscilloscope automatically detect this issue and then automatically engage the proper termination? This seems like something that would also show up on an FFT output which some scopes are able to provide.
What type of coax adapter are you using on your probe,I have not found an adapter with the BNC that will fit onto a probe?
+William Colvin They are pretty common. You can buy them by themselves or a lot of probes that aren't the absolute cheapest you can find come with them.
www.amazon.com/Cal-Test-Electronics-CT3655-Oscilloscope/dp/B009QBSKE2/ref=sr_1_2?ie=UTF8&qid=1443499425&sr=8-2&keywords=oscilloscope+probe+bnc+tip
Here is a 10 pack of just the connectors.
www.amazon.com/Precision-PR150SA-Oscilloscope-Probe-Insulated/dp/B00TTOWEX4/ref=sr_1_6?ie=UTF8&qid=1443499425&sr=8-6&keywords=oscilloscope+probe+bnc+tip
Here is a probe that comes with the bnc tip.
I have never used either of the products and so am not vouching for their quality at all by linking them, they were just the first to examples that popped up on Amazon.
Thank you very much.
+lbrieden1 Thank you very much.
+William Colvin Very much welcomed
It makes a good notch filter :)