EEVblog #652 - Oscilloscope & Function Generator Termination Demo

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.

Check out Doug Ford's Oscilloscope probe article which explains a lot of this stuff, and why x10 probes are better than x1.

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[jimharmon]

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!

[samcast]

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!

[waew]

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).

[tonybell]

Cracking explanation, condensed a tedious 2 hour lecture into 30 mins with practical demo of effects, superb, as always, thanks Dave.

[waew]

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

[petergrain]

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.

[Tangobaldy]

I wish i had a teacher like you at school. I watch loads of your videos and learning so much. Thank you

[worldoflard]

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!

[newfablesam]

I've never watched a video of yours and walked away empty handed. Fantastic

[carentanbr]

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!

[oriole]

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!

[SilverCoreLabs]

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. 

[zebsbro]

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!

[SteveAull]

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   

[nachomahn]

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.  

[oraz.]

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.

[Xrispy]

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!

[DavidLeeMenefee]

What w2aew said. ie., DIDO. He knows his stuff. He has several videos that deals with this issue. Thank, THUMBS UP.

[BenChilds]

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.

[barry]

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).