EEVblog #652 - Oscilloscope & Function Generator Measurement Trap - Page 1
Forum section: electronics / general

author: EEVblog
author_rank: Unknown
timestamp: August 14, 2014, 10:01:17 pm
content: 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 is thrown in as well.

author: apelly
author_rank: Unknown
timestamp: August 14, 2014, 10:46:05 pm
content: Useful background/primer from w2aew:How to measure coax velocity factor VF and impedance ZTransmission Line Terminations for Digital and RF signals - Intro/TutorialEdit: Oops. Forgot to say thanks, Dave. Please excuse my manners.

author: Co6aka
author_rank: Unknown
timestamp: August 14, 2014, 11:02:37 pm
content: Yo Stubby!Yous shoulda also shorted da enda da stub 'n show'd 'em what appens den -- fuhgeddaboudit!

author: HP-ILnerd
author_rank: Frequent Contributor
timestamp: August 14, 2014, 11:22:25 pm
content: Dave,Great video! It's a nice one particularly since there's a lot for viewers to try at home (assuming they have appropriate kit).This is actually the sort of thing that lured me back into the hobby from the software side of things. In software, everythingis so perfect and pristine--i.e, the ones and zeroes don't ring, reflect or otherwise distort. But that's only because some EEmade sure they were that way in the end product. At the bottom, the electronics are entertainingly messy, and there is physicsinvolved, not just logic.I suppose it's normal practice when debugging something complicated to try your probing setup against known voltages, etc.to make sure you are seeing real data and not an artifact of your measuring arrangement?Anyway,

author: w2aew
author_rank: Super Contributor
timestamp: August 14, 2014, 11:31:45 pm
content: Quote from: apelly on August 14, 2014, 10:46:05 pmHa, you beat me to it! Dave's now playing in my RF world... Here's a few more besides those that you mentioned...This one is on how to make your own TDR (take advantage of the reflection to measure the coax length):My initial video on how to measure length and impedance of coax using scope:This one goes into more detail on 1/4 wavelength transmission line (stub filters):

author: martinv
author_rank: Contributor
timestamp: August 15, 2014, 12:05:38 am
content: It seems you can think of this in both frequency and time domain. On the white board at 16:00 on the portion of the pulse labeled "Out of Phase Cancels", I tend to think (perhaps abnormally): what if that pulse is the first pulse? There is no reflection yet, as the pulse has only just reached the oscilloscope input. The oscilloscope sees the signal of the 50 Ohm signal generator terminated into 2 50 Ohm loads (25 Ohm), one being the terminator and the other being the characteristic impedance of the stub cable. The level at the scope remains about the same as the signal travels down the open ended cable. When the first reflection coming back from the open stub cable hits the oscilloscope input, revealing the open stubs termination impedance (infinite) and the voltage jumps up (labeled "IN PHASE" on the white board). Beyond that you get more back and forth reflections adding and subtracting.It's kind of fun to view it on the scope, then model it in LT Spice and see how the results agree, but the real measurements are what I tend to remember and make the concepts sink in. I also found w2aew's videos on this subject very good and it's always interesting to see other perspectives.

author: apelly
author_rank: Unknown
timestamp: August 15, 2014, 12:20:14 am
content: Quote from: w2aew on August 14, 2014, 11:31:45 pmSorry mate. Just happened to be online.And I really like your videos.

author: EEVblog
author_rank: Unknown
timestamp: August 15, 2014, 12:31:20 am
content: Quote from: martinv on August 15, 2014, 12:05:38 amYes, and ultimately there are many ways to think of and explain this sort of stuff.

author: EEVblog
author_rank: Unknown
timestamp: August 15, 2014, 12:33:06 am
content: Quote from: HP-ILnerd on August 14, 2014, 11:22:25 pmYeah, it's a fun and easy way to play with transmission line theory without actually having to build anything.

author: KD0RC
author_rank: Regular Contributor
timestamp: August 15, 2014, 03:53:53 am
content: Thanks Dave. As someone who likes to play with RF, but without an EE degree, I don't usually think of the termination when connecting a pair of BNC connectors with coax. This kind of tutorial really helps keep me from falling into those famous traps for young players!

author: hamster_nz
author_rank: Super Contributor
timestamp: August 15, 2014, 04:25:19 am
content: At 14:21, the 'bench' on the rising edge is caused by the signal being split, not the reflection. The short stub looks like a parallel resistance until it is 'loaded up'.If the stub cable was infinitely long, the 'bench' would be infinitely long too, as a portion of the signal's energy sails off down the stub...Or to look at it another way, it can't be a reflection, as the signal hasn't reached the unterminated end of the stub yet!

author: motocoder
author_rank: Unknown
timestamp: August 15, 2014, 04:58:44 am
content: Quote from: EEVblog on August 14, 2014, 10:01:17 pmNice video, although I was a bit disappointed that you didn't whip out a Smith Chart.

author: BravoV
author_rank: Super Contributor
timestamp: August 15, 2014, 05:11:49 am
content: Thanks for this video Dave !It will be a good reference for helping "tons" of young players that were facing this exact problem that we've encountered and helped in this forum alone.Just to complement this video, you also can use a cheap 50 Ohm terminator + a T connector instead of using feed through terminator as Dave used in the video as usually its harder to find and more expensive.

author: miguelvp
author_rank: Super Contributor
timestamp: August 15, 2014, 05:14:54 am
content: And if the terminator is color coded, then Green is 50 Ohms, Purple is 75 Ohms

author: OilsFan
author_rank: Regular Contributor
timestamp: August 15, 2014, 09:14:36 am
content: Thanks Dave good info though I don't usually play up in that Hz range being an audio guy. I have a few side questions related to coax for a noob like me.1. What exactly is termination and what does a terminator do? Does it simply ground the signal wire to the shield?2. Where do you get good 50 ohm coax leads? Also can a person make them easily?

author: SNGLinks
author_rank: Regular Contributor
timestamp: August 15, 2014, 09:21:34 am
content: I wish my 'scope (DSO-X 2024A) had switchable input Z (50/75/1M) as I do loads of RF and video work.I have to use T pieces and terms - very messy - easy to use the wrong term.Using a spectrum analyzer and a tracking generator is a great way to show the effect of a stub.

author: HP-ILnerd
author_rank: Frequent Contributor
timestamp: August 15, 2014, 09:32:31 am
content: Quote from: OilsFan on August 15, 2014, 09:14:36 amThis is a broader question than you may realize. Not to step on a question directed at Dave, but this will answer your question pretty effectively:

author: jesuscf
author_rank: Frequent Contributor
timestamp: August 15, 2014, 01:07:14 pm
content: Transmission line reflections could happen at any frequency. For instance at 50/60Hz. This is a big issue with power grids where the transmission lines could be hundreds of kilometers long. Just imagine what would happen in a a 400kV power line if the voltage doubles...

author: w2aew
author_rank: Super Contributor
timestamp: August 15, 2014, 01:19:59 pm
content: Quote from: SNGLinks on August 15, 2014, 09:21:34 amHere's the video I did on that exact topic:

author: w2aew
author_rank: Super Contributor
timestamp: August 15, 2014, 01:22:36 pm
content: Quote from: OilsFan on August 15, 2014, 09:14:36 amTo answer #1, a termination is usually a resistive load connected at the end of a transmission line (from center to ground), whose resistance matches the impedance of the transmission line. Here's a video I did on the topic of terminations and reflections, from both a digital and RF perspective:

author: w2aew
author_rank: Super Contributor
timestamp: August 15, 2014, 01:24:21 pm
content: Quote from: HP-ILnerd on August 15, 2014, 09:32:31 amThat AT&T video is one of my favorites - absolutely brilliant way to visualize wave propagation and reflections on a transmission line.

author: rstoer
author_rank: Regular Contributor
timestamp: August 15, 2014, 02:42:00 pm
content: This whole topic brings up a question I've had for awhile:When do you need a feed-thru terminator vs. the end-cap type?

author: eendje
author_rank: Newbie
timestamp: August 15, 2014, 02:46:40 pm
content: Hi All,please also add a bit about different cables, and connectors, there is a world of difference.maybe this wuld be a good read for your young ones as wellhttp://www.l-com.com/content/Coaxial-Cabling-Tutorial.htmlmy two cents,Eendje

author: w2aew
author_rank: Super Contributor
timestamp: August 15, 2014, 03:58:10 pm
content: Quote from: rstoer on August 15, 2014, 02:42:00 pmA feedthrough is used when connecting the end of a transmission line to something that has a high input impedance (scope input, counter input), and it is placed between the end of the coax and the input jack on the instrument/device. It provides a proper termination to the line, with ideally minimal issues from the short "stub" connecting the high-impedance device.A end-cap type termination is used when you need to terminate the end of a length of transmission line to prevent reflections. In many applications, this would be called a "dummy load". For example, it may take the place of an antenna so that transmitter tests can be run into a proper load, but not radiate. It may be used at the end of an old Ethernet coax run. They may be used to terminate unused outputs of an amplifier, or unused halves of a differential output device. In a pinch, they can be used along with a T connector at an instrument/device input to substitute for a through terminator when one isn't available.

author: rstoer
author_rank: Regular Contributor
timestamp: August 15, 2014, 05:51:05 pm
content: Quote from: w2aew on August 15, 2014, 03:58:10 pmGot it. Thanks for the clear explanation.BTW: I subscribe your U-Tube channel and always enjoy your videos. I used to work as an electronics technician but switched to software development over 20 years ago. Now as I approach retirement I'm back to electronics as a hobby, trying to understand more about the design process. Videos from you, Dave and others are making this possible. Your efforts are appreciated!!!

author: plesa
author_rank: Frequent Contributor
timestamp: August 15, 2014, 06:19:18 pm
content: Quote from: SNGLinks on August 15, 2014, 09:21:34 amTry to ask in the MSOX/DSOX 2k/3k hack thread, there is several options which can be enabled in firmware. I'm also not sure if the HW (relay) is present.But 75 Ohm is rare on scopes nor is it available on 3K/4k/6k.

author: PA0PBZ
author_rank: Super Contributor
timestamp: August 15, 2014, 06:25:17 pm
content: I wonder if in the last part where the yellow wave peaks when the blue one is at its lowest... is this maybe the function generator increasing the voltage? If it tries to keep a constant level by measuring the output it will certainly be influenced by the reflected signal.

author: briandorey
author_rank: Regular Contributor
timestamp: August 15, 2014, 07:54:43 pm
content: You can also use the stub as a notch filter to attenuate certain frequencies which can be useful to block radio receivers from strong interference sources.I found this sitehttp://www.arcticpeak.com/antennapages/quaterwavestub.htmwhich has a useful calculator to find the coax length needed to block a frequency.

author: HP-ILnerd
author_rank: Frequent Contributor
timestamp: August 15, 2014, 09:33:45 pm
content: Quote from: w2aew on August 15, 2014, 01:24:21 pmYeah, and I like the broad applicability of it . You'll never look at a wave in a pool in quite the same way again.It's a very clear and watchable video, too. As are yours! Between them, a beginner's train of thought can go from "what the heck is going on?" to "that needs to be terminated."Understanding is powerful stuff.

author: VanitarNordic
author_rank: Frequent Contributor
timestamp: August 16, 2014, 03:20:41 pm
content: Please also make a video on differential signal measurement (like CAN ...)

author: SNGLinks
author_rank: Regular Contributor
timestamp: August 16, 2014, 10:02:14 pm
content: Quote from: plesa on August 15, 2014, 06:19:18 pmI've got most of the options enabled. Never heard of different terms being an option. As the 'scope has many video trigger modes I'm surprised it doesn't offer the video term.

author: OilsFan
author_rank: Regular Contributor
timestamp: August 17, 2014, 02:08:03 am
content: Thanks HP-ILnerd and w2aew for the videos. Very informative.

author: coppice
author_rank: Super Contributor
timestamp: August 17, 2014, 05:55:55 am
content: Quote from: SNGLinks on August 15, 2014, 09:21:34 amWhy would anyone put a 75 ohm terminator on a 50 ohm connector. There are 75 ohm BNCs, although they are rare these days. Beware of these things, as they cross-mate mate too easily with a 50 ohm BNC, and bust it.Adding 50 ohm termination is not cheap if you do it right. You need a switch, and it probably needs to be a physical switch. If you switch in a 50 ohm terminator while the maximum permitted voltage is applied to the input there would be considerable power dissipated, so you also need protection too.When the input switching of scopes was largely mechanical a 50 ohm terminator setting was much more common. Quite a few scope didn't have proper protection, though, and dealing with overdissipation in the terminator was a common reason for repairs.

author: SNGLinks
author_rank: Regular Contributor
timestamp: August 17, 2014, 11:24:13 am
content: Quote from: coppice on August 17, 2014, 05:55:55 amWhen working with video you have to use a 75 ohm term. I use 75 ohm cable and a 75 ohm term plugged into a 50 ohm T-piece into the 'scope. The T-piece doesn't appear to suffered having the wrong BNC plugged into it. I would not risk a 75 ohm BNC plugged directly into the 'scope.If you weren't meant to look at video then why give us all those video triggering modes?

author: coppice
author_rank: Super Contributor
timestamp: August 17, 2014, 03:35:28 pm
content: Quote from: SNGLinks on August 17, 2014, 11:24:13 amOscilloscopes are meant for the viewing of many types of signal, but the oscilloscope doesn't provide every possible interface needed for that.If you have a real 75 ohm BNC it has a thinner pin in the middle, so its impedance is higher. They easily mate with a 50 ohm BNC, but the narrower socket will be stretched by the thicker pin. I have no idea when they didn't make the outside part of the 75 ohm version fatter to get the right impedance in a way that wouldn't allow false mating.These days its common to find a 50 ohm BNC on the end of 75 ohm cables. YMMV.

author: vk6zgo
author_rank: Super Contributor
timestamp: August 19, 2014, 05:12:18 am
content: Quote from: coppice on August 17, 2014, 03:35:28 pmSorry,but that doesn't make sense!If a 75 Ohm BNC centre pin was narrower than that of a 50 Ohm one, how could that damage the latter?Perhaps the other way round!.Over many years of dealing with 75 Ohm video systems in analog TV Broadcasting,I have found it was standard practice to use "50 Ohm" BNCs on the end of 75 Ohm coax--- they make them to fit RG59,etc.With the advent of Digital TV,& the necessity of piping baseband digital video around the Studio,the racks were rewired with "real" 75 Ohm BNCs,as were any test leads,etc.At the time,(1999),the only difference I could discern with the plugs was that the insulation on the 50 Ohm ones was brought right up to the front of the connector,whereas with the 75 Ohm one it was not.There was never any question of incompatibility.Looking at a manufacturer's site,unless I misread,the part number for the centre pin is the same for both versions,but the different insulation coverage is plain in the diagrams.http://www.gordontech.com.au/catalog.htmThere is a definite difference in centre pin size in"N"connectors,where the 75 Ohm one will cause damage to the 50 Ohm version,but that is a different matter.

author: coppice
author_rank: Super Contributor
timestamp: August 19, 2014, 06:43:01 am
content: Quote from: vk6zgo on August 19, 2014, 05:12:18 amOne way around you get poor contact. The other you get damage. Neither is a happy solution.Quote from: vk6zgo on August 19, 2014, 05:12:18 amI suspect the industry has completely abandoned the use of 75 ohm BNCs because of the cross mating problems. Analogue video isn't very wide bandwidth, and you kind of get away with having a small length of 50 ohm discontinuity in a 75 ohm cable. Try it at 300 MHz, and the results can be pretty sad.

author: vk6zgo
author_rank: Super Contributor
timestamp: August 19, 2014, 07:18:29 am
content: Quote from: coppice on August 19, 2014, 06:43:01 amThe "Impedance discontinuity" isn't that great,& only occupies a very small fraction of a wavelength,even at 300MHz..So-called "UHF" connectors which don't haveanydeclared Impedance "get away with it"at 438MHz!Either there was an earlier version of 75 Ohm BNC which I am unaware of (& I've been around a long time),or the compatibility problem isn't as bad as you think.

author: Slothie
author_rank: Regular Contributor
timestamp: August 19, 2014, 11:53:12 am
content: I have a small box full of T pieces and terminators somewhere because I'm old enough to have been working with computers when thinwire coax was the network cabling of choiceThis video shows WHY accidently disconnecting a cable could shut down the network for a whole section!!

author: ConKbot
author_rank: Super Contributor
timestamp: August 19, 2014, 04:31:13 pm
content: If youre that concerned about ithttp://www.minicircuits.com/MCLStore/ModelInfoDisplay?14084643973520.760222021982059650 ohm end goes into scope, 75 ohm end goes to your 75 ohm cable. Scope has to be in 50 ohm mode, and just remember to account for the 5.7db of loss.

author: peteroakes
author_rank: Contributor
timestamp: August 20, 2014, 02:29:24 am
content: nice video Dave, many things I had not mentioned in my video when describing how use the same properties to measure the length of various cables and how the termination can mess you up my video for reference is here,http://youtu.be/JPnKfPzj3E4, it is not talking about the pitfalls from a measurement perspective, I think you did an admirable job of that, but more about how to make use of those pitfalls to determine where a fault lies and also how to measure the length of a cable even if you dont have access to the other end of it.I also only used a Sig gen and an Ebay special scope so as you said, its all about the fast rise times of the signal source and does not require anything too expensive to see the affectPeter from the Breadboard.caKeep up the excellent work Dave.

author: larry42
author_rank: Regular Contributor
timestamp: August 28, 2014, 06:26:56 pm
content: Quote from: coppice on August 17, 2014, 05:55:55 amI used to think so as well (damage between 50 and 75Ohm BNC), but this seems to *not* be the case for modern 75Ohm BNC connectors. I checked this for a project that used DIN1.0/2.3 and BNC connectors (both 50 and 75Ohm). According to Rosenberger"The optimum working frequency of BNC and TNC 50 ? connectors isupto4GHz(max.10GHz),of75?connectorsisupto1GHz (max. 4 GHz). 50 ? and 75 ? connector types are intermateable without any restrictions within BNC and TNC series."http://www.rosenberger.com/documents/headquarters_de_en/ba_communication/catalog_coax/12_Chapter_BNC_TNC.pdfI believe that this change was made in the 70s or 80s.NB this is *not* the case for 75Ohm N-type, but these are thankfully quite rare (I've never seen one).