What do you know about Standing Wave and How to Measure it? - Page 1
Forum section: electronics / beginners

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 11, 2012, 09:24:24 am
content: i want to study standing (or reflected) wave in my circuit, want to know how improperly matched the impedance is etc etc. not RF circuit with antenna, just normal circuit with few cm of transmission lineor loose wire(copper wire) running at 10-100MHz from source (IC or opamp) to destination (another IC or opamp). do you guys know any method (preferably simple one) how to measure or at least have a rough figure on what the value like? thanks.

author: BravoV
author_rank: Super Contributor
timestamp: November 11, 2012, 09:43:29 am
content: Maybe this Alan's video could help ?

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 11, 2012, 11:55:24 am
content: i've watched that, but that is to measure coax impedance and length. maybe i asked incomplete question sorry. what i want to know is how to measure standing wave ratio SWR (voltage or power) and the idea behind it (how to get the ratio). i know the idea its supeimpose of node and antinode signal, but when it come to practical measurement, i'm stumbled a bit. the line length is not in meters, but only a few cm, eg 10-20 cm, i believe its quite a challenge if using allan's method with my ds1052e.i saw a method from rcgroup...http://www.rcgroups.com/forums/showthread.php?t=1555674but its specific to radio (RF) using kindof RF peak detect mode and probing two points in circuit end to end, not sure what kind of circuit is that. can that be applied to my case? does VSWR meter need to probe 2 points in circuit? is measuring peak both at source and destination and dividing the value will simply get the ratio?edit: this is another one but i'll need time to study that circuit...http://electronics-diy.com/electronic_schematic.php?id=952http://namham.phpbb8.de/homebrew-corner-f17/swr-meter-diy-t45.html(also still, its for radio to antenna (RF or HAM) type of circuit measurement)

author: robrenz
author_rank: Super Contributor
timestamp: November 11, 2012, 01:53:27 pm
content: Can you just convert the frequency (pick one of the range) to wavelength and see if your trace length is an exact multiple or sub multiple? then do that to the entire range of frequencies.But when i check the wavelength of 100MHz it is 299.79cm so even 1/4 of that is immensely longer than any of your traces. I don't think it applies at this low of a frequency and this short of a distance. Remember this is a machinist talking not an EE

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 11, 2012, 03:12:00 pm
content: yes, doing the 1/10X or 1/4X wavelength calculation seems easy and my circuit should be well within the safety limit. but in practical i see alot of ringing, signal distortion, mystery stuffs i cant explain. for example, probing low freq to 10MHz is ok, and then 10MHz-30MHz the signal got attenuated, and then goes back up again at higher frequency etc etc.maybe with SWR knowledge i can plot some graph and see the corelation? and have better tool of knowledge? i dont know. but lets not going offtrackwhether its the problem or not... how to measure SWR in real life?

author: PA0PBZ
author_rank: Super Contributor
timestamp: November 11, 2012, 08:14:53 pm
content: I think measuring SWR in such a short trace will be almost impossible. Just to state the obvious: source, transmission line and destination impedance should be the same. Source and destination are in the datasheet, and the transmission line you can calculate.

author: Bambur
author_rank: Regular Contributor
timestamp: November 14, 2012, 02:51:13 pm
content: A bit off topic, but still worth looking at. Wave, media impedance, impedance matching, standing wave ratio --

author: robrenz
author_rank: Super Contributor
timestamp: November 14, 2012, 03:11:57 pm
content: In trying to learn about this to have some usefull input I have come to this highly uneducated opinion.Standing wave is not what you are concerned about. It is proper impedance matching of the source, the connecting conductor, and the receiving device. If that is properly done there will be, the least attenuation possible, minimal reflection, therefore no appreciable standing wave with a VSWR of close to 1:1. FWIW

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 14, 2012, 03:56:42 pm
content: actually this got something to do with differential probe. i saw fluke or any other diff_probe use simple wire as probes and the length is quite significant. example is this...that one goes up to 20MHz, 1.5m loose wire probe length. 20MHz 1/10x wavelength is 150cm, fair! from my experience, i only use 20cm loose wire length and i got nasty stuff at that frequency and nearby. actually i'm not even sure if i ask the right question sigh. but certainly i would like to gather as much knowledge as i can, hence this SWR thing came to mind.

author: robrenz
author_rank: Super Contributor
timestamp: November 14, 2012, 04:39:41 pm
content: I thought you were talking about within your circuit. All three of my differential amplifiers use scope probes for inputs with grounds usually connected at the probes. IMO especially for 100MHz you would want the shield effect of the coax of the probe all the way to the probe tip to minimize interference. The CMRR on all diff amps drops with increased frequency so you need to still use best practices to minimize common mode noise. The impedance mismatch of the two points probed with a diffamp also greatly affects the effective CMRR but that is a different issue than attenuation from SWR. FWIW, remember machinist not EE

author: poptones
author_rank: Frequent Contributor
timestamp: November 14, 2012, 05:37:35 pm
content: yes, doing the 1/10X or 1/4X wavelength calculation seems easy and my circuit should be well within the safety limit. but in practical i see alot of ringing, signal distortion, mystery stuffs i cant explain. for example, probing low freq to 10MHz is ok, and then 10MHz-30MHz the signal got attenuated, and then goes back up again at higher frequency etc etc.There are many ways to measure SWR, though they are pretty much the same. Simplest way I know is to make a wheatstone bridge with the coax as one node and measure the voltage across it. But I don't know any old engineers tricks to do what you're trying to do except suggest making sure you're using a 10x probe and use the ground lead directly at the driving end of your circuit.I suspect the ringing and such you are seeing is related to trace distortion. Do you have parallel signal and ground paths? If your trace is meant to carry vhf you're going to want to either have a ground plane or you'll need to run a ground trace alongside your signal from source to destination. Do you have an impedance matching resistor at the beginning of your signal run? Is your trace terminated at the other end with a resistor?You might try looking into ECL troubleshooting methods. Here's a paper on ECL and design constaints that might get you startedhttp://www.analog.com/static/imported-files/tutorials/MT-097.pdf

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 14, 2012, 05:51:00 pm
content: Quote from: robrenz on November 14, 2012, 04:39:41 pmthe probe... it is within my circuit, i consider it that way. its just no ground plane no impedance control.Quote from: robrenz on November 14, 2012, 04:39:41 pmimpedance control and rejecting interference are another stories, consider that done! (not done actually). my question implied that in case we cannot control the impedance (and there is no interference), how do we measure SWR?Quote from: robrenz on November 14, 2012, 04:39:41 pmthis is certainly offtopicQuote from: robrenz on November 14, 2012, 04:39:41 pmi know!

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 14, 2012, 06:16:27 pm
content: Quote from: poptones on November 14, 2012, 05:37:35 pmwhat is trace distortion?Quote from: poptones on November 14, 2012, 05:37:35 pmnot necessarily. do i have to? what if i cant?Quote from: poptones on November 14, 2012, 05:37:35 pmoff topic againor maybe this ground plane is very closely related to SWR? maybe to fix high SWR right? my question is not how to fix it but how to measure it. no offense intendedyou've answered some of my question in the beginning of your post thanks i'll look into it.Quote from: poptones on November 14, 2012, 05:37:35 pmthanks!

author: poptones
author_rank: Frequent Contributor
timestamp: November 14, 2012, 06:23:35 pm
content: You're going to have to stop saying "off topic" or you're never going to get your question answered.100MHz is VHF. So is 30MHz. Apparently you are trying to shuffle VHF around a board without a ground plane, without proper design methods, and wondering why it won't work. Then when someone tries to help you tell them their response is off topic.If you want to move 100mhz more than just a couple of cm, you have to treat it as a transmission line. That means it has two conductors, a signal and a ground. Your ground has to run next to your signal. You can do this one of two ways: with a ground plane, or with thin parallel traces. Either way, your ground MUST be able to follow the electromagnetic path - that means every curve, twist and bend. Same with your scope probe: use the x10 setting on the probe because that minimizes the effect of the leads, and make sure your ground is attached as close to the signal pin as possible - ideally the ground lead of the component generating the signal. Otherwise you're going to get ringing and all sorts of nonsense.

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 14, 2012, 07:35:40 pm
content: Quote from: Bambur on November 14, 2012, 02:51:13 pmexactly on topic! i've watched that (after 2 hours download) it is exactly what i'm looking for, the basic of standing wave and impedance matching. very educational thanks i already have few ideas in mind. @poptones please dont take it seriously, thanks for your advice i still struggling with this waves behaviour, i thought the ground plane have something to do with capacitance coupling or such, still cant put all the ideas in one piece yet

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 14, 2012, 07:47:05 pm
content: i think i got the "idea" about SWR already if not completely... ok make me thinking (from the at&t video)... we know a transmission line has a characteristic impedance... a track on pcb with a ground plane below or a cable with surrounding shield, if properly placed at some distance with some proper material in between (fr4 or that white plastic in coax) will have certain characteristic impedance and if terminated properly will avoid reflection. so the new question is... whats the characteristic impedance of a single loose wire? and how to terminate it properly?(again i'm talking about when a ground plane or parallel line is not feasible) pls bear with me i'm trying to understand here.

author: poptones
author_rank: Frequent Contributor
timestamp: November 14, 2012, 07:52:38 pm
content: You have to keep it short. If you can't keep it short then you have to use coax or properly designed pc board. There are no other options.

author: jahonen
author_rank: Super Contributor
timestamp: November 14, 2012, 08:02:47 pm
content: I think the most important thing to do here is to first to try to understand the problem - it might not even be on the PCB. First thing to do is to verify that the measurement setup used for the diagnostics gives sane results. Then try to isolate the problem a little step at a time, verifying each amplifier stage independently.Few centimeters of short unterminated transmission line (use LTSpice and Tline component to get an idea of that, see attached simulation, 50ns transmission line delay will exhibit its first resonance when signal period is 4*50 ns=200 ns, which equals 200 ns => 5 MHz, which is indeed the case) will probably show a little rise due to mismatch, but that rise continues monotonically up to ?/4, which is the peak point and then the amplitude recovers. However, 50 ns equals about 10 meters of ordinary coax cable, so one has to really make things long to get such pronounced behaviour. If the delay is reduced to 100 ps, then the effect is barely noticeable.Since you are experiencing wobbly frequency response here, I'd guess that the problem it is not on the PCB, at least not with the PCB transmission line mismatch. If I had to make a guess, it is in the measurement setup.Regards,Janne

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 14, 2012, 08:32:55 pm
content: thanks allit will be until next week before i can make another diagnostics.

author: poptones
author_rank: Frequent Contributor
timestamp: November 14, 2012, 08:43:42 pm
content: It's not frequency it's risetime. OP specifically mentioned looking at square waves. A 10MHz square wave can have 100pS rise time... then what?

author: PA0PBZ
author_rank: Super Contributor
timestamp: November 14, 2012, 09:06:01 pm
content: It would be of great help if you posted the actual circuit you are experiencing these problems with. It looks like we are all in the dark about that...

author: jahonen
author_rank: Super Contributor
timestamp: November 14, 2012, 09:20:39 pm
content: That rise time thing is most certainly true, in fact the actual pulse repetition frequency does not matter at all. Reflections show up even if the transition is made just once, no frequency at allBut what I have inferred so far from various postings is that Mechatrommer has built some kind of an active differential probe and it shows a wobbly frequency response. In that case that rise time thing might not be relevant. But I might be wrong. I'm not at all sure that Fluke DP120 shown earlier exhibits a flat frequency response from DC to 20 MHz.Regards,Janne

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 14, 2012, 10:07:29 pm
content: its very hard to show picture right now i cant get my bench organized i have something else non-electronics urgent to do right now i only can afford talking. i think the problem is not on the pcb, but on the loose wire used as the probe, which i consider apart of the "circuit", hence i want to steer away from impedance control discussion.the situation is like this, i have the pcb (diff probe), i used loose wire as the probe, no shield no twist no parallel no nothing, just a single 20cm length wire strand in rubber or plastic or whatever that is, ie normal wire. last time diagnosing with impedance tight control (using coax or direct pcb to pcb or bnc or sma connection showed no issue (or very slight woobling), thing got worse when i used loose wire, the respond start go up and down at only few tens MHz. feeding square wave from 50ohm FG showing ringing on the diff_probe output (to scope), that indicates not-nice frequency respond which is confirmed when feeding sine wave at the verge of 20-30MHz or so. using coax or other "better" setup will attenuate the effect. so it got to be in loose wire. and the HF loading (or reflection or distortion?) to the source (FG) feeding square wave is quite significant, that is probed at the FG end using terminated coax to tee, the other side of tee is where the differential probes connected. the HF loading will got worst if both diff probe input connected to the same source. it got halved when one probe tied to the FG gnd. soi'll post picture when i got the chance. as janne said, i need to prove my measurement setup and each amplifier stage first then its worth further discussion about "the matter that need to be fixed".

author: PA0PBZ
author_rank: Super Contributor
timestamp: November 15, 2012, 03:11:43 pm
content: Just to give you an idea what a piece of wire does, here are some pictures of measurement from 10 - 200MHz.S1 - direct connection, readout normalized on this.S2 - short piece of wire.S3 - famous crocodile clip wire.Of course the results will depend on the thickness of the wire, the height above the PCB and such.And this is dB scale, if you look at the absolute voltage it will look a lot worse.

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 16, 2012, 12:53:33 am
content: thanks for that photos. janne did that for me too last year, up to GHz span, but your are clearer in the region of interest. opening them back they all seem broken now, i should have saved them to my local HDD. -1 to SMF forum.https://www.eevblog.com/forum/beginners/how-to-better-wire-the-bnc-connector-and-ghz-issue/msg37545/#msg37545yes from your picture its woobling up and down at the region 10-100MHz probably what i have experienced, can anybody explain why is this? is it due to reflection (standing wave?) or other reason?

author: ptricks
author_rank: Frequent Contributor
timestamp: November 16, 2012, 01:06:02 am
content: One of the best videos I ever saw on standing waves was done with water, a laser line level , and a tank.The laser line level was aimed so that it projected a line on the wall reflected off the water surface just under the water level. The wall then showed a waveform about like an oscilloscope display with the water modulating the laser line. For people trying to understand standing waves this made it clear almost instantly where hours of lectures failed.Waves generated at one side would propagate across the surface and as you kept generating the same wave on the left , after about 5-10 seconds, you would suddenly see a waveform appear going several times as fast as the original but in the opposite direction of the first, it would stand still for a second a little off center then would suddenly be overtaken by a new wave from the left until again the reflected waves would return, causing the standing wave to disappear.I realize this doesn't address the needs of the OP, but if anyone is trying to understand transmission lines and SWR maybe thinking of the above example and how a wave travels might help.

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 16, 2012, 01:18:31 am
content: Quote from: ptricks on November 16, 2012, 01:06:02 amwhy do you think is that? or is my english really broken? this is exactly what i asked for from the title. duh! can anybody suggest whats the title should be? can you provide the link to the water laser video ptrick?ps: maybe i have mistake in what "transmission line" is. which does not include a loose cooper wire, hence i edited the OP to include that.

author: ptricks
author_rank: Frequent Contributor
timestamp: November 16, 2012, 12:52:33 pm
content: Quote from: Mechatrommer on November 16, 2012, 01:18:31 amNo , your english is fine. I meant that I don't address how to do the measurement in electronics. I don't have a link for the laser video, but there are good videos on youtube that shows the same sort of thing.For general wave information

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 16, 2012, 05:05:28 pm
content: thanks for the link... i've made a video about this issue, i thought it will be easier for people to understand albeit my broken spoken and written english. i hope someone can give some comment if my method is flawed...as usual... if you like it, press the "thumb up" button, if not then no need to

author: PA0PBZ
author_rank: Super Contributor
timestamp: November 16, 2012, 08:55:38 pm
content: After looking at your video I'm even more confused... What exactly is your question?

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 17, 2012, 06:57:24 am
content: hehe what i tried to demonstrate....opamp1 output is benchmark signal.opamp2 output should = opamp1 output if no significant loading.my question... why we got bigger signal when probing after the yellow wire? compared to opamp1 (benchmark signal). i've proved that probing before the yellow wire ie at opamp2 output is equal to opamp1 output, not bigger.so this should rule out my measurement setup (i think) and the suspect is in the yellow wire. what i tried to demonstrate is as Janne's statement said that i have to diagnose each stage, each output step by step to pinpoint where the problem is. since i cannot see any woobling respond in any output sma 1,2 and 3... so it must be in the yellow wire (sma 4). with this test setup, i am certain that the problem is in the wire, hence i'm asking standing wave question, not impedance control question. and my mind also buggered with why "1/10X wavelength rule" didnt work. ie if the cable length is less than 1/10X of the frequency wavelength, then we can neglect reflection? but not in my case here.maybe you still dont get the point of the circuit that i've built. it a "highly evolved highly thought" "scientific instrumentation" test, kinda like impedance analyzer, bode plotter circuit etc, except its simple, diy, and my own idea and way. this picture may help you to get to the idea. i beg you to give it some thought.it is much like the internal components (i guess, conceptually) of the device that you have used to test the green aligator clips... the Agilent Technology "Stimulus Respond - Two Port Insertion Loss" test, except my circuit is way way cheap. you connect the aligator clips to the "Two Port" of your device... but for my device below, i need to solder the DUT to the "Two Port" as you can see at the bottom most of the pcb.

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 17, 2012, 07:35:55 am
content: or maybe to explain it more simpler and more reliably....how can you explain and why the woobling (jumping up and down) effect occured in your own picture?it is actually the same question i keep asking from the beginning.

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 17, 2012, 07:59:48 am
content: from my video, the most important point i want to demonstrate is at 11:15 - 11:39. please note the difference the oscilloscope showing when i change from DUT signal to the opamp2 (input signal) output. the rest of video are just to verify my "measurement setup".

author: PA0PBZ
author_rank: Super Contributor
timestamp: November 17, 2012, 01:11:41 pm
content: I see what you mean now. I was confused by the word "problem", as if something unexpected happens.What you see is not SWR but group and phase delay. Don't ask me to explain exactly, but you can have a look here:http://en.wikipedia.org/wiki/Group_delay_and_phase_delayI made a model in LTSpice so you can see the effect and play with it yourself, just save this as an .asc file:Code:Version 4SHEET 1 904 680WIRE 96 80 -112 80WIRE 800 80 656 80WIRE -112 112 -112 80WIRE 96 144 96 80WIRE 176 144 176 80WIRE 256 144 256 80WIRE 336 144 336 80WIRE 416 144 416 80WIRE 496 144 496 80WIRE 576 144 576 80WIRE 656 144 656 80WIRE -112 208 -112 192WIRE 800 208 800 160FLAG -112 208 0FLAG 96 208 0FLAG 176 208 0FLAG 256 208 0FLAG 336 208 0FLAG 416 208 0FLAG 800 80 VoutFLAG -112 80 VinFLAG 496 208 0FLAG 576 208 0FLAG 656 208 0FLAG 800 208 0SYMBOL voltage -112 96 R0WINDOW 39 0 0 Left 2SYMATTR InstName VSYMATTR Value ""SYMATTR Value2 AC 1SYMBOL ind 80 96 R270WINDOW 0 32 56 Invisible 2WINDOW 3 5 56 VBottom 2SYMATTR InstName L1SYMATTR Value 2µSYMBOL cap 112 208 R180WINDOW 0 24 56 Invisible 2WINDOW 3 19 54 Left 2SYMATTR InstName C1SYMATTR Value 2pSYMBOL cap 192 208 R180WINDOW 0 24 56 Invisible 2WINDOW 3 19 54 Left 2SYMATTR InstName C2SYMATTR Value 2pSYMBOL cap 272 208 R180WINDOW 0 24 56 Invisible 2WINDOW 3 19 54 Left 2SYMATTR InstName C3SYMATTR Value 2pSYMBOL cap 352 208 R180WINDOW 0 24 56 Invisible 2WINDOW 3 19 54 Left 2SYMATTR InstName C4SYMATTR Value 2pSYMBOL cap 432 208 R180WINDOW 0 24 56 Invisible 2WINDOW 3 19 54 Left 2SYMATTR InstName C5SYMATTR Value 2pSYMBOL cap 512 208 R180WINDOW 0 24 56 Invisible 2WINDOW 3 19 54 Left 2SYMATTR InstName C6SYMATTR Value 2pSYMBOL cap 592 208 R180WINDOW 0 24 56 Invisible 2WINDOW 3 19 54 Left 2SYMATTR InstName C7SYMATTR Value 2pSYMBOL cap 672 208 R180WINDOW 0 24 56 Invisible 2WINDOW 3 19 54 Left 2SYMATTR InstName C8SYMATTR Value 2pSYMBOL ind 160 96 R270WINDOW 0 32 56 Invisible 2WINDOW 3 5 56 VBottom 2SYMATTR InstName L2SYMATTR Value 2µSYMBOL ind 240 96 R270WINDOW 0 32 56 Invisible 2WINDOW 3 5 56 VBottom 2SYMATTR InstName L3SYMATTR Value 2µSYMBOL ind 320 96 R270WINDOW 0 32 56 Invisible 2WINDOW 3 5 56 VBottom 2SYMATTR InstName L4SYMATTR Value 2µSYMBOL ind 400 96 R270WINDOW 0 32 56 Invisible 2WINDOW 3 5 56 VBottom 2SYMATTR InstName L5SYMATTR Value 2µSYMBOL ind 480 96 R270WINDOW 0 32 56 Invisible 2WINDOW 3 5 56 VBottom 2SYMATTR InstName L6SYMATTR Value 2µSYMBOL ind 560 96 R270WINDOW 0 32 56 Invisible 2WINDOW 3 5 56 VBottom 2SYMATTR InstName L7SYMATTR Value 2µSYMBOL res 784 64 R0WINDOW 0 36 40 Invisible 2SYMATTR InstName R1SYMATTR Value 1000KTEXT -112 24 Left 2 !.ac dec 100 10000000 100000000See the picture...