Scope input VSWR figure at 50 ohm, what is that ? - Page 1
Forum section: electronics / beginners

author: BravoV
author_rank: Super Contributor
timestamp: February 18, 2013, 08:12:51 am
content: Searched at the internet and all I found are "antenna" related, really, my poor brain just can't find the correlation between radio transmitting world vs signal measurement at oscilloscope.For example an excerpt from my scope's datasheet :Input at 50 Ohm :- VSWR (DC to 400 Mhz) <= 1.5:1- Max input voltage : 5 V RMS averaged for 1 second +/- 50 V peakPlease be gentle, preferably in more "intermediate hobbyist's" grade electronic speak/dialect if possible.Thanks in advance.PS : Alan (w2aew), really wish you could jump in here to have few words on this matter, I'm absolutely sure you know this subject very well..

author: TerraHertz
author_rank: Super Contributor
timestamp: February 18, 2013, 10:38:31 am
content: VSWR is a measure of the ratio of power traveling in each direction on a transmission line. Ideally (such as on a 50 ohm coax going to the input of a high bandwidth scope), there should be power going in only one direction - towards the scope. If the scope's input isn't correctly matched to the incoming coax, then some of the signal energy is going to be reflected back towards the source (the thing you're trying to measure.) The result of that reflected energy can't be anything other than a degradation of the signal you're trying to measure.So ideally, a scope's specified VSWR should be 1:1, meaning no reflections.It's kind of surprising your scopes stated VSWR is so high. I'm looking at the specs for a high bandwidth 50 ohm input scope I have, and they say "Inputs: percent reflection less than or equal to 5% (for 30pS risetime.)"Converting:www.telestrian.co.uk/cgi-bin/www.telestrian.co.uk/vswr.pl]http://cgi.[url=http://www.telestrian.co.uk/cgi-bin/www.telestrian.co.uk/vswr.pl]www.telestrian.co.uk/cgi-bin/www.telestrian.co.uk/vswr.plVSWR to return loss conversion5% reflection is a VSWR of 1.105 : 1While VSWR of 1.5 :1 is a reflection coefficient of 0.2 ie 20%.What scope is it?

author: w2aew
author_rank: Super Contributor
timestamp: February 18, 2013, 03:48:31 pm
content: TerraHertz gave you a good explanation of VSWR, which stands for Voltage Standing Wave Ratio. When discussing the quality of a 50ohm termination, it gives you a measure of how well matched the termination impedance is to the transmission line impedance. Any mismatch causes a reflection of the signal. When the reflected signal adds to the incoming signal, a standing wave is produced.I like seeing things visually, as you've seen from my videos. One of the best videos I've seen that describes wave action, terminations, reflections and standing waves is this old video from the Bell Labs archives. Absolutely brilliant:

author: mamalala
author_rank: Unknown
timestamp: February 18, 2013, 04:38:48 pm
content: Quote from: TerraHertz on February 18, 2013, 10:38:31 amThat doesn't sound right. Pretty much every table and online calculator that i know of gives a different value here. A reflection coefficient of 0.2 soes not mean 20% reflected power.Here's a table:http://www.firestik.com/Tech_Docs/SWRLOSS.htmAnother table, as PDF:www.markimicrowave.com/menus/data/return%20loss%20to%20vswr.pdfHere's a claculator:http://www.csgnetwork.com/vswrlosscalc.htmlThere are plenty more out there, but at least all the ones that i know of agree: A VSWR of 1.5 means 4% loss, not 20%. To get to 20%, the VSWR is a tad over 2.6.Greetings,Chris

author: Marco
author_rank: Super Contributor
timestamp: February 18, 2013, 06:01:09 pm
content: Quote from: mamalala on February 18, 2013, 04:38:48 pmThe problem is that you are looking at sources which concern themselves with RF transmission/reception and not measurement. They care about power, but for measurement that is irrelevant (as long as you don't blow up the input protection). You should care about amplitude.Terrahertz got it right.

author: Marco
author_rank: Super Contributor
timestamp: February 18, 2013, 06:19:46 pm
content: Quote from: w2aew on February 18, 2013, 03:48:31 pmApart from mixing up short circuit and open circuit electrical reflection ...

author: mamalala
author_rank: Unknown
timestamp: February 18, 2013, 06:22:46 pm
content: Quote from: Marco on February 18, 2013, 06:01:09 pmHmm, maybe i'm missing something then. If i use this page:http://www.eeweb.com/toolbox/rf-unit-converterand start by entering 100 into the watts field, and then entering 96 there (that would be the initiall 100 wats minus 4% in power), the difference in either Vpeak or Vrms is nowhere near 20%, but is 2% instead. Even the other way round, changing Vpeak from 100 to 96 does not give 20% difference, neither in Vrms nor in Watts.So what's wrong with those numbers then, if there is supposedly a 20% loss somewhere, either in amplitude or power?Greetings,Chris

author: PA4TIM
author_rank: Super Contributor
timestamp: February 18, 2013, 06:23:56 pm
content: VSWR is a ratio, the correct use is writing the VSWR is 1.5 ( not 1.5:1)VSWR is about reflection of EM waves. It has nothing to do with loss, but it can cause loss, for instance by multiple reflections in lossy transmission line. So a VSWR of 1.5 will cause a huge loss at 1GHz in 10 meter cheap RG58 but there will be allmost no loss in 10 cm of high quality 50 Ohm airline.VSWR is 1 to infinity, allways positive. So you see there is a numerated mismatch but not if the load impedance it to low or to high.The reflection cooficient is more usefull. It is 0 when matched, an open is 1, a short is -1Or you could say a match is 0% reflection, and 1 and -1 are 100 % reflection.So a RC (or Rho) that is 0.2 is 20 % reflection and the load impedance is higher as the transmission line.I will not do the math but you can convert Rho to impedanceRho = reflected wave / incident waveRho = (Z-1) / (Z+1) where Z is the reference impedance, in this case 50 OhmA VSWR of 1.499 is indeed 19.953% reflection ( Rho is 0.19953) or a reflection of 14dBVSWR = (Umax /Umin) or (1+|s11| ) / ( 1-|s11| ), |s11| is the magnitude of the reflection coefficient.This is indeed very bad, the impedance of your scope is somewhere between 33.4 and 79.9 Ohm and this is a value that is not realistic. This is a toy scope value.Problem is, many tables and formulas are from the HAM world and are to much simplified and only true for Real resistance, so a impedance with a imaginairy part of jX = 0 OhmThey also mix VSWR with cable loss, many people think it has something to do with the source impedance but it is only about matching a transmission line to a load.The VSWR of the 50 Ohm input of a 150 MHz Tek 2445. The red numbers are Rho and green is VSWR. Even at twice the rated bandwith it is still rather good VSWR is 1.13 max.

author: mamalala
author_rank: Unknown
timestamp: February 18, 2013, 06:34:42 pm
content: Damn RF stuff, always gets me lost ...My bad, i was assuming that whatever is reflected equals to a loss. Thanks for clearing that up for me, PA4TIM.Greetings,Chris

author: BravoV
author_rank: Super Contributor
timestamp: February 18, 2013, 06:35:57 pm
content: Wow ... this is getting confusing, I might need times to digest what you guys posted here, btw, thanks to you all for the contribution.@w2aewThank you Sir, that is one hell of an excellent education video, downloaded and will be kept as my learning videos collection, really good material.Guys ... my scope is old analog Tektronix 2465B 400Mhz capable, here the page on it's spec, see for yourself :The question is, is this Tek scope is so bad ?Quote from: PA4TIM on February 18, 2013, 06:23:56 pmIf you called even a Tektronix 2465B is toy scope, then what do you call those cheap Chinese DSOs and your 2445 ?

author: PA4TIM
author_rank: Super Contributor
timestamp: February 18, 2013, 06:41:50 pm
content: I added the vswr of a 2445. Tek is a typical brand that uses worst case specs. It will in real life be much better but they guaranty it will never be higher as 1.5 ( in a tropical rainseason at 45 degrees or in a dust storm in the dessert ;-) )

author: jahonen
author_rank: Super Contributor
timestamp: February 18, 2013, 07:10:04 pm
content: Similar specs can be found on R&S FSV spectrum analyzer, which is definitely a RF instrument. In fact the VSWR is pretty lousy when input attenuator is bypassed (0 dB attenuation):Regards,Janne

author: TerraHertz
author_rank: Super Contributor
timestamp: February 19, 2013, 09:33:18 pm
content: w2aew that's a wonderful old video. Thanks for posting it.Now, something as clear as that, explaining Smith charts for the mathematically challenged.... I wish.

author: BravoV
author_rank: Super Contributor
timestamp: February 20, 2013, 05:30:15 am
content: Quote from: w2aew on February 18, 2013, 03:48:31 pmSo ... at the end, in the context of measurement domain instead of radio transmission, just wonder how bad is it when the VSWR 1.5:1 figure or better, say like 1.1x:1 might degrade or distort the original signal with the extra unwanted standing wave ?Really, I just can't imagine at the signal's visualization in the scope's screen when this factor kicks in, is it in form of signal jittering ? or other artifacts or else ? or even thicker line ? cause I'm visualizing from that tutorial video about the standing wave's shape.Alan, really wish you could make a tutorial video about this and yes, I love to learn and understand stuffs much-much better visually as well.

author: PA4TIM
author_rank: Super Contributor
timestamp: February 20, 2013, 07:44:18 am
content: It causes for instance ringing and overshoot

author: BravoV
author_rank: Super Contributor
timestamp: February 21, 2013, 01:28:15 am
content: Quote from: PA4TIM on February 20, 2013, 07:44:18 amThanks, so only extra ringing and overshoot ? No other types of artifacts ?Quantitatively, say with VSWR of 1.13 like your Tek 2445, how much in percentages these extra unwanted signal will "contaminate" the real signal ?Also another questions :- Why this scope's input VSWR topic is not much discussed or raised ? Especially its clearly can degrade the measured signal integrity ? Is it just trivial issue ? Ask this cause I just can't find straight forward resources about this in Google, maybe my noob googling skill. All google's results are swamped with only antenna/high frequency radio transmission topics, not on the measurement equipment's side.- Since its clearly affects the measured signal, if its not trivial, what kind of situations or conditions that this scope's VSWR factor will affect signal integrity so badly that the measurement results is almost useless ? Examples ?

author: uprightsquire
author_rank: Contributor
timestamp: February 21, 2013, 02:34:47 am
content: w2aew,That vid was great. I've used a version of that wave machines in the classroom, but using duct tape, wooden sticks and gummy bears.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: February 21, 2013, 04:25:18 am
content: Hi BravoV and the group,A lot of the poor VSWR performance comes from the fact that most low frequency scopes have high impedance attenuators for a 1M Ohm input.The 50 Ohm input is made by switching directly, or with a relay, a 50 Ohm resistor across the input.Scopes which are 50 Ohms only have much better VSWR because they have 50 Ohm attenuators.Here is a section from the Tektronix 7A29 specifications. Option 4 is an adjustable delay line that is used to de-skew the input. The adjustable delay degrades the VSWR.The other thing to note in the specifications is that the 10mV setting, which is no attenuation, is worse that the other setting. In the 20mV and above setting the VVSWR is helped by the attenuator. At 10mV/div the VSWR is that of the amplifier.Jay_Diddy_B

author: TerraHertz
author_rank: Super Contributor
timestamp: February 21, 2013, 06:17:55 am
content: Something to remember is that for relatively low bandwidth scopes, the input VSWR value has little significance because easily visible effects of reflections typically fade away after 2 or 3 reflections back and forth on the input coax. So say you have a 1 meter length of coax, the transmission time (one way) will be roughly 3nS. If the scope can't see much happening faster than 10nS (a 100 MHz scope for instance) then the distortions of the edges of fast square wave signals due to poor scope input impedance match will be more or less invisible.Many years ago (1986 actually) I made a little circuit to easily demonstrate reflections on transmission lines (coax or any old wire). I've been preparing to post it as relevant here, with signal pics, but hit a slight snag. The scope I'd like to use (Tek 500MHz R7903) I forgot had a power supply problem recently. Started to fix it, but it's turning out to be slightly mysterious.Using my garden variety 100MHz scope does work, but mainly as a demonstration of how inadequate a 100MHz scope is for seeing transmission line effects.And unfortunately my 20GHz scope is unsuitable too, for various reasons. The primary one being that the signal levels the pulser produces would destroy the 20GHz scope input.

author: BravoV
author_rank: Super Contributor
timestamp: February 22, 2013, 04:12:47 am
content: Sorry for late response, its just I've been reading quite some materials gathered from the net and really appreciate the replies, thank you.Quote from: TerraHertz on February 21, 2013, 06:17:55 amTerraHertz , thanks, actually your explanation here made it simpler for me to see the bigger picture that relatively low speed scope < 500 Mhz, this VSWR factor though will have impact, still not significant, anyway realistically scope's accuracy is good enough at 1 or even 2% right ? Especially at analog scope.Also I've discovered these two nice application notes that I believe are related to this topic :- Understanding and Dealing with High Frequency Error Sources in Oscilloscope Calibration by Paul Robert, Fluke Precision Measurement Ltd ->PDF download- Implementing Automated Oscilloscope Calibration Systems by Richard Roddis, Wavetek Ltd. ->PDF downloadCMIIW, merged & quoted two interesting graphs from two documents above, this charts below should summarized this topic pretty well right ? Or did I understand it wrongly ?

author: TerraHertz
author_rank: Super Contributor
timestamp: February 23, 2013, 04:16:56 am
content: Now you've got it!When I posted that comment, I was only thinking of square waves and transient signals. Had completely forgotten about CW signals, and the amplitude errors resulting from reflections. That 'Figure 4' diagram is neat.The other diagram, "Uncertainty (V) arising from Unit Under Test mismatch vs frequency" - I'm curious why it has such discontinuities. I definitely don't understand. Guess I should read the articles you mentioned.I'm guessing it has something to do with the measurement system.Speaking of measurement systems, I'm so pissed off. I really want to post pics of that cable pulser in operation. I have several Tek R7903 500MHz scopes, all of which worked a while ago. Of the first three I've tried, all three are non-starters. Grrr.... More chores.And the first one I've started trying to fix, turns out to have a quite mysterious problem.Oh well. Rainy weekend, canceled camping trip, what better way to spend it than fixing Tek scopes?

author: BravoV
author_rank: Super Contributor
timestamp: February 23, 2013, 12:26:21 pm
content: Quote from: TerraHertz on February 23, 2013, 04:16:56 amThanks, this thread started just because I was curious reading scope's spec and ended up quite fruitful, imo.Quote from: TerraHertz on February 23, 2013, 04:16:56 amPlease, post the cable pulser on work once you've settled down those problems.