devise an experiment that will show the effect of guard traces. - Page 1
Forum section: electronics / projects

author: ftransform
author_rank: Frequent Contributor
timestamp: May 26, 2013, 08:53:19 am
content: Can someone here devise some kind of experiment that will show the effect of a guard trace?I.E. I want to see some kind of circuit that is impossible to use with out guard traces, a design that won't function with them or in which they willmake a detectable difference in.I would like to make two of these circuits, one with a guard trace and one with a empty space (same distance) instead of the guard trace.The whole idea of guard traces just seems nutty to me......I don't really wanna hear stories of how they helped you, I just want to building a test jig.Please don't start an argument or call me stupid, if they work then there must be a easy way to prove that they do.I don't even care how they work, so don't waste your time explaining it and instead devise a test setup that I could prove it to myself, as simple as possible.im going to revert to calling it audiophool crap unless I can prove it to myself.I will etch my own pcbs for this experiment, and im inclined to call it bullshit if the experiment requires the pcb to be made with some kind of crazy ass shit that works at 15 gigahertzthis is for people who suggest guard trace use in sane applications (voltage reference, mcu clock)NOT electrometers

author: Bored@Work
author_rank: Super Contributor
timestamp: May 26, 2013, 09:15:36 am
content: Hire a slave, or if that is illegal in your neck of the woods, hire a servant. You can then push him around in any way you like and dictate how he should do the work for you.

author: EEVblog
author_rank: Unknown
timestamp: May 26, 2013, 09:17:48 am
content: Here is some info:http://www.sigcon.com/Pubs/news/15_02.htm

author: Bored@Work
author_rank: Super Contributor
timestamp: May 26, 2013, 09:23:02 am
content: Quote from: EEVblog on May 26, 2013, 09:17:48 amFail. He doesn't want information:Quote from: ftransform on May 26, 2013, 08:53:19 am

author: AndyC_772
author_rank: Super Contributor
timestamp: May 26, 2013, 09:24:03 am
content: Firstly: full marks for using actual science to test, and possibly debunk, a 'rule of thumb' that gets endlessly recycled from one engineer to another without being properly validated from time to time. I applaud you.I suspect you'll find it a difficult experiment to carry out, because I've seen some theoretical modelling which shows that in many cases, it is indeed the physical separation between "aggressor" and "victim" nets that actually has the beneficial effect, not the grounded trace in between. If that trace isn't properly connected to an underlying ground plane at regular intervals, it's not going to be quiet with respect to that ground plane for much of its length anyway.If you want to just demonstrate the shielding effectiveness, I'd etch a PCB with some parallel traces, one of which would be connected to a high speed single-ended driver, and another which would be connected to a suitable RF connector that you can attach to your scope. Duplicate the layout with equal spacings and with/without a guard trace. If you can, also lay out a circuit with a guard trace that has a GND via at regular intervals to see whether it can be made more effective this way.As to 'real world' applications where it's needed, I think you'll struggle to produce an example that categorically doesn't work without a guard trace. Reducing crosstalk is all about improving noise margin, and this varies from board to board with tolerances and silicon process variations. You might find a really thoroughly engineered design goes from 99.99% functional yield down to 98% without a (required, useful) guard trace, but that's not something you'll be able to see in a lab prototype.

author: BravoV
author_rank: Super Contributor
timestamp: May 26, 2013, 09:27:35 am
content: Quote from: Bored@Work on May 26, 2013, 09:23:02 amWhat he "need" is sort of this kind of treatment like I've done ->HEREto get the result.

author: ftransform
author_rank: Frequent Contributor
timestamp: May 26, 2013, 09:27:47 am
content: Quote from: AndyC_772 on May 26, 2013, 09:24:03 amI like this idea. I figure making the boards and then cleaning the top with nice clean acetone should be enough for the integrity of the experiment?

author: ftransform
author_rank: Frequent Contributor
timestamp: May 26, 2013, 09:29:21 am
content: Quote from: BravoV on May 26, 2013, 09:27:35 amwhat he "need" is people like andyC and not you.I don't understand, have you used alot of guard traces in your design and now you are scared of looking like a fool?If you don't know then you don't need to respond here.I like to prove things for myself to get a better feel for the importance of design considerations instead of blindly following rules of thumb.People on this forum are interested in this type of stuff, which is why dave jones and mike had like 6 videos about the effect of adding additional solder to PCB traces... it got down to a thermal imager.I want the guard trace saga. Don't derail this shit with my old function generator posts. The tone of my post is like that because I have a very specific request. I know people will start linking articles but I am looking for a very specific experiment.

author: jahonen
author_rank: Super Contributor
timestamp: May 26, 2013, 09:35:31 am
content: Also note that the result might be completely different if there is no ground plane below the traces (i.e. just loose "wires"). Near means within trace width or less (i.e. impedance of ~75 ohms or less for glassfiber-epoxy FR4). For experiment, you could take a look at Johnsons HSSD book, chapter 5.Regards,Janne

author: Bored@Work
author_rank: Super Contributor
timestamp: May 26, 2013, 09:35:49 am
content: Quote from: ftransform on May 26, 2013, 09:29:21 amOf course you are happy that you found a bitch.QuoteBut you are to stupid to devise you own experiment. And instead you like to order people around to do the work for you.QuoteNo, the tone of your request is because you want to treat us as your bitches.

author: ftransform
author_rank: Frequent Contributor
timestamp: May 26, 2013, 09:38:22 am
content: Quote from: Bored@Work on May 26, 2013, 09:35:49 amordering you? you make me sound like the cable guy.PROFANITY REMOVED BY MODwhy --------- should I execute a potentially flawed experiment without the consultation of the forum? You are free to ignore the topic. I'd rather have people brain storm before I waste my resources performing a experiment. I am sure that some people are interested in seeing guard trace demonstrations and this topic is for people with those ideas.get out of here patrol man,PROFANITY REMOVED BY MOD

author: jahonen
author_rank: Super Contributor
timestamp: May 26, 2013, 09:46:57 am
content: Quote from: AndyC_772 on May 26, 2013, 09:24:03 amI agree, my personal favourite topics are paralleling different valued bypass caps and use of ground plane splits.In many times people tend to exclude second order effects without considering if it is valid to do so. Then they often get offended when I show them my measurements which show just the opposite effect that they are trying to achieveThen I ask if they have done any tests which show opposing results, deep silence falls in the room.In fact I have just done some measurements on a split ground which showed quite interesting results. In fact the split made situation considerably worse for some frequency range but provided next to nothing useful elsewhere.Regards,Janne

author: EEVblog
author_rank: Unknown
timestamp: May 26, 2013, 09:49:28 am
content: Quote from: ftransform on May 26, 2013, 09:38:22 amWhy not? You learn a lot when you just go and do stuff. Often a lot of unexpected stuff.You won't get that with a spoon feed guaranteed to work experiment.

author: ftransform
author_rank: Frequent Contributor
timestamp: May 26, 2013, 09:50:48 am
content: Quote from: EEVblog on May 26, 2013, 09:49:28 amI know I won't but it will put me on the right track and save me money if people find some obvious guidelines that I may fail to find out. I don't want to waste money in failed etches and time in bad designs. If someone here with 20 years of electronics design experiance can help me out why not?

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 26, 2013, 09:51:12 am
content: from what i read (theoritical knowledge) guard trace is to guard noise leaking from outside to inside a sensitive circuit. a sensitive circuit implying high impedance circuit. i believe the easiest demo board is something like....signal ----- hi-val-resistor ------------------ (signal that needs guarding) --------------------- output (buffer)the most important keypoint there is the "high value resistance" to simulate hi impedance circuit. during the test, switch on any noisy source available in the surrounding to see the difference. hope it help and "provable"edit: protecting surrounding noise by using 2 guard traces on the side could be dumb. so i agree with others, you need to understand the concept, once you do, building test setup is (should be) as easy as ABC.

author: ftransform
author_rank: Frequent Contributor
timestamp: May 26, 2013, 09:53:08 am
content: Quote from: Mechatrommer on May 26, 2013, 09:51:12 amYes, it makes sense to me when dealing with high impedance (electrometers) but I have seen it suggested before for circuits like precision voltage references (I.e. the zener oven stabilized LTZ9000 or whatever it is thread).

author: Bored@Work
author_rank: Super Contributor
timestamp: May 26, 2013, 09:55:36 am
content: Sorry for interfering with your attempt to recruit some bitches. I see you now try to make Dave your bitch by asking him to do a video about it. Did you grow up as spoon-feed only child, maybe with servants at home you could push around?Here is what I expect from you:- You apology for your initial posting in this thread and your other postings- You go out and do your home work:- You read the available material- You present a summary of the available material here- You device an experiment to show the validity or invalidity of what you learned- You present the experiment here, and politely ask for feedback (that includes using the word 'please')- You listen to feedback,- you go out to learn more if the feedback suggests so,- and you change your experiment according to the feedback- You execute the revised experiment and report backNotice how often I refer to you doing something, instead of trying you recruiting bitches doing the work for you?

author: ftransform
author_rank: Frequent Contributor
timestamp: May 26, 2013, 09:56:43 am
content: MORE PROFANITY REMOVED BY MOD. PLEASE KEEP IT CIVIL

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 26, 2013, 10:01:00 am
content: Quote from: ftransform on May 26, 2013, 09:53:08 amso what do you expect guard trace is for?

author: C
author_rank: Super Contributor
timestamp: May 26, 2013, 11:18:40 am
content: A very old parallel IDE disk drive cable was 40 wires in a ribbon cable. To get the speed of the last generation of parallel IDE disk drive required required a cable with 80 wires with every other wire ground, but you may not call this a guard.Guard rings protect against leakage, and you should be able to see the results if during your test you make the leakage much greater.A capacitor charge hold time with a carbon pencil trace between the terminals should work if pencil's still use carbon. A salt water trace should also work.

author: KJDS
author_rank: Super Contributor
timestamp: May 26, 2013, 11:19:48 am
content: I was going to post something sensible here, however if the OP can't be polite, and indeed is going to be abusive then I'm not inclined to help him.

author: plesa
author_rank: Frequent Contributor
timestamp: May 26, 2013, 11:25:54 am
content: I created the explaining post in different thread. When I saw this thread with his reaction I decided to delete it and not support him.

author: AndyC_772
author_rank: Super Contributor
timestamp: May 26, 2013, 01:01:16 pm
content: Quote from: jahonen on May 26, 2013, 09:35:31 amIf there's room on the PCB, it might be worth layout out multiple versions of the same layout with different spacings between the various nets, so as to get an idea of just how close "near" has to be in order to make crosstalk effects significant. The trace width isn't really that important IMHO - what's being investigated is the effect of capacitive and/or transformer coupling between one net and another, and I wouldn't expect trace width to be nearly as important as spacing.It's definitely worth putting a space on the board for a series resistor or inductor on the output of the buffer that drives the "aggressor" net, and others between the "victim" net and GND at both ends. Investigating the effect the value of these parts has would be well worth writing up IMHO.I wouldn't personally bother with an experiment that doesn't have a GND plane. The benefits of having a solid, uninterrupted GND plane in a PCB adjacent to sensitive or high speed signals are already so well documented and understood that I'd regard "always use a ground plane" as being sufficiently well proven in theory and in practice as not to need validating again. (Don't come crying to me if you leave the ground plane out of a board and then have a bad day at the EMC lab - unless you want to pay me to lay the board out for you properly!!)

author: free_electron
author_rank: Super Contributor
timestamp: May 26, 2013, 02:16:55 pm
content: HOLD IT BOYS !There is a lot of confusion going on in this topic.To satisfy the original posters request : the test is very simple. All you need are a 2 resistors of 10 megaohm , a simple opamp and a dirty finger.Set the opamp in gain x2 using the resistors. Make one layout with and one without guard.Then apply ground at the input. The output should be 0 volt.. 0x2=2.Next apply a greasy finger touching the area between the pins. Do experiment with and without guard ring. The guarded one will stay at 0. The non guarded one will jump to some voltage. Use a straw to blow some air around the i puts( nice conductive acid laden air from your mouth). Watch the non-guarded one go crazy..As for guarding crystals .. I'll come back to that in a moment. We need some explanation first on what guarding is and ,judging by the 'blabla' already posted in this topic, more importantly what it is NOT !The definition of a guard :1) A guard is a shield that prevents leakage currents from injecting into a sensitive node.2) the potential on the guard is at the same potential as the sensitive node.Of only one condition is met it is NOT a guard ! Condition 2 is the kick in the pants.A trace connected to ground between two signals is not a guard ! It is merely a shield. Why is it not a guard ? Because, as long as there is a potential difference there is still a chance for leakage current to flow !If i have two signals , one at 1 volt and one at 5 volt. And there is a leakage current flowing (lets say we have a leakage resistance of 1 megaohm).so 5-1 / 1 meg = 4uA. Placing a ground trace between them doesnt solve that problem. There will still be 1uA flowing from 1 v to gound , and 5uA from 5v to ground ! If that 5 volt line is the sensitive node, i have actually made the problem worse ! It was only 4uA... Adding the ground trace pulled it to 5uA !Now, if instead of placing ground there, i place 5 volt on that guard line...5-5 = 0... You can have all the leak you want. There is no voltage, so no current flows.The 1 volt is still leaking , but that was not the sensitive node.A third important point about a guard is that it must be able to absorb the current. So a low impedance is required.Guards are therefore always made using active circuitry (opamps or buffers)Now, a guard has another effect : it collapses parasitic capacitance. Hows that work ?Lets take a look at a simple shielded cable first. The shield goes to ground, the center gets a signal applied from a 100kiloohm source. We sweep frequency. We basically have a low pass filter. As frequency increases the stray capacitance of the cable will start conducting a current. That current flows because there is a potential difference accress the stray capacitor. See where i'm headed ? No voltage is no current ....If i throw a guard layer inbetween the center conductor and the outer ground shield, there are now two stray capacitances. One from center to guard, one from guard to ground.Since there is no voltage delta from centre to guard .. The stray capacitance there has no impact . No delta voltage means no current through the stray cap.The current through the stray cap is frequency driven as the impedance of a cap is determined by frequency. For a fixed potential the current follows the 1/(2 pi f c) formula.My guard made sure there is no voltage delta... So the capacitor has no effect as no current flows through it...So that is another thing a guard does : it has the capability to increase bandwidth ! The limiting factor is the speed at which you can make the guard follow the input. Any speed (read phase) delta there and the rule is no longer valid. If you have a very fast opamp amplifying a high impedance source : applying a guard will increase system bandwidth closer to what the opamp can do as you collapsed the stray capacitance in the system.Now, back tothe original requesters question about crystal guarding..This has to do with stray capacitance. The capacitive load on acrystal alters its oscillation frequency.If you have a finely tuned oscillator and you inject some greasy finger you will slightly alter the frequency.Lets say you make a clock circuit. The non guarded one will be accurate to 10 seconds per year. The guarded one will get 1 second a year...If a fly , or ant walks over the board , or the board gets dusty or greasy over time, the non guarded one will drift more. The guarded one doesn't give a rats ass...For guards to be effective : they need to be applied on both top and bottom of the pcb (or also on inner layers for multilayers ) the guard traces should be stitched together using vias.Got it now ?any circuit where you are fishing for accuracy will benefit of garding becasue it reduce errors introduced by leakage. that's why you see that in all decent multimeters around the input dividers. you have a reistance ladder making a 1 / 10 /100 ratio. apply 100 volt and the voltage on tap 1 is 10 volts. , but there is a 'leakage from the 100 volt to the 10 volt. the delta there is 90 volts !. if the leakage across the surface off the pcb is a constant ( which it is not ) you will get more leakage there than from the 10 volt to 1 volt range. ( the resistors are the same mechanical size.... as the leakage is determined by length ( ohms per millimeter ) the relative leakage on the high range will be larger. guarding solves that misery. even on a 3 1/2 digit multimeter !

author: mzzj
author_rank: Super Contributor
timestamp: May 26, 2013, 08:06:19 pm
content: Quote from: free_electron on May 26, 2013, 02:16:55 pmBetter yet, use 1 gigaohm resistors in >90% relative humidity environment and try to measure currents below femtoampere.

author: JVR
author_rank: Regular Contributor
timestamp: May 27, 2013, 01:54:37 pm
content: Quote from: free_electron on May 26, 2013, 02:16:55 pmWhy not just buffer the signal and then transmit it? Why have a "high power" signal shielding its exact mate, but at lower power?

author: free_electron
author_rank: Super Contributor
timestamp: May 27, 2013, 03:05:44 pm
content: Quote from: JVR on May 27, 2013, 01:54:37 pmBecause mechanically you can't.Guarding takes place around the input pins of an opamp, or around the two pins of a single resistor ! You can't stuff another opamp in there. There's no room. Actually, the smaller the parts are the more important guarding becomes.Leakage increases as distance decreases. Lets say you have a surface conductivity of 1 gigaohm per cm. at 1 mm you are down to 100 meg ... If the input impedance of the system (input of an opamp) is 100 megaohm . You just blasted 50% error in there.... Purely by surface leakage.So , if you swap that dil package for a so-8... See where i'm headed ?You will find that for high precision opamps with extreme input impedances the pins will not be next to each other.( not pin 2 and 3... But pin 1 and 8 for example.) the reason being mechanical distance.'In the old days' (actually we still do that) we had opamps in metalcan (to99 or to100 or to5). You would spread out the input legs and conmect them to teflon standoffs. The can would be ground. On some specialty opamps the can was actually isolated from the chip and had its own leg. In precision circuitry that leg would be tied to the output. So the entire body of the opamp already became a guard. You will find that analog devices still produces opamps in those metal cans. Those are meant for specialty apps to condition the signal coming from high impedance sensors.Guarding in a cable collapses stray capacitance and improves system bandwidth and input impedance (as the stray capacitor impedance is gone)Guarding on a board eliminates current injection or current leakage by creating a low impedant ring at the same level of the sensitive node. The ring will absorb al injected , or deliver any parasitic current. The high impedant node doesnt see it.Guarding around voltage dividers in laboratory multimeters is common. Any dirt or grease accumulated over its lifetime has no impact on the precision. The guard prevents that.Take a look at a fluke 87 handheld or any bench multimeter. It's full of guarding around the input divider.