When not to use a ground plane? - Page 1
Forum section: electronics / beginners

author: cwalex
author_rank: Frequent Contributor
timestamp: June 10, 2012, 04:49:21 am
content: Hi, I have been lurking for a while, love the forum and Dave's videos. I have been trying to learn to make PCB's and I have a question about ground planes. I am mainly interested in audio circuits, specifically guitar effects circuits. I have been practicing by trying to make some PCB designs for some classic guitar effects likebig muffetc, is there anything wrong with making the bottom layer a ground plane and running the rest of the traces on the top layer? is there any time when I shouldn't use a ground plane when cost of PCB manufacture isn't an issue?I have looked at examples of PCB's for the big muff from places likegeneral guitar gadgetsbut they are all single sided. Their designs never seem to use copper pours and I was wondering if there is a reason for it because a lot of the PCB designs I have looked at that were for digital stuff like microcontrollers seem to use copper pours a lot even on single sided designs and I wonder why.Thanks for reading

author: Rerouter
author_rank: Super Contributor
timestamp: June 10, 2012, 05:05:16 am
content: ground planes and large copper planes are for failry higher frequencies, audible 22-22KHz doesnt really need any high frequency consideration, most micro's run at 16Mhz or higher, thats where you start to worry about RF signal integrity and copper pours,not to mention single sided can be done at home with tone transfers,if cost wasnt a concern, i would say a full ground plane wouldnt be that bad, if only just to mop up half of the EMI the amplifiers and other audio gear would likely be spitting out,

author: free_electron
author_rank: Super Contributor
timestamp: June 10, 2012, 05:11:41 am
content: Its got Nothing to do with frequency. Frequency is irellevant. Its all about edge rate (dv/dt) ( edge rate is sometimes called rise time and fall time, both of which are edge rates )A triangle wave of 1MHz has totally different edge rate than a sqquare wave of 1MHz.

author: cwalex
author_rank: Frequent Contributor
timestamp: June 10, 2012, 05:40:07 am
content: Thanks for your replies, that was really fast.If I understand correctly the digital circuits use ground planes because the signals are higher freq and more square so the ground plane will absorb more of it.So there would be nothing wrong with using ground plane and copper pours in the audio circuits but it would be unnecessary.Am I understanding this correctly?

author: gxti
author_rank: Frequent Contributor
timestamp: June 10, 2012, 04:23:00 pm
content: Correct, there's no reason not to use one if it doesn't cost you anything.

author: jahonen
author_rank: Super Contributor
timestamp: June 10, 2012, 04:46:31 pm
content: Having a ground plane has basically an effect that it lowers the impedance of the traces above it, if trace width is about same or wider than dielectric thickness. Although DC-like applications like audio won't benefit from that, there is a benefit that traces become more immune to external interference than traces "in the air".Regards,Janne

author: free_electron
author_rank: Super Contributor
timestamp: June 10, 2012, 06:49:16 pm
content: Actually ground planes can cause unwanted oscillations due to stray capacitance in audio circuits....First audio systems were built dead bug style by stringin parts between vacuum tube pins. No shield in sight... Works perfectly fine.Once again, it is NOT frequency ! The determining parameter is edge rate. Edge rate is not frequency.A triangle wave of 1 MHz at 1 volt has an edge rate of 2 volts per second.Take a 1 MHz 1 volt square wave with picosecpnd risetime and you have an edge rate of 1 volt per picosecond.. And that spells trouble !

author: Baliszoft
author_rank: Frequent Contributor
timestamp: June 10, 2012, 07:17:40 pm
content: Quote from: free_electron on June 10, 2012, 06:49:16 pmTotally out of topic, but if you got a high rise time square and a low rise time square, which one will contain more and louder harmonics in the hf range?!

author: w2aew
author_rank: Super Contributor
timestamp: June 10, 2012, 07:27:43 pm
content: Quote from: Baliszoft on June 10, 2012, 07:17:40 pmThe faster the risetime, the more high frequency harmonic content.

author: Baliszoft
author_rank: Frequent Contributor
timestamp: June 10, 2012, 07:35:19 pm
content: So at the end it is something about frequency ;-) ?

author: ejeffrey
author_rank: Super Contributor
timestamp: June 10, 2012, 07:59:38 pm
content: Ground planes increase parasitic capacitance but lowers inductance. They also reduce capacitive cross-talk between nearby signals since the capacitance to the ground plane overwhelms and screens the trace-to-trace capacitance. For instance, the data sheets /application notes for most high speed CFB opamps recommend relieving the ground plane under the inverting input, as even a small capacitance can compromise stability.When constructing a two layer board it is fine to use the bottom side as a ground pour, but keep in mind that it is 1.6 mm away from the signal traces. This means it will have limited effectiveness as a ground plane if your traces are much narrower or closer than 1.6 mm. It (probably) won't hurt, but if you are expecting it to have magical properties you will probably be disappointed. If you are only doing it because you can, and it is convenient to have 0VDC available everywhere, knock yourself out.Finally, ground planes on 2-layer boards are somewhat uncommon because they are so hard to implement. Doing so restricts your layout to a single side. As your circuit becomes more complicated it gets rapidly impractical to do that successfully. As soon as your start cutting up the ground plane you have defeated the purpose. At that point it may be better to ditch the ground plane and focus on doing a good 2 layer layout.

author: free_electron
author_rank: Super Contributor
timestamp: June 10, 2012, 08:04:48 pm
content: no . has nothing to do with frequency. a 1KHz 1 picosecond rise time wil produce more crap in the spectrum than a 1 Mhz 500nS rise time ...and ejeffry has a point too : capacitive loadig everywhere AND since your pcb is 1.6mm thick it doesnt do snot for RF ...ground planes work only in multilayers and you need to create the stack very carefully. simply using equidistant thickness for the pre-preg gets you nowhere...

author: Baliszoft
author_rank: Frequent Contributor
timestamp: June 10, 2012, 08:08:46 pm
content: Quote from: free_electron on June 10, 2012, 08:04:48 pmYou did not get the point at all, but thats OK. Just dont do any signal analysis and you will be fine :-P

author: jahonen
author_rank: Super Contributor
timestamp: June 10, 2012, 08:11:31 pm
content: Quote from: w2aew on June 10, 2012, 07:27:43 pmHow about single pulses without repetition frequency then? I think it is difficult to think those in the frequency domain. Better approach is to think how long the edge is physically on the PCB.Regards,Janne

author: Mechatrommer
author_rank: Super Contributor
timestamp: June 10, 2012, 11:32:19 pm
content: Quote from: Baliszoft on June 10, 2012, 08:08:46 pmsure you can somehow convert rise time to "equivalent effective frequency" (highest harmonics), but stating it in term of frequency alone can be misleading esp for noobs. take a single pulse (LO-HI or vice versa) for eg, too short rise time will create visible reflection and SI/RF issue. and its hard to tell in term freq for a pulse, unless, you know what you talking about

author: Baliszoft
author_rank: Frequent Contributor
timestamp: June 10, 2012, 11:55:10 pm
content: Oh no, im not an rf guy. Also wanted to be strictly off topic, BUT stating that rise and fall times have nothing to do with the spectrum (therefore frequencies) of the signal is just not true. If i got it wrong, let me apologize.For me a signal is not described solely by its fundamental frequency, though i agree that you dont have to care about it unless you are in the hf/rf range. F.eg why do they limit (slow down) rise and fall times to reduce emi? Not for dampening the radiation of the fundamental freq. for sure.

author: w2aew
author_rank: Super Contributor
timestamp: June 11, 2012, 01:38:16 am
content: Quote from: Baliszoft on June 10, 2012, 11:55:10 pmThe only signals that are described solely by their fundamental frequency are sinusoids. Faster risetimes lead to higher frequency components, regardless of the frequency of the fundamental. It doesn't matter if the signal is periodic or not. Consider the following complementary relationships:A pure sinusoidal signal is represented by a single "line" in the frequency domain (contains only one frequency component),andA single "impulse" in the time domain is represented by a broad, flat horizontal line in the frequency domain (very broad frequency content), just not for very long.A simple digital clock (square wave), is ideally composed of frequency components at the fundamental, and its higher order odd harmonics. The more odd harmonic components added, the faster the risetime. Picture it this way, if you pass a squarewave through a lowpass filter, you remove the higher order harmonics, and can reduce the signal back to a sinusoid.

author: cwalex
author_rank: Frequent Contributor
timestamp: June 11, 2012, 03:05:42 am
content: Hey, thanks for all the great replies. Looks like I'm wasting my time doing a ground plane other than it is convenient for gnd connections. I'll have a better read through the replies when I get home from work. Cheers

author: free_electron
author_rank: Super Contributor
timestamp: June 11, 2012, 04:46:36 am
content: Quote from: Baliszoft on June 10, 2012, 11:55:10 pmwell ,you still got it wrong !a pure sinewave has no harmonics. so it does not radiate in the spectrum. you will only find it at its fundamental frequency.any other signal is a compound of base frequecies + harmonics.the sharper a signal rises or falls ( dv/dt) , which is called edge rate , the more harmonics there are present in the signal. the more harmonics the more energy isdispersed throughout the spectrum.the base frequency of the signal doesnt matter for this.a 1 hz signal with 1 ps risetime produce an equal amount harmonics as a 1Mhz ignal with 1pS risteime. the base freuqency isshited, thatis all. the number of harmonics does not change.therefore : it is purely edge rate that determines how much energy is blasted in the spectrum. the frequency if the edge determines where the harmonics begin.a 1Mhz 1vpp squre wave with an edge of 100nS does not produce as many harmonics as a 1MHz 1vpp with a 1pS edge rate.simply becasue you don;t need that many harmonics to get that edge to be so fast...

author: Baliszoft
author_rank: Frequent Contributor
timestamp: June 11, 2012, 09:27:19 am
content: I think we are just talking about the same thing. I am doing signal processing for more than fifteen years (worked for several big companies in this topic in the past), so i do not really need a lesson how a sine, square, pulse, ect looks like in the frequency domain. You say that rise/fall time is what counts, but not the frequency (spectrum) and it is completely wrong! The two is the same thing, one comes from other, it's just the point of view. If it is hard to admit this for you (in your current position/reputation on this forum), it still does not change anything on the facts.Just a last thoguht; You have a low frequency square and a high frequency square with the same rise/fall times (as you state). If the frequency is too high, you wont be able to talk about a square anymore, but only about a triangle instead. When i say my signal is a square, it is a square, no matter how high its freq. is.

author: beaker353
author_rank: Unknown
timestamp: June 12, 2012, 06:53:09 am
content: Ground planes and grounded metal enclosures in pro audio gear have almost nothing to do with signals playing with each other inside the case. It's all about keeping stray electric fields in the environment out of the case and signal paths. The inherent nature of the tiny output from a guitar and the magnitude of gain to get that signal to a usable level makes it super sensitive to fields in the environment. Try running a guitar with a unshelled cable within a few feet of a fluorescent light fixture transformer and you will see what I mean. There are very good reasons why most true pro audio gear has thick metal chassis with high gauge wire connecting everything together...- EM

author: free_electron
author_rank: Super Contributor
timestamp: June 12, 2012, 07:14:13 am
content: Quote from: Baliszoft on June 11, 2012, 09:27:19 amNo it is NOT ! This is a common mistake that is made over and over amd is very hard to stomp out.The 'frequency' is not the 'spectrum'. The spectrum is the sum of all frequencies.Fouriers theorem : a square wave is made by adding a fundamental sinewave with its harmonics. Every harmonic increases the edge rate.The frequency of the signal is still only the base frequency ! Adding harmonics does not change the time required to pass through one full period of the signal. 'frequency' means 'base frequency'.If you make an assymetric edge controlled square wave , lets say a rise time of 1us, 3 us high, 1us fall time, 95 us low.The period is 100us or 10khz.... The crap in the spectrum is still generated by the edges. In case of an assymetrical signal the spectrum distribution is going to be very weird since the superposition theorem doesnt hold.... That only works with symmetric 50% duty cycle signals...If i take the same period for this pattern but i pump up the edge rates to picoseconds. 1 picosecond up , 5 nanosecond (-2 ps) high, 1 ps down.. The period of this signal has not changed, so its frequency has not changed! Yet it radiates waaaay further in the spectrum. So , edge rate is not the same as frequency ! The only relation is that your edge rate can never be slower than 1/2 period. If you edge rate is 1/2 period you have a triangle wave.Its got nothing to do with position on the forum. I'm the first to admit that there is at least 90% of electronics i don't know anything about. The above is how it is defined in all literature about emc and emi.I followed the course given by Montrose ( one of the gurus in this field) at UCSC a couple of years ago (uni of California - Santa Cruz) Very interesting stuff. Final exam was a pcb stack and layout that had to be optimized for emc. The damn thing was full of traps.. Aced itwalked away with 100%

author: Baliszoft
author_rank: Frequent Contributor
timestamp: June 12, 2012, 08:19:07 am
content: Im not arguing with you, it is an odd conversation. You are just proving your own truth.

author: Write_to_Smokegenerator
author_rank: Regular Contributor
timestamp: June 12, 2012, 12:01:10 pm
content: Well I'm defently not an Expert, but you I think it's a really bad idea to use a ground plane under a switch mode power supply (at least I heard that some months ago)

author: free_electron
author_rank: Super Contributor
timestamp: June 12, 2012, 02:03:07 pm
content: It can be bad...There is only one answer in anything related to signal integrity and EMC , and that answer is : it all depends.If you have a coil that leaks magnetic field you may want to remove the plane underneath it so it induces no current. ( here is another common mistake : lets put copper to shield the inductor..... Copper does not shield magnetics ! )

author: Neilm
author_rank: Super Contributor
timestamp: June 12, 2012, 06:59:21 pm
content: Quote from: free_electron on June 12, 2012, 02:03:07 pmA copper band around a transformer / inductor can act as a shorted turn and will absorb any RF energy that is leaking out. This will reduce the RF and is used to reduce the emissions for EMI tests.Neil

author: free_electron
author_rank: Super Contributor
timestamp: June 12, 2012, 07:44:55 pm
content: ahh. that is something different. you are shorting the field by giving it a closed electrical path. this may work for smps transformers ( there you see that often , but they dont use a hard short... there is some resistance in the path , )but for a simple ferrite coil like a 1210 package this doesnt work.

author: jerry507
author_rank: Regular Contributor
timestamp: June 12, 2012, 08:33:38 pm
content: Quote from: Baliszoft on June 12, 2012, 08:19:07 amYea he is, it is very very strange

author: free_electron
author_rank: Super Contributor
timestamp: June 12, 2012, 10:27:22 pm
content: Quote from: jerry507 on June 12, 2012, 08:33:38 pmit's not my truth. this is how it is explained in courses given Howard Johnson ( Black magic for High speed deisgns ) and Montroses' EMc and Signal integrity courses.

author: jerry507
author_rank: Regular Contributor
timestamp: June 13, 2012, 02:27:52 am
content: After rereading what you said, I think I see the confusion. You're trying to say that the use of ground planes has nothing to do with the highest fundamental frequency in use, and everything to do with the highest harmonic frequency seen?

author: w2aew
author_rank: Super Contributor
timestamp: June 13, 2012, 02:37:35 am
content: Quote from: free_electron on June 11, 2012, 04:46:36 amNot that you need another viewpoint here - but the statement above that says that the number of harmonics doesn't change with the base frequency of a squarewave (assuming equal risetimes) is simply not true.A square wave is composed of energy at the fundamental frequency, and energy at the ODD harmonic frequencies. The more high order harmonics included, the faster the edge rate. Thus, an ideal 10Hz square wave will have energy at 10Hz, 30Hz, 50Hz, 70Hz, etc. An ideal 1MHz squarewave will have energy at 1MHz, 3MHz, 7MHz, etc. If both of these squarewaves have equal risetimes, then the harmonic energy of them will roll off at the same high frequency value, but the NUMBER of harmonic components will be dramatically different between the two since the spacing is so different.

author: jahonen
author_rank: Super Contributor
timestamp: June 13, 2012, 07:27:22 am
content: Quote from: w2aew on June 13, 2012, 02:37:35 amThis discussion has been off-topiced somewhat but I think it might interest someone anyway...Purely odd harmonics will only occur if square wave has precisely 50/50 duty cycle. If we change the duty cycle to something else like 1/3 or 2/3, then 3rd harmonic and multiples of will disappear, but we get second harmonic. In any case regardless of the duty cycle, the spectrum envelope obeys sin x/x (sinc) shape. Having 50/50 duty cycle means that nulls of the sinc functions will be at even harmonic points, and odd harmonics are exactly at lobe peak points of the sinc function. At very extremes of the duty cycle, the sinc main lobe gets wide (approaching constant when duty cycle is shrunk into dirac delta impulse), so all harmonics will be of same amplitude.The edge rate is significant in sense that it introduces second inflection point on the spectrum envelope. For ideal theoretical squarewave, harmonics always go down 1/f fashion, i.e. 6 dB/octave ad infinitum. Having a finite edge rate means that when certain frequency is reached, the roll-off increases to 1/f^2, i.e. 12 dB/octave. That particular frequency is also considered to be the critical high limit for EMC.Edge rate is significant parameter in sense that signals which have steep edge rate, must be cared for regardless of their repetition frequency. For single-transition edge-sensitive signals, corruption of edge will ruin the signal even if there is no frequency. Now this means that if we get a single edge transition cleanly from the source to the load, it means absolutely nothing how many of these transitions are sent per time unit, they will always get transmitted cleanly (assuming that the edge does not slow down along the way). We can even put several transitions simultaneously on the line without any problems.Thus I understand free_electron's point of view perfectly and I share his opinion. Purely thinking as harmonics, for example one might think that having 10 harmonics of fundamental is enough to transmit the signal. This is not the case if we have very extreme duty cycle. That can be misleading if we are operating at odd parameters. Thinking via edge rate, doesn't fail even on extreme cases.Regards,Janne

author: free_electron
author_rank: Super Contributor
timestamp: June 13, 2012, 09:29:36 am
content: And there you go. Janne explained it in more detail. The problem is that signals in real life are never 50% duty cycle... So if you focus at frequency you can only handle 1% of the problems. Databuses and serial transport systems like PCIx , SATA , HDMI, are very critical. When it comes to signal integrity there is only one thing that matters : getting the edge across undistorted.When it comes to EMC the problem is 'trapping' all the energy so it gets absorbed in the receiving end and does not reflect back onto the line and transmitter.if it reflects back it will radiate ( there goes you compliance testing...) and it will intermodulate...( there goes your signal integrity... )...EMC and signal integrity are two completely different things, but one can cause the other... And solving one can cure the other, or it can harm the other.Fact of the matter is that fast processors have slew rate control for both reasons (integrity and emc).Series termination and co trolled current sources on the output. Some transceiver cells are even adaptive. When tansitioning they turn on two parallel current sources. When a certain voltage is reached one is turned off , when final voltage is reached the second one is turned off and only a 'maintenance' source is remaining. Fpgas have the cells programmable and you set them up by selecting the transceiver standerd you need.I have seen asics that have auto-tune. At powerup they transmit a sequence of pulses and they reprogram the current sources to get the right shape. Used for very fast transceivers to overcome hassles on the board.

author: Baliszoft
author_rank: Frequent Contributor
timestamp: June 13, 2012, 11:02:21 am
content: I wasnt telling you that rise/fall times have nothing to do with this. I was just stating, that rise and fall times affect (control) the spectrum of the signal (go and read all this from the beginning). But you do not want to be aware of this, you just stick to that the fundamental frequency does not count (which is true unless the frequency is too high), and that edge rise/fall times are only important. It is also true (no doubt), but they are just an "indirect" parameter - as they control the harmonic content of the signal. It is a simplified explanation/solution of the actual problem for those who are thinking in the time domain.The harmonic content is causing most of the electromagnetic radiation!Setting a relatively slow rise/fall time is just one (undoubtly easy and effective) way of controlling EMI, but it is not the only way. Where it is not possible, f.eg a low pass filter can be used. It will have the same result.Quote from: free_electron on June 12, 2012, 10:27:22 pmYou have learned the lesson well, you just do not understand it.

author: free_electron
author_rank: Super Contributor
timestamp: June 13, 2012, 01:29:05 pm
content: Quote from: Baliszoft on June 13, 2012, 11:02:21 amYes, rise and fall times affect the spectrum. You got that right.But you can not look at the frequency of a signal. You want to rip a signal apart into a base frequency and harmonics and then strip off the harmonics with a filter. There is two problems with that :1) It only works for simple repetitive signals. Have a complex waveform like a modulated signal and you just made it fubar...Especially when complex modulations are used like qam.2) any form of filtering affects signal integrity! Can't do that because you distort the signal ! Signal integrity is lost. If you start filtering the harmonics you will affect rise and fall time and this can , and will, screw up things.For a lot of signals there is no base frequency. There is only a base frequency if a signal is repetitive in nature.Good luck finding a fundamental for a databus or address line, or a modern switching regulator like a constant on-time or a hysteretic regulator... There is none. It dances all over the place. So your filtering sceme doesnt work. Slap filters on a databus and you will change the edges causing data corruption because sometimes you do not meet the required level in time when the memory is latching in.The only solution is to control dv/dt at the source, or impedance match everything so there is no loss of energy along the path. Loss of energy radiates...In order to do this work you need to know dv/dt (edge rate) and you need to impedance match.Frequency is! For the most part, irrelevant.

author: Baliszoft
author_rank: Frequent Contributor
timestamp: June 13, 2012, 01:38:08 pm
content: Yes, electron. You are right. I am wrong.

author: free_electron
author_rank: Super Contributor
timestamp: June 13, 2012, 03:53:50 pm
content: you're just being sarcastic now...it is not a matter of being right or wrong. it's a matter of learning a technique that is applicable when doing a board.

author: jerry507
author_rank: Regular Contributor
timestamp: June 13, 2012, 04:13:17 pm
content: Why are you focusing on the fundamental as if it's some fixed number? Any realistic signal like you describe can be easily thought of as a band + harmonics of that band. It's exactly the same concept as a single fundamental frequency + harmonics. If you don't have that connection, than it's difficult to turn an edge rate into frequencies you can actually manage. If you just want to manually back down your edge rates until you hit EMI targets, that's fine. But if you want to actually DESIGN to a target, you need to have a better understanding of the spectral content of the signal you're working on, even if it's non-repetitive.Even if you back this discussion away from the edge rate problem (which actually has nothing to do with copper pours directly), I don't actually know of many situations where pours are inappropriate. As free_electron stated, there are cases where they add stray capacitance, but I'd still say that it's better to have the pour there and manage those effects where necessary. But in a lot of cases, pours aren't going to net you a whole lot. In fact, nice tight layouts with well thought out current paths are probably just flat out better than copper pours in any case.Unless they're heat sinks.

author: free_electron
author_rank: Super Contributor
timestamp: June 13, 2012, 04:40:05 pm
content: on the subject of copper pours : those can be actually very ineffective and cause of trouble... as frequency increases the return path is flowing right beneath the coupled source trace. ( for low frequencies the electrons kinda flow wherever they like in the ground plane , for high frequencies they follow the shape of the trace above...)here is a potential problem on a multilayer board :top layer trace in horizontal directioninner one is a ground planeinner two is anotehr signal plane where there is again a trace in horizontal direction..due to the tight coupling there now is intermodulation betwene the two signals... it happens because of the ground plane !so , for signals contaiting high frequencies : DO NOT run them on top of each other shielde by a groundplane. you still need to cross at 90 degree angles. simply because the return current will cause intermodulation in the ground ( ground bounce )i have sene designs where a bus transceive was routed with half the traces above the ground plane , half below the ground plane... and all the return currents nicely intermodulated in the plane itself.. causing havoc.you always need to analyze your current loops. currents should never share a common pathway ( sometimes you cannot avoid , and sometimes you can ). that is the principle between having 'analog and 'digital' grounds. you keep the noise in the digital ground. digital circuits have really good noise margin ( don't overdo it though)the analog ground is anothe rmatter then. and you can only use a piece of copper as a shield if there is NO current flowing in it.other fun tidbits : planes canradiate tremendously. example : 4 layer board : inner two layers are a power and a ground.parts are concentrated in the center. with large 'open area's' towards the edge of the board.. this is effectively a dipole.... two pieces of copper sticking out with noewhere for the current to return... radiation galore !so , if you have large open spaces on a board : create a return path : slap a few capacitors close to the edge so any ac current (dc current does not radiate) is shunted through the caps and the dipole is effetively shorted.we had this problem in a Co rack. one of the boards was a diagnostic card. since all boards are the same size this board was mainly empty apart from a littel lump of parts( some cpu and assorted parts ) close to the back end card connector ( these boards slide in large 24 inch telecoms racks. i was working on ADSL central office boards at the time )only a few signals travel from the far end to the front. some led's and an rs232 connector.this board was radiating as hell... we thought its the rs232 , its the leds... tried everything. ferrite beads , whatever.until we simply placed a few 10nF caps at the front of the board. that solved the problem. the planes were simply long antennas radiating .So her : remove the plane under the void area's or shunt it with a cap.

author: Omicron
author_rank: Regular Contributor
timestamp: June 13, 2012, 04:53:35 pm
content: At the risk of making matters worse let me try to give another point of view.Firstly there seems to be confusion about dV/dt and rise-fall times. These things are not the same! dV/dt, in general, is useless as an indicator for high frequency signal content because dV/dt is scaled with amplitude. I can create any kind of dV/dt I want using a 1 KHz sine wave, simply by increasing or lowering the amplitude. What matters in terms of the spectral content is rise and fall times, which are independent of amplitude. If you limit yourself to digital signals that have a fixed set of levels then you can arguably talk about dV/dt or edge rate and use it interchangeably with rise- and fall time, because in that case you obviously have just one fixed signal amplitude as a reference.In the "black magic" book that free refers to it is shown that for digital signals the spectral bandwidth of the signal is related to the rise- and fall times. The smaller the rise or fall times the larger the bandwidth of the signal. The fact that you can translate rise and fall times into a bandwidth shows that there is both a time domain explanation and a frequency domain explanation and those two are perfectly equivalent. So arguing that you need to look at rise times or arguing that you need to look at the spectrum is moot, just take whichever of the two is more convenient in a particular situation. When dealing with digital signals looking at the rise times is more convenient because you can easily find these in data sheets etc. When you are dealing with complex comms signals it may perhaps be easier to use the spectrum (not my cup of tea so I wouldn't really know).Well, my 2 cents anyway ;-)

author: jahonen
author_rank: Super Contributor
timestamp: June 13, 2012, 05:32:37 pm
content: Well, data/address buses as well as serial links behave essentially as random noise sources as far as spectral content is concerned. So if we have a 100 Mbit/s random data flowing in a simple digital signal. What kind of spectrum this kind of signal has? It turns out that the spectrum is initially flat, and drops to null around data rate, 100 MHz (excluding a non-ideal clock feedthrough which should not occur in theory, however even small imperfection, like asymmetry, on the signal gives rise to clock feedthrough). Then we have again a continuous lump until 200 MHz where we have another null and so on. So voltage spectrum is essentially a continuous (measurement made by me some years ago, using Cyclone II EP2C8):And regarding to emissions, it really is the current what causes the radiation (acceleration/deceleration of an electron causes electrons to emit photons, which are in this case at radio frequency) so one must think how to make a low-pass filter for a current source, not voltage source for the filter to be effective. That was one of eureka-moments when I figured that out. Current spectrum is very different and has spiked form in the time domain, and it usually has little resemblance to voltage spectrum. This is kind of bad thing since it can lead to very wrong conclusions. Too bad almost all tools are voltage oriented and there are few small wide bandwidth probes to visualize the current spectrum of a signal.I find it interesting that crosstalk could happen across ground plane. I have been in belief that skin effect prevents the current flowing very deep in the copper. For fastest transitions, the current flows only on few µm deep layer so we have plenty of margin in typical 35/17 µm thick copper inside of the PCB. Nevertheless, I haven't personally ever seen crosstalk problems happening via groundplane, but of course I can't deny that it couldn't happen. I have been evaluating Sonnet professional lately, maybe I'll give it a go and see what it says...Regards,Janne