Re: [PEDA] EMC question
> Give Henry's book a read and look suspiciously around high current areas. If > anyone wishes to have the ISBN number they can email me off line, if someone > does not post it first. Noise Reduction Techniques in Electronic Systems, 2nd Edition by Henry W. Ott ISBN: 0471850683 . Zim * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * To post a message: mailto:[EMAIL PROTECTED] * * To leave this list visit: * http://www.techservinc.com/protelusers/leave.html * - or email - * mailto:[EMAIL PROTECTED]?body=leave%20proteledaforum * * Contact the list manager: * mailto:[EMAIL PROTECTED] * * Browse or Search previous postings: * http://www.mail-archive.com/proteledaforum@techservinc.com * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
Re: [PEDA] EMC question
Jon has many good points on Greg's problem. After the fact, what one presumes as square waves is not often the case since edges are never perfectly square. I have seen so called linear RF amps produce -30 dBc and -10 dBc second and third order products in my time. EMI is a real problem when excessive logic speed is used when maybe not necessary. This puts a jolt on a ground plane that everyone must ride like a surf board. There is no such thing as ground because any ground plane has inductance and resistance. Grounds are always relative to me. There is much to be said about sectioning ground planes to steer high ground currents. Anyone who has worked with data acquisition will agree. The other rule of RF thumb is keep distances short and decoupling is crucial. Henry Ott's book on Noise Reduction Techniques Electronic Systems is the best reference I have ever found to working around EMI concerns. Jon's suggestion of killing the buss smell for a problem is a great idea. Like Jon, I do not know what the clock source is. If there is access to a high band width scope or spectrum analyzer a sniffer coil can be used as a beagle (hunter). Shielding is an important issue on times that should not be overlooked. Give Henry's book a read and look suspiciously around high current areas. If anyone wishes to have the ISBN number they can email me off line, if someone does not post it first. Fabe Fabian Hartery Guigne International Paradise, Newfoundland * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * To post a message: mailto:[EMAIL PROTECTED] * * To leave this list visit: * http://www.techservinc.com/protelusers/leave.html * - or email - * mailto:[EMAIL PROTECTED]?body=leave%20proteledaforum * * Contact the list manager: * mailto:[EMAIL PROTECTED] * * Browse or Search previous postings: * http://www.mail-archive.com/proteledaforum@techservinc.com * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
Re: [PEDA] EMC question
Greg Olson wrote: > Hi all.. > > I've got a 2.5"x3.5" plug on daughterboard which contains my processor, memory, >flash, etc... The processor is a 20MHz AM186 driven by a 20MHz crystal. I just had >this system in for FCC EMC testing and found that the 20MHz crystal is bleeding its >harmonics (especially 80MHz) all over everything! It's getting onto the IO lines from >the daughterboard to the main board and from there onto just about every wire leaving >the enclosure! Since the crystal is not running all by itself, but is clocking many other circuits, it is not absolutely positive that the harmonics are coming from the oscillator only, but it could be many other signals, like memory strobes that is causing it. You might be able to lock the processor in reset and see if the 80 MHz emission drops, to tell whether it is the oscillator alone. It seems to me that square waves are supposed to be composed of ODD harmonics. One fix I have heard of is to put a small series resistor on the output line of the oscillator. Also, switching to an oscillator with a lower output current may be possible. But, it sounds like you have a built-in oscillator in the chip, and are using a bare crystal? You might be able to lower the drive level to the crystal with a series resistor (this is necessary in 32 KHz crystals, which will self destruct if given too much drive). Do you have capacitors to ground to resonate the crystal? A common circuit uses 2 caps of about 15 - 33 pF from each side of the crystal to ground. The 'sense' side is fine, but on the 'drive' side, the output driver of the crystal oscillator is now trying to pump a (5 V) square wave into a 33 pF cap to ground. This will cause large current pulses unless the output driver is specially designed to have 'weak' output. It may be that placing a resistor of a couple hundred ohms in series from the drive pin to the cap/xtal junction will relieve the chip of that heavy capacitive load, and stop the problem. As you note it is a 4-layer board, with power and ground planes, it should have pretty good grounding of the power nets, so that shouldn't be a problem. (I've frequently had nasty EMI gremlins on 2-layer boards.) But, you might check the power, ground and decoupling layout on chips that are likely to sources of the 80 MHz emission, to see if the routing of the tracks could be shortened. Minimum track length (or, optimally, zero length by use of thermal reliefs on the pin's through holes) of the power and gnd connections is desirable, as CMOS chips tend to draw power pulses right at clock edges. Do you have DRAM on this board? CMOS static RAM can also pull large pulses on the power lines when address lines change (sync SRAM can pull pulses when clocked). But, DRAMs are phenomenal EMI sources when the strobes are pulses, as they can switch 300+ mA PER CHIP! (The newer stuff is usually a lot less than that.) I hope this gives you some ideas of where to look, and a few tricks that may help isolate the specific chip or region of the circuit that is the source. Jon * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * To post a message: mailto:[EMAIL PROTECTED] * * To leave this list visit: * http://www.techservinc.com/protelusers/leave.html * - or email - * mailto:[EMAIL PROTECTED]?body=leave%20proteledaforum * * Contact the list manager: * mailto:[EMAIL PROTECTED] * * Browse or Search previous postings: * http://www.mail-archive.com/proteledaforum@techservinc.com * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
Re: [PEDA] EMC question
Make sure that the ground side of the decoupling caps for the crystal go straight to the GND of the processor with as thick a trace as possible. Preferably a polygon fill throughout the processor & crystal area. If you are using just a normal trace here, believe it or not, you may have just made a tuned antenna which can multiply the signal strength of the 3rd harmonic coming from the crystal's decoupling current through those caps, 80Mhz. Brian Guralnick - Original Message - From: "Greg Olson" <[EMAIL PROTECTED]> To: "Protel EDA Forum" <[EMAIL PROTECTED]> Sent: Thursday, July 26, 2001 10:41 AM Subject: [PEDA] EMC question Hi all.. I've got a 2.5"x3.5" plug on daughterboard which contains my processor, memory, flash, etc... The processor is a 20MHz AM186 driven by a 20MHz crystal. I just had this system in for FCC EMC testing and found that the 20MHz crystal is bleeding its harmonics (especially 80MHz) all over everything! It's getting onto the IO lines from the daughterboard to the main board and from there onto just about every wire leaving the enclosure! Can anyone give suggestions as to the best way to isolate this clock signal in a relatively tight space? This is a 4 layer board, top layer signal, next layer ground, then Vcc then signal again. Thanks in advance, Greg Olson DSX Access Systems * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * To post a message: mailto:[EMAIL PROTECTED] * * To leave this list visit: * http://www.techservinc.com/protelusers/leave.html * - or email - * mailto:[EMAIL PROTECTED]?body=ave%20proteledaforum * * Contact the list manager: * mailto:[EMAIL PROTECTED] * * Browse or Search previous postings: * http://www.mail-archive.com/proteledaforum@techservinc.com * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * To post a message: mailto:[EMAIL PROTECTED] * * To leave this list visit: * http://www.techservinc.com/protelusers/leave.html * - or email - * mailto:[EMAIL PROTECTED]?body=leave%20proteledaforum * * Contact the list manager: * mailto:[EMAIL PROTECTED] * * Browse or Search previous postings: * http://www.mail-archive.com/proteledaforum@techservinc.com * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
Re: [PEDA] EMC question
Re: [PEDA] EMC question
What I do is lay a ground (polygon plane) plane directly underneath the crystal and surrounding components. This plane is additional to the ground plane on the internal layer. So the polygon is on the same layer as the component. Also, very close placement to the processor on the same side, no vias, then I try to make the lengths the same. I hope this helps you, Brad Greg Olson wrote: > Hi all.. > > I've got a 2.5"x3.5" plug on daughterboard which contains my processor, memory, >flash, etc... The processor is a 20MHz AM186 driven by a 20MHz crystal. I just had >this system in for FCC EMC testing and found that the 20MHz crystal is bleeding its >harmonics (especially 80MHz) all over everything! It's getting onto the IO lines from >the daughterboard to the main board and from there onto just about every wire leaving >the enclosure! > > Can anyone give suggestions as to the best way to isolate this clock signal in a >relatively tight space? This is a 4 layer board, top layer signal, next layer ground, >then Vcc then signal again. > > Thanks in advance, > > Greg Olson > DSX Access Systems * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * To post a message: mailto:[EMAIL PROTECTED] * * To leave this list visit: * http://www.techservinc.com/protelusers/leave.html * - or email - * mailto:[EMAIL PROTECTED]?body=leave%20proteledaforum * * Contact the list manager: * mailto:[EMAIL PROTECTED] * * Browse or Search previous postings: * http://www.mail-archive.com/proteledaforum@techservinc.com * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
Re: [PEDA] EMC question
Greg, We are using the AM186 on one of our boards with a 25MHz crystal. What I did was put the crystal as close to the microprocessor and put a ground plane under the crystal that covered the traces and filters for the crystals on the top layer. I also made sure the traces where the same length. It help do the reduce the harmonics to pass CE. Ted -Original Message- From: Greg Olson [mailto:[EMAIL PROTECTED]] Sent: Thursday, July 26, 2001 9:41 AM To: Protel EDA Forum Subject: [PEDA] EMC question Hi all.. I've got a 2.5"x3.5" plug on daughterboard which contains my processor, memory, flash, etc... The processor is a 20MHz AM186 driven by a 20MHz crystal. I just had this system in for FCC EMC testing and found that the 20MHz crystal is bleeding its harmonics (especially 80MHz) all over everything! It's getting onto the IO lines from the daughterboard to the main board and from there onto just about every wire leaving the enclosure! Can anyone give suggestions as to the best way to isolate this clock signal in a relatively tight space? This is a 4 layer board, top layer signal, next layer ground, then Vcc then signal again. Thanks in advance, Greg Olson DSX Access Systems * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * To post a message: mailto:[EMAIL PROTECTED] * * To leave this list visit: * http://www.techservinc.com/protelusers/leave.html * - or email - * mailto:[EMAIL PROTECTED]?body=leave%20proteledaforum * * Contact the list manager: * mailto:[EMAIL PROTECTED] * * Browse or Search previous postings: * http://www.mail-archive.com/proteledaforum@techservinc.com * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
Re: [PEDA] EMC question
ensure that the xtal and its associated decoupling caps are as close as possible to the processor clock pins. Put ground planes around it on top /bottom layers and locally stitch top / bottom planes with as many vias as you can fit in (I tend to use 1mm via / 0.5mm hole for this) You may also need to decouple all your I/O lines with a few pF - I use SM capacitor or C-R networks for this purpose and it works well - look at the Bourns or Phillips websites for these parts. Hope this helps... -Original Message- From: Greg Olson [mailto:[EMAIL PROTECTED]] Sent: 26 July 2001 15:41 To: Protel EDA Forum Subject: [PEDA] EMC question Hi all.. I've got a 2.5"x3.5" plug on daughterboard which contains my processor, memory, flash, etc... The processor is a 20MHz AM186 driven by a 20MHz crystal. I just had this system in for FCC EMC testing and found that the 20MHz crystal is bleeding its harmonics (especially 80MHz) all over everything! It's getting onto the IO lines from the daughterboard to the main board and from there onto just about every wire leaving the enclosure! Can anyone give suggestions as to the best way to isolate this clock signal in a relatively tight space? This is a 4 layer board, top layer signal, next layer ground, then Vcc then signal again. Thanks in advance, Greg Olson DSX Access Systems * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * To post a message: mailto:[EMAIL PROTECTED] * * To leave this list visit: * http://www.techservinc.com/protelusers/leave.html * - or email - * mailto:[EMAIL PROTECTED]?body=ave%20proteledaforum * * Contact the list manager: * mailto:[EMAIL PROTECTED] * * Browse or Search previous postings: * http://www.mail-archive.com/proteledaforum@techservinc.com * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * To post a message: mailto:[EMAIL PROTECTED] * * To leave this list visit: * http://www.techservinc.com/protelusers/leave.html * - or email - * mailto:[EMAIL PROTECTED]?body=leave%20proteledaforum * * Contact the list manager: * mailto:[EMAIL PROTECTED] * * Browse or Search previous postings: * http://www.mail-archive.com/proteledaforum@techservinc.com * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
