Here is an excellent summary article on attaching circuit boards to chassis for control of EMI. These techniques may have an impact on your signal integrity as well. http://www.compliance-club.com/archive/article.asp?artid=3D90 The source of your radiated energy may be the frequency content in your signal edge rate (rise and fall time), rather than the frequency of your clock. The source of your problem may well be a trace with poor signal integrity.=20 Curt Curt McNamara. P.E.=20 Senior Electrical Engineer=20 Logic Product Development=20 411 Washington Ave. N Suite 101=20 Minneapolis, MN 55401=20 Tel 612-436-5178=20 Fax 612-672-0443=20 curtm@xxxxxxxxxxx=20 www.logicpd.com=20 This message (including any attachments) contains confidential information intended for a specific individual and purpose, and is protected by law. If you are not the intended recipient, you should delete this message and are hereby notified that any disclosure, copying, or distribution of this message, or the taking of any action based on it, is strictly prohibited. -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Americomrh@xxxxxxx Sent: Wednesday, August 17, 2005 2:27 PM To: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Radiated Emissions continued Content-Type: text/plain; charset=3D"US-ASCII" Content-Transfer-Encoding: 7bit Thanks for the informative replies with regard to the radiation emissions =20 question. Here is more info detailing the problem. The chassis, backplane, and front panel are all grounded. The PCB has frame=20 ground floods on both the upper and lower layers. The front to back =20 distance of the cavity is about 5.5 inches. The left to right width of the cavity=20 is about 15 inches, but there is some intervening metal for card guides. It=20 is mostly open. The top to bottom size is 1.3125 inches.=20 The other anomoly is where it comes from. I believe the frequency is=20 actually 687.5MHz. We have 125MHz on the board. At one time, I divided it by 2 to=20 get 62.5MHz to drive some logic so I assumed it was the 11th harmonic. I=20 changed the 62.5MHz to 41.67MHz, but the 687.5MHz did not move. Several other=20 lower level spikes did move, but they are not a concern. The 687.5MHz=20 appears to be the 5 1/2 harmonic. Fourier says that can't be. Perhaps a chip =20 does a divide by 2 internally which I don't know about. We tried bypass caps on=20 the power supplies as small as 100pf without effect. =20 Robert Hanson -- Attached file included as plaintext by Ecartis -- Return-Path: <Gary.Miller@xxxxxxxxx> Received: from rly-xa04.mx.aol.com (rly-xa04.mail.aol.com [172.20.64.40]) by air-xa03.mail.aol.com (v107.10) with ESMTP id MAILINXA31-67430337d224a; Wed, 17 Aug 2005 09:12:54 -0400 Received: from mimesweeper2.hubbell.com (mimesweeper2.hubbell.com [63.111.141.185]) by rly-xa04.mx.aol.com (v107.10) with ESMTP id MAILRELAYINXA43-67430337d224a; Wed, 17 Aug 2005 09:12:50 -0400 Received: from CRPORGCTNW03-IA.HUBBELL.COM (gwise.hubbell.com [63.111.141.139]) by mimesweeper2.hubbell.com (8.12.11/8.12.11) with ESMTP id j7HDvhe8009428 for <Americomrh@xxxxxxx>; Wed, 17 Aug 2005 09:57:43 -0400 (envelope-from Gary.Miller@xxxxxxxxx) Received: from HUBBELL_GWGATE-MTA by CRPORGCTNW03-IA.HUBBELL.COM with Novell_GroupWise; Wed, 17 Aug 2005 09:12:49 -0400 Message-Id: <s302ff91.086@xxxxxxxxxxxxxxxxxxxxxxxxxxx> X-Mailer: Novell GroupWise Internet Agent 6.0.4 Date: Wed, 17 Aug 2005 09:12:40 -0400 From: "Gary Miller" <Gary.Miller@xxxxxxxxx> To: <Americomrh@xxxxxxx> Subject: Re: Response to EMI Questions Mime-Version: 1.0 Content-Type: text/plain; charset=3DUS-ASCII Content-Transfer-Encoding: quoted-printable Content-Disposition: inline X-AOL-IP: 63.111.141.185 Robert, The resonant cavity is an interesting thought. What is the Q of a reson=3D ant cavity? The chassis, backplane, and front panel are all grounded. The PCB has fr=3D ame ground floods on both the upper and lower layers. The = front to back dis=3D tance of the cavity is about 5.5 inches. The left to = right width of the cav=3D ity is about 15 inches, but there is some = intervening metal for card guides.=3D It is mostly open. The top to bottom size is 1.3125 inches.=3D20 The other anomoly is where it comes from. I believe the frequency is ac=3D tually 687.5MHz. We have 125MHz on the board. At one time, I divided it by=3D 2 to get 62.5MHz to drive some logic so I assumed it = was the 11th harmonic.=3D I changed the 62.5MHz to 41.67MHz, but the 687.5MHz did not move. Several=3D other lower level spikes did move, but they are not a = concern. The 687.5MH=3D z appears to be the 5 1/2 harmonic. Fourier says that can't be. Perhaps a=3D20=3D chip does a divide by 2 internally which I don't know about. We tried bypas=3D s caps on the power supplies as = small as 100pf without effect. Perhaps we can discuss this when you arrive. For now, we are moving tow=3D ard a plastic front panel as a solution. regards Gary Miller Pulsecom >>> <Americomrh@xxxxxxx> 08/16/05 07:42PM >>> =3D20 Gary, =3D20 My first thought is that a resonance frequency(s) may be inherent in your=3D20 chassis. A quarter wave length at 688 MHz is about 4.36 in. which can exis=3D t=3D20 within your structure. Any harmonics of 688 = MHz (assuming they exist) woul=3D d=3D20 have wave lengths of decreasing distance. Your front panel is grounded,=3D20 how-wire, Cu, braid, = other and how long? I assume that the enclosure is al=3D so=3D20 metal, but = it is floating. Therefore can I assume it is isolated from the=3D20=3D front=3D20 panel? Also, what is "ground," i.e. chassis, earth, green wire, etc.? =3D20 When the front panel is removed the cavity is = eliminated which stops the=3D20 build up of resonant modes. Now this is all conjecture on my part in that=3D20=3D I=3D20 have not experienced any = of your development knowledge; therefore, it is ju=3D st an=3D20 educated = guess. =3D20 Robert =3D20 message dated 8/16/2005 8:01:22 A.M. Pacific Daylight Time, =3D20 Gary.Miller@xxxxxxxxx writes: Robert, We have just experienced an unexpected EMI phenomena, and I would like to=3D20 add a question to our list of questions. One of our units had an emission problem at 688MHz. The device is roughly=3D20 5" x 5" x 1.5" with a grounded, metal front panel. When = we removed the fro=3D nt=3D20 panel (leaving a gaping hole) the overall emission reduced by 20dB. How is=3D =3D20 this possible? We are = looking at isolating the panel from ground as our=3D20 current solution. Gary Miller Pulsecom ******************************************************************* This email and any files transmitted with it are confidential and intended solely for the use of the individual or entity to whom they are addressed. If you have received this email in error please notify the system manager. This footnote also confirms that this email message has been swept for the presence of computer viruses. www.hubbell.com - Hubbell Incorporated ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org List archives are viewable at: =20 //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu =20 ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu