Chris, it may be that the internals of that chip are generating the 40 MHz harmonic and it is getting out onto the ground and/or power planes. First order of business is to have a solid ground plane. Second, is to prevent noise from the DSP chip from getting on the power plane. Create a +5V island for the DSP chip with a moat surrounding it. The power from the island is routed through a ferrite bead/cap filter to the main power plane. Of course good bypass is required on the island. Good luck.
Richard Woods ---------- From: Chris Chileshe [SMTP:[email protected]] Sent: Friday, March 02, 2001 11:13 AM To: [email protected] Subject: CPU clock emissions Hi group, It's Friday and thinking hurts the head on a Friday, so I'll take the easier option and just ask. I have just returned from emissions testing on a new product. The product uses a DSP which runs off an 8Mhz oscillator and internally 'ups' this to 40Mhz. It also has a CLOCK_OUT pin which is currently floating. I have 'beyond the limit' emissions at 40Mhz and higher harmonics of this frequency. Has anyone got any good simple, cost effective ideas how to suppress these emissions which are apparent in both radiated and conducted emissions? Would appreciate your personal experiences on suppressing these narrowband emissions are welcome. Regards - Chris Chileshe - Ultronics Ltd ________________________________________________________________________ This message has been checked for all known viruses, by Star Internet, delivered through the MessageLabs Virus Control Centre. For further information visit: http://www.star.net.uk/stats.asp ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson: [email protected] Dave Heald [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.rcic.com/ click on "Virtual Conference Hall," ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson: [email protected] Dave Heald [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.rcic.com/ click on "Virtual Conference Hall,"
