Hi Derek, 33 MHz is a standard clock frequency for PCI bus. Possible sources are; motherboard bridge to PCI connectors; motherboard bridge to PCI IC devices on the motherboard such as on board LAN, USB or audio; sometimes a PCI card will have an onboard bridge that redistributes the PCI bus to several devices on the PCI card. Typically, PCI clocks go to a single device so clock distribution IC is required. These some possible sources if it is PCI bus. Isolating the component blocks or clock lines can be a little tricky because it may shut down the complete system. Having the ability to control the firmware is very helpful. Hope this helps and good luck. RICK LINFORD [email protected]
From: [email protected] [mailto:[email protected]] Sent: Wednesday, July 02, 2003 10:39 AM To: [email protected] Subject: Source of noise Morning folks.. I find myself wrestling with PC emissions again. No names this time :-) Above 500 MHz, I'm seeing a bunch of reasonably stable harmonics with about a 33 MHz spacing. Any thoughts as to what they may be from? I've played with all the case openings etc, and also added a ferrite bead to every cable leaving the MB... only minimal effect on this particular set of emissions. Ironically, the cooling fan is a big problem, it's powered from a 5 volt source that's very noisy. Consequently, the wires going to it radiate like crazy... I believe the MotherBoard is clocking around 1 GHz. Cheers, Derek N. Walton Owner L F Research EMC Design and Test Facility Poplar Grove, Illinois, USA www.lfresearch.com
