Re: Unterminated Busses
Don Borowski at Schweitzer Engineering Labs wrote: Loading up the PCI slots puts more capacitance on the PCI bus, slowing the logic transistions and thus reducing emissions. Quite likely loading up the ISA slots will do the same thing, though emissions could go up. Some years ago, a computer product I worked on generated a terrible 105 MHz emission. This was without too much trouble traced to the unterminated ISA bus, which was resonant in that range; the bus drivers were tri-stated to save power, and only turned on when a signal was detected, but the resulting high impedance allowed noise to thereby trigger oscillation at the bus resonant frequency. As soon as a card was inserted, things quieted down. An aside: Had we tested this only with the maximum configuration we would have missed the problem - but 5 volts in the FM broadcast band would surely have come to our notice later! Cortland Cortland 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: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
Re: Unterminated Busses
Loading up the PCI slots puts more capacitance on the PCI bus, slowing the logic transistions and thus reducing emissions. Quite likely loading up the ISA slots will do the same thing, though emissions could go up. To reduce keyboard emissions, try wrapping the cable multiple times through a ferrite torroid. Inductance is proportional to the square of the number of turns, so just a few turns can be a lot more effective than a single pass through a bead. If the problem emissions are in a narrow range of frequencies, you can adjust the number to turns to tune the point of parallel resonance between the interwinding capacitance and the inductance to make a large series impedance at the frequency of interest. Don Borowski Schweitzer Engineering Labs Pullman, WA, USA Derek N. Walton wrote: Hi All, recently I posted about my tribulations with a noisy motherboard. I was testing with just the bare minimum of components, which included cards in the slots. For grins I loaded up the PCI slots with cards, and the system got quieter. Note, there were no cables, just the cards plugged in. And yes, the back panel holes were covered when the cards were not in. :-) This is an older MB, with ISA slots too. I have no cards for these. Could they be ringing, and causing the few spikes I have left? If so are there dummy loads I could plug in the slots to damp the ringing? The same board has multiple uses.. Also noted that adding the keyboard dramatically adds to the emissions, the mouse not so much. The Fe bead on the cable doesn't help much... Thoughts anyone? Thanks, Derek N. Walton Owner L F Research EMC Design and Test Facility Poplar Grove, Illinois, USA www.lfresearch.com 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: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
Unterminated Busses
Hi All, recently I posted about my tribulations with a noisy motherboard. I was testing with just the bare minimum of components, which included cards in the slots. For grins I loaded up the PCI slots with cards, and the system got quieter. Note, there were no cables, just the cards plugged in. And yes, the back panel holes were covered when the cards were not in. :-) This is an older MB, with ISA slots too. I have no cards for these. Could they be ringing, and causing the few spikes I have left? If so are there dummy loads I could plug in the slots to damp the ringing? The same board has multiple uses.. Also noted that adding the keyboard dramatically adds to the emissions, the mouse not so much. The Fe bead on the cable doesn't help much... Thoughts anyone? Thanks, Derek N. Walton Owner L F Research EMC Design and Test Facility Poplar Grove, Illinois, USA www.lfresearch.com
