Hi group, Thank you everyone for your great responses on the subject.
Sorry I couldn't acknowledge your postings earlier than this and sorry I haven't acknowledged your postings individually. Had to go away somewhere - at short notice. Only just returned. I believe the gist of your responses was that it is a bit of a tall order to try and get a filter that will let 25kHz signals through and reject RF at 150kHz. To this end, I am thinking I may have to design a separate filter circuit for the output while keeping my original filter strategies on the DC lines. Any fundamental flaws in this reasoning? Gert and others raise a few points I would like to comment/pursue/query etc. Note that I have to assume screened cable is not an option initially and further that the output it is a PWM square wave with a maximum fundamental frequency of 25kHz and not 25kbit.. >> 2/ It has not been proved that the sensor actually degrades when exposed to the signals. My fault. I didn't supply you with enough information. I have the analog version of the sensor. Before addition of the pi-filters it was very susceptible. With only LC filtering, it was much better but still showed mild susceptibility during conducted immunity tests although the radiated immunity was vastly improved. With PI filters, it was immune over the full test spectrum 150k-1G. >>3/ A CM coil may be applied with a asym signal if certain conditions are met (esp. grounding ) I have had bad experiences with trying to use CM chokes on asym. lines, but you make a point about grounding, so maybe worth another look? >> 5/ standard EMI filters of the pi-type will probably do the job. I recommend using off the shelve parts available easily. I have heard conflicting reports on these devices. I have heard from certain 'well-informed' sources that the SMT PI-filters which one can get from the big manufacturers do not work quite as well as filters made from discrete SM devices. Any experiences worth sharing - anyone? Is it all about 'horses for courses' i.e. the right filters for the job. >> 7/ The value of reasonable impedance does not need to be that high 300- 500 Ohms is sufficient. Don't over do. That is a good point. I work to a minimum of 300 Ohms - and where I have to squeeze every last bit of the spectrum into the useable range on a ferrite, will accept slightly lower (usually means using larger caps-to-case to maintain the impedance ratio). Thank you again for the wonderful responses. - Best regards - Chris Chileshe - Ultronics Ltd -----Original Message----- From: [email protected] [mailto:[email protected]]On Behalf Of Chris Chileshe Sent: Thursday, August 22, 2002 11:31 AM To: 'emc-pstc' Subject: Designing for low power conducted and radiated immunity Dear all, On the evidence of the quality of responses I have seen on more difficult queries, I think this should be fairly straightforward for all you RF engineers out there. My product is a small 3-wire pressure sensor which sends out a digital bit stream encryption of the pressure measured. It's current draw is around 30mA max. The bit stream is around 25kHz, 5V TTL. The 3-wires are simply supply (5-10VDC), Output (5V logic) and Ground. I am trying to make the product pass conducted RF immunity tests to EN 61000-4-6 (150kHz - 80Mhz) at 10V and radiated immunity tests to EN 61000-4-3 (80Mhz - 1Ghz) at 30V/m. There is an intrinsic safety limit on the overall capacitance I can use in the product. My general thoughts are to use a PI-filter on each of the 3 lines with the vertical section of the filter being ceramic capacitors to the metal case enclosure. A limit of about 10nF exists (for other reasons) on these capacitors. The horizontal section of the PI will be made up of a series connection of a ferrite (for high frequency suppression) and an inductor for the lower frequencies where the ferrite is transparent - in that order. My problems (sorry .. challenges) are as follows: 1 - I need to let a bit stream through at 25kHz, but reject RF at 150kHz without a common mode choke (output is not differential). Is this a tall order? 2 - The inductors that give me a reasonable value of impedance at these sub 1Mhz frequencies tend to be largish and have Self Resonant Freqs. in the test spectrum albeit some of them (SRFs) are in the radiated immunity band where I expect the ferrite to be in charge of attenuation. Is this likely to cause me problems? 3 - Can I find a single ferrite that will cover the entire RF test spectrum of EN 61000-6-2 ( 150kHz - 1Ghz) or is it generally accepted that even the so called 'wideband' ferrites (SMT 0603/0805 max) are good down to about 5MHz but no lower? Any relevant comments welcome. Best regards - Chris Chileshe - Ultronics Ltd ________________________________________________________________________ This e-mail has been scanned for all viruses by Star Internet. The service is powered by MessageLabs. For more information on a proactive anti-virus service working around the clock, around the globe, visit: http://www.star.net.uk ________________________________________________________________________ ------------------------------------------- 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: Ron Pickard: [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://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list" ------------------------------------------- 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: Ron Pickard: [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://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list" ________________________________________________________________________ This e-mail has been scanned for all viruses by Star Internet. The service is powered by MessageLabs. For more information on a proactive anti-virus service working around the clock, around the globe, visit: http://www.star.net.uk ________________________________________________________________________ ________________________________________________________________________ This e-mail has been scanned for all viruses by Star Internet. The service is powered by MessageLabs. For more information on a proactive anti-virus service working around the clock, around the globe, visit: http://www.star.net.uk ________________________________________________________________________ ------------------------------------------- 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: Ron Pickard: [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://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list"
