Chris, from what you described, it seems that a common-mode (CM) filter on all three lines would be a good approach. That will not reject your single-ended signal.
Coupled RF will mostly come as common-mode carried by the wire, thus that is what you want to reject in the first place. A good three-line CM choke, plus maybe some extra HF filters (e.g. caps/ferite beads in the signal line and between Vcc and GND) seems to be a way to go. Neven > > 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"
