I was going to make most of Ing. Gremmen's points but he beat me to the punch. The most important point is that the injection, whether CI or RI IS common mode, thus a common mode choke may be used with no resulting signal degradation. Mr. Woodgate's comment about 10 Volts at 150 kHz is very apropos, EN6100-4-6 is extremely conservative in estimating field-to-wire coupling efficiency. I do believe the baseline document, IEC 1000-4-6 makes allowances for shorter wires to be exposed over a smaller range of frequencies, meaning a higher start frequency. That should be investigated. I don't believe either of these requirements will upset a TTL-type digital bit stream, especially as it will no doubt be shielded. I think the Achilles heel of this circuit is the bias potential fed to the sensor, that bias must be as clean as the lowest output potential of the sensor that carries useful information. Twisting these three wires together and shielding them ought to suffice. A ferrite over the whole assembly will reduce shield currents at higher frequencies, where field-to-wire coupling is more efficient.
on 8/22/02 10:04 AM, Gert Gremmen at [email protected] wrote: > > Hi Chris, > > Let's start to state that you are probably over > estimating the effects of immunity test signals. > > 1/ The signals are common mode and present on the screen > initially and couples into the signal and powersupply > lines (DM) by the transfer impedance. 10 V of CDN couples signal > won't have to create more then 100 mV DM signal on a screened > and twisted pair. > > > > 2/ It has not been proved that the the sensor actually > degrades when exposed to the signals. > > 3/ A CM coil may be applied with a asym signal if certain > conditions are met (esp. grounding ) > > 4/ 10 nF will be sufficient in all filters. > > 5/ standard EMI filters of the pi-type will probably > do the job if cutoff freuqncy is 150 Khz or higher. > I recommend using off the shelve parts available easily. > > 6/ Make the best ground connection you ever designed for > those filters: use SMD and less then 10 mm to enclosure. > > 6/ Your bitstream at 25 Khz may be actually running at 12 Khz > if you specified 25 Kbit as 25 Khz. > > 7/ The value of reasonable impedance does not need to be > that high 300- 500 Ohms is sufficient. Don't over do. > The cap + ground connection needs to do the job. > No concern if commercila filters are used. make sure > you get your ground OK; that's your REAL challenge ! > > Gert Gremmen > ce-test, qualified testing > > http://www.cetest.nl > > > -----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" > -- Ken Javor EMC Compliance Huntsville, Alabama 256/650-5261 ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. 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