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. 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