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
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