Derek,
At 200MHz you should suspect all shunt capacitors; they may be above resonance
and so inductive!
Consider the possibility of the SMPS producing this 200MHz due to spurious
oscillation, and this being emitted DIFFERENTIAL mode down the power cable,
then converted to Common mode by impedance unbalance at the chamber exit and
reflected back onto the EUT cable within the chamber.
Good Luck!
Richard
Richard Marshall Limited
30 Ox Lane, Harpenden, Herts.
AL5 4HE UK
Phone +44 (0)1582 460815 www.design-emc.co.uk
Member of the EMC Industry Association
----- Original Message -----
From: Derek Walton
To: [email protected]
Sent: Saturday, December 07, 2013 3:27 PM
Subject: [PSES] Puzzle
Good day folks,
I'm faced with a challenge that's confusing me a bit, and I'm concerned I'm
overlooking something.
I have a small box, about 4" by 6" by 2" that contains some Ethernet
circuitry. Normally there are cables that come and go from this box.
During RE testing, there are several very strong emissions ( above 200 MHz )
that exceed all other noise made by the box. As yet, we have not been able to
link them with any of the anticipated frequencies one would expect. In chasing
the source down I have taken a number of steps which are:
1) Disconnect all cables except the power. There was no change on the large
spikes, other noise came down as anticipated.
2) Added several ferrites on the power cable. No change.
3) Added two feed through type caps at the power entry point. No change.
4) Wrapped the box completely in Aluminium foil. No change.
5) Checked Ambients... Nothing, quiet.
6) Replaced power source. No change.
7) Rotated box, very minor change in amplitude.
This box is a new version of an existing design that does not have the same
issue and close field probing is very difficult inside the box. My suspicion is
that a small daughter board is the source, but this cannot be removed from the
box without destruction. A new box has been ordered build without that board.
That may not arrive for a few weeks.
In the meantime, can anyone make suggestions about what may cause this noise,
besides parasitic oscillations/faulty parts, and what radiating mechanisms can
be uninfluenced with the protection measures I took.
Many thanks,
Derek Walton.
L F Research.
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Attachments are not permitted but the IEEE PSES Online Communities site at
http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used
formats), large files, etc.
Website: http://www.ieee-pses.org/
Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe)
List rules: http://www.ieee-pses.org/listrules.html
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