Jeeez, I have just the opposite problem GROUNDING a PCB at a certain
location.
The device is a metal enclosed digital device, about the size of a
video cassett tape. Its powered by one of those small AC/DC power supplies like
you have on your laptop computer.
The DC power cable does have a shielded cable, with a drain wire in
contact with the foil. Both are connected at shielded 9 pin D and grounded to
the case. The other end however, is just a drain wire into the AC/DC supply.
Its a plastic box.
The PCB is grounded in several locations to the bottom of the metal
enclosure, at roughly 37 cm (5 inches). At about 800 Mhz the thing radiates
above class B by 3 or 4 dB. If I remove one of the ground points the signal
drops 5 to 8 dB. Re-ground and its up, unground and its down. Guess, I should
feel lucky. I can remove the ring and via so that it doesn't get grounded there
and the problem is gone. I just get real nervous with that answer, but it works
so the engineer boss is not to crazy about making any mods.
Gary
-----Original Message-----
From: Robert Wilson [mailto:robert_wil...@tirsys.com]
Sent: Monday, February 25, 2002 12:19 PM
To: emc-p...@majordomo.ieee.org
Subject: RE: PCB floating area layout
Floating "grounds" on PCBs tend to be problematic, especially at high
frequencies. Minimizing the effective capacitive reactance between the
floating "ground" and the real ground will ensure that the floating
ground is "AC Cold". I don't just mean bypassing it with (say) a few
thousand uF of electrolytic capacitance, but instead ensuring minimal
capacitive reactance to ground across the entire frequency band of
interest. This usually entails (as an example) paralleling something
like a 0.1uF cap, and with an NP0 1000pF cap (or similar).
If the floating "ground" not properly decoupled to the main ground, and
it is a significant proportion of a HF wavelength, then it can have very
high AC voltages superimposed and act as a marvelous antenna.
Reducing the size of the floating "ground" is always a good plan.
Increasing it merely means that you have a larger potential antenna.
Bob Wilson
TIR Systems Ltd.
Vancouver.
-----Original Message-----
From: Paolo Peruzzi [mailto:paolo.peru...@esaote.com]
Sent: February 25, 2002 7:34 AM
To: emc-p...@majordomo.ieee.org
Subject: PCB floating area layout
Hi all,
I'm dealing with a PCB that has a floating section isolated from the
rest
of the board for safety purposes (patient applied part).
I found out some problems with emissions, due to the coupling between
the
floating part and of the PCB and the earthed one.
My questions are concerning the layout design of the floating area:
1) Is it best to minimize the HF capacitive coupling between the earthed
ground and the floating ground or to maximize it?
2) Is it best to reduce the amount of the floating ground or to increase
it?
Does it depend on the goodness of the "main ground", i.e. how much it is
"cold" ? (I see the board as a dipole with one end connected to earth,
and
the other floating).
Thanks,
p.p.
-------------------------------------------------------------
ESAOTE S.p.A. Paolo Peruzzi
Research & Product Development Design Quality Control
Via di Caciolle,15 tel:+39.055.4229306
I- 50127 Florence fax:+39.055.4223305
e-mail: paolo.peru...@esaote.com
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