[SI-LIST] Re: Split gnd planes - for/against?
- From: "Kevin G. Rhoads" <kgrhoads@xxxxxxxxxxxx>
- To: Sol Tatlow <Sol.Tatlow@xxxxxxxxxxxxxxxxxxxx>
- Date: Wed, 08 Apr 2009 18:04:57 +0000
>Have you ever had 2 variants of the same layout manufactured on the
>same panel, where the only difference between the two is that one
>has a solid ground plane, and the other has some form of moating
>and/or ferrite (or otherwise) isolated ground islands, where one of
>the variants could be indisputably shown to perform better, with
>respect either to functionality or EMI? If so, which one?
Not exactly the case for which you asked, but somewhat similar,
so I will describe below.
The board in question is a pre-amp board for a detector head.
Details are: Hemispherical analyzer for an electron detector
(modified Top Hat design) using a micro-channel plate as the
first amplification stage, so single electrons making it through
the analyzer get multiple in the MCP and become a charge burst
onto an anode.
The anodes (30 in all) feed a through resistors on a "ring and
spokes" board (one spoke per anode, five rings) becoming a
Gray-coded binary on the rings. Each ring feeds a BJT amplifier.
The pre-amp board is the the third of three in the stack, (anode
board directly under the Top Hat, then the ring and spokes, then
the preamp).
Because of the high voltage used for MCP bias, there is a HV Ground
plane on the anode and on the ring and spokes boards which floats
at MCP bias. On the pre-amp board there is a small section of HV
ground where the leads from the ring and spokes come in, then an
HV isolation moat, then the rest of the preamp board with ground plane.
The preamp board is effective a 2 layer board, but it is made with
others on one panel with a 4 layer process.
We tried two versions of the pre-amp board. One with null inner layers
and only copper flood ground-plane on the outer surfaces, tied with multple
vias. The other had inner ground planes underlying the surface copper
flood regions -- all tied together with vias.
We saw no behavioral differences between the 2 version and the 4 version
of this board.
Because of the high gain required, and the closeness of the five
channels on the preamp board, this circuit is prone to RF pickup
and cross-channel regeneration oscillation. We have found that
taking the five signal coaxes an the LV power coax and cutting
through the outer insulation and bonding the braid together electrically
is key to making the final assembly fairly well behaved.
We have also found that putting coarse screening over the entrance
apertures to the TOp Hats was also necessary. These detectors are
flown on NASA sounding rockets and often the TM antenna is radiating
into the Top Hat entrance aperture. Screening the aperture reduces
the interference pickup significantly.
SO while this is not a case of split vs. non-split ground planes,
it is a case of varying numbers of multi-conductor ground planes
in the board.
FWIW
Kevin
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