Datasheet

TRACKWIDTH
GROUNDPLANE
(w)
(h)
UPPER TRACK LAYER
INNER LAYER A
INNER LAYER B
BOTTOM TRACK LAYER
TRACK WIDTH
HEIGHT OVER
GROUND PLANE
TOTAL
PCB
HEIGHT
LMH6559
www.ti.com SNOSA57C –APRIL 2003–REVISED MARCH 2013
Figure 47.
Using a trace over a ground plane has big advantages over the use of a standard single or double sided board.
The main advantage is that the electric field generated by the signal transported over this trace is fixed between
the trace and the ground plane e.g. there is almost no possibility of radiation (see Figure 48).
Figure 48.
This effect works to both sides because the circuit will not generate radiation but the circuit is also not sensible if
exposed to a certain radiation level. The same is also noticeable when placing components flat on the printed
circuit board. Standard through hole components when placed upright can act as an antenna causing an electric
field which could be picked up by a nearby upright component. If placed directly at the surface of the pcb this
influence is much lower.
The Effect Of Variation For ε
r
When using pcb material the ε
r
has a certain shift over the used frequency spectrum, so if necessary to work with
very accurate trace impedances one must taken into account for which frequency region the design has to be
functional. Figure 49 (Courtesy of Islola Corporation) gives an example what the drift in ε
r
will be when using the
pcb material produced by Isola. If working at frequencies of 100MHz then a 50Ω trace has a width of 3.04mm for
standard 1.6mm FR4 pcb material, and the same trace needs a width of 3.14mm. for frequencies around 10GHz.
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