LMH6560 Datasheet, PDF (20/23 Pages) National Semiconductor (TI) – Quad, High-Speed, Closed-Loop Buffer

Application Notes (Continued)
Discontinuities in a Ground Plane
A ground plane with traces routed over this plane results in
the build up of an electric field between the trace and the
ground plane as seen in Figure 13. This field is build up over
the entire routing of the trace. For the highest performance
the ground plane should not be interrupted because to do so
will cause the field lines to follow a roundabout path. In
Figure 17 it was necessary to interrupt the ground plane with
the blue crossing trace. This interruption causes the return
current to follow a longer route than the signal path follows to
overcome the discontinuity.
If the overall density becomes too high it is better to make a
design which contains additional metal layers such that the
ground planes actually function as ground planes. The costs
for such a pcb are increased but the payoff is in overall
effectiveness and ease of design.
Ground Planes at Top and Bottom Layer of a PCB
In addition to the bottom layer ground plane another useful
practice is to leave as much copper as possible at the top
layer. This is done to reduce the amount of copper to be
removed from the top layer in the chemical process. This
causes less pollution of the chemical baths allowing the
manufacturer to make more pcb’s with a certain amount of
chemicals. Connecting this upper copper to ground provides
additional shielding and signal performance is enhanced.
For lower frequencies this is specifically true. However, at
higher frequencies other effects become more and more
important such that unwanted coupling may result in a re-
duction in the bandwidth of a circuit. In the design of a test
circuit for the LMH6559 this effect was clearly noticeable and
the useful bandwidth was reduced from 1500MHz to around
850MHz.
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FIGURE 17.
If needed it is possible to bypass the interruption with traces
that are parallel to the signal trace in order to reduce the
negative effects of the discontinuity in the ground plane. In
doing so, the current in the ground plane closely follows the
signal trace on the return path as can be seen in Figure 18.
Care must be taken not to place too many traces in the
ground plane or the ground plane effectively vanishes such
that even bypasses are unsuccessful in reducing negative
effects.
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FIGURE 19.
As can be seen in Figure 19 the presence of a copper field
close to the transmission line to and from the buffer causes
unwanted coupling effects which can be seen in the dip at
about 850MHz. This dip has a depth of about 5dB for the
case when all of the unused space is filled with copper. In
case of only one area being filled with copper this dip is
about 9dB.
FIGURE 18.
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PCB BOARD LAYOUT AND COMPONENT SELECTION
Sound practice in the area of high frequency design requires
that both active and passive components be used for the
purposes for which they were designed. It is possible to
amplify signals at frequencies of several hundreds of MHz
using standard through hole resistors. Surface mount de-
vices, however, are better suited for this purpose. Surface
mount resistors and capacitors are smaller and therefore
parasitics are of lower value and therefore have less influ-
ence on the properties of the amplifier. Another important
issue is the pcb itself, which is no longer a simple carrier for
all the parts and a medium to interconnect them. The pcb
board becomes a real component itself and consequently
contributes its own high frequency properties to the overall
performance of the circuit. Sound practice dictates that a
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