driving
large
bus
systems.
Alternatively,
high-speed
signal
lines
can
be
terminated
with
resistors
or
even
active
terminations
to
reduce
high
frequency
radiation
originating
from
overshoot
and
ringing
on
these
lines.
If
dielectric
layers
are
thick
compared
to
the
line
width,
route
ground
traces
between
the
signal
lines
to
increase
shielding.
This
is
especially
important
if
only
two
layer
boards
are
used
(see
Figure 14
).
Bad: Excessive Radiation
Good: Radiation terminated
Figure 14: Terminating radiation of signal lines
1.6.2.3 Decoupling
Capacitors
Use
a
sufficient
number
of
decoupling
capacitors
in
parallel
between
power
and
ground
nets.
Small
size,
small
capacitance
types
reduce
high-frequency
emissions.
Large
size,
high
capacitance
types
stabilize
low
frequency
variations.
It’s
preferred
to
have
a
large
number
of
small
value
capacitors
in
parallel
rather
than
having
a
small
number
of
large
value
capacitors.
Every
capacitor
has
an
internal
inductance
in
series
with
the
specified
capacitance.
In
addition
to
resonance,
the
capacitor
will
also
behave
like
an
inductor.
If
many
capacitors
are
connected
in
parallel,
total
inductance
will
decrease
while
total
capacitance
will
increase.
Figure 15
shows
the
impedance
dependence
of
SMD
capacitors.
Figure 15: Impedance of 0805 size SMD capacitors vs. frequency, MuRata
If
the
power
and
ground
plane
are
not
connected
by
an
efficient
capacitor
network,
the
power
plane
may
act
as
a
radiating
patch
antenna
with
respect
to
the
ground.
Furthermore,
ceramic
capacitors
come
with
different
dielectric
materials.
These
materials
show
different
temperature
behavior.
For
industrial
temperature
range
applications,
at
least
a
X5R
quality
should
be
selected.
Y5V
or
Z5U
types
may
lose
almost
all
of
their
capacitance
ailure
at
low
temperatures
because
of
excessive
noise
emissions
from
the
digital
part.
Tantalum
capacitors
show
good
thermal
stability,
however,
their
high
ESR
(equivalent
series
resistance)
limits
the
usable
frequency
range
to
some
100
kHz.
at
extreme
temperatures,
resulting
in
potential
system
f
GPS
Modules
-
System
Integration
Manual
(SIM)
(incl.
Reference
Design)
GPS
Fundamentals
GPS.G4-MS4-05007-A1
Page 24
