NEO-5
-
Hardware
Integration
Manual
Preliminary
Product
Testing
GPS.G5-MS5-08003-A2
u-blox
proprietary
Page 44
C Interference Issues
A
typical
GPS/GALILEO
receiver
has
a
very
low
dynamic
range.
This
is
because
the
antenna
should
only
detect
thermal
noise
in
the
GPS/GALILEO
frequency
band,
given
that
the
peak
power
of
the
GPS/GALILEO
signal
is
15
dB
below
the
thermal
noise
floor.
This
thermal
noise
floor
is
usually
very
constant
over
time.
Most
receiver
architectures
use
an
automatic
gain
control
(AGC)
circuitry
to
automatically
adjust
to
the
input
levels
presented
by
different
antenna
and
pre-amplifier
combinations.
The
control
range
of
these
AGC’s
can
be
as
large
as
50
dB.
However,
the
dynamic
range
for
a
jamming
signal
exceeding
the
thermal
noise
floor
is
typically
only
6
to
12dB,
due
to
the
one
or
two
bit
quantization
schemes
commonly
used
in
GPS/GALILEO
receivers.
If
there
are
jamming
signals
present
at
the
antenna
and
the
levels
of
these
signals
exceed
the
thermal
noise
power,
the
AGC
will
regulate
the
jamming
signal,
suppressing
the
GPS/GALILEO
signal
buried
in
thermal
noise
even
further.
Depending
on
the
filter
characteristics
of
the
antenna
and
the
front
end
of
the
GPS/GALILEO
receiver,
the
sensitivity
to
such
in-band
jamming
signals
decreases
more
or
less
rapidly
if
the
frequency
of
the
jamming
signal
moves
away
from
GPS/GALILEO
signal
frequency.
We
can
conclude
that
a
jamming
signal
exceeding
thermal
noise
floor
within
a
reasonable
bandwidth
(e.g.
100
MHz)
around
GPS/GALILEO
signal
frequency
will
degrade
the
performance
significantly.
Even
out-of-band
signals
can
affect
GPS/GALILEO
receiver
performance.
If
these
jamming
signals
are
strong
enough
that
even
antenna
and
front-end
filter
attenuation
are
not
sufficient,
the
AGC
will
still
regulate
the
jamming
signal.
Moreover,
very
high
jamming
signal
levels
can
result
in
non-linear
effects
in
the
pre-amplifier
stages
of
the
receiver,
resulting
in
desensitizing
of
the
whole
receiver.
One
such
particularly
difficult
scenario
is
the
transmitting
antenna
of
a
DCS
handset
(max.
30
dBm
at
1710
MHz)
in
close
proximity
to
the
GPS/GALILEO
antenna.
When
integrating
GPS/GALILEO
with
other
RF
transmitters
special
care
is
necessary.
If
the
particular
application
requires
integration
of
the
antenna
with
other
digital
systems,
one
should
make
sure
that
jamming
signal
levels
are
kept
to
an
absolute
minimum.
Even
harmonics
of
a
CPU
clock
can
reach
as
high
as
1.5
GHz
and
still
exceed
thermal
noise
floor.
On
the
receiver
side
there’s
not
much
that
can
be
done
to
improve
the
situation
without
significant
effort.
Of
course,
high
price
military
receivers
have
integrated
counter-measures
against
intentional
jamming.
But
the
methods
employed
are
out
of
the
scope
of
this
document
and
might
even
conflict
with
export
restrictions
for
dual-use
goods.
The
recommendations
and
concepts
in
this
section
are
completely
dependent
on
the
specific
applications.
In
situations
where
an
active
antenna
is
used
in
a
remote
position,
e.g.
>1
m
away
from
other
electronics,
interference
should
not
be
an
issue.
If
antenna
and
electronics
are
to
be
tightly
integrated,
the
following
sections
should
be
read
very
carefully.
C.1 Sources of Noise
Basically
two
sources
are
responsible
for
most
of
the
interference
with
GPS
receivers:
1.
Strong
RF
transmitters
close
to
GPS
frequency,
e.g.
DCS
at
1710
MHz
or
radars
at
1300
MHz.
2.
Harmonics
of
the
clock
frequency
emitted
from
digital
circuitry.
The
first
problem
can
be
very
difficult
to
solve,
but
if
GPS/GALILEO
and
RF
transmitter
are
to
be
integrated
close
to
each
other,
there’s
a
good
chance
that
there
is
an
engineer
at
hand
who
knows
the
specifications
of
the
RF
transmitter.
In
most
cases,
counter
measures
such
as
filters
will
be
required
for
the
transmitter
to
limit
disruptive
emissions
below
the
noise
floor
near
the
GPS/GALILEO
frequency.
Even
if
the
transmitter
is
quiet
in
the
GPS/GALILEO
band,
a
very
strong
emission
close
to
it
can
cause
saturation
in
the
front-end
of
the
receiver.
Typically,
the
receiver's
front-end
stage
will
reach
its
compression
point,
which
will
in
turn
increase
the
overall
noise
figure
of
the
receiver.
In
that
case,
only
special
filtering
between
the
GPS/GALILEO
antenna
and
receiver
input
will
help
to
reduce
signal
levels
to
the
level
of
linear
operation
at
the
front-end.
