14
July 6, 2010
AN1559.0
GPS Application Setup
The following setup should be used for testing QHx220
noise cancellation ability for a GPS application. It is
important to maintain the identical core setup used in the
“GPS Optimization Setup (Static Coupling Channel)”
section on page 12. Otherwise, the optimization values
will not be valid.
Begin with the aggressor source (signal generator) and
QHx220 board powered off. Turn on the GPS simulator
and GPS receiver and record the receiver’s signal quality
parameters (C/N, etc.) once a satellite lock is achieved.
Find the weakest GPS signal power level that will still
maintain a lock. Next, set the aggressor signal generator
to the same GPS frequency and set the power level to
roughly 10dB above the peak of the broadcast GPS
signal. The aggressor signal needs to be high enough to
show degradation of the receive signal strength reported
by the GPS receiver. Remember not to exceed ~ -45dBm
at the input of the QHx220. Once an aggressor power
level is found that degrades the receive signal strength
reported by the GPS receiver, turn on the QHx220 board
and set the optimized I and Q values that were
determined in the “GPS Optimization Setup (Static
Coupling Channel)” section on page 12. This should
result in a <15dB recovery of the receive signal strength
power reported by the GPS receiver if the board was
optimized correctly. However, since the setup was slightly
modified from the original optimization setup, it should
be possible to fine tune the QHx220 to improve the
in-band noise cancellation. Please note that any changes
to the cable lengths or slight impedance changes will
slightly impact the optimal gain and phase of the
QHx220. Therefore, in the GPS application setup, adjust
the gain and/or phase one step at a time and monitor the
GPS receiver sensitivity (or any other signal quality
parameter (i.e. C/N, etc.). Continue to adjust the gain
and phase one step at a time until no further
improvement to the receiver sensitivity can be achieved.
FIGURE 14. GPS TESTING SETUP DIAGRAM
GPS
AGGRESSOR
PATH
SIGNAL
GENERATOR
GPS
SIMULATOR
QHX220
GPS
TUNER
CANCELLATION
NODE
GPS RECEIVE PATH
POWER
SPLITTER
ATTENUATOR PAD
TO SET COUPLING LOSS
POWER
SPLITTER
Application Note 1559
