I/Q Gain Imbalance - Agilent Technologies ESA-E Series User And Programming Manual

Making Modulation Analysis Measurements
Interpreting Measurement Results

I/Q Gain Imbalance

I/Q gain imbalance is a result of non-equal gains in the I and Q parts of
the network. This can be caused by differences in components such as
filters, amplifiers, and DACs. For non-scrambled or spread QPSK
signals, the symptom of this imbalance is indicated by
rectangular-shaped groupings of symbol decision points in each
quadrant.
Press
1. Use the I/Q polar constellation diagram first to identify this problem.
The long side of the rectangle appears along the I axis, but this will
NOTE
shift 90° along the Q axis and back again in random fashion. This is
normal.
The analyzer arbitrarily assigns the demodulated symbols to the I and
Q channels for each new time record; the analyzer acts as an
asynchronous receiver. For this reason, it is not possible to determine
the correct carrier phase reference.
Figure 4-15 Polar Constellation Showing an I/Q Gain Imbalance
36
Meas Control, Pause
Press
View/Trace, I/Q Measured Polar Constln
gain imbalance is shown in
indicated with the symbol decision points at the rectangle corners,
instead of the square shape you would normally see with a properly
balanced signal. A gain imbalance of 2 dB was used to obtain this
result.
Although the display indicates a Wideband CDMA format, the signal
shown is a non-scrambled or spread QPSK signal.
to view a single time record.
. A signal with an I/Q
Figure 4-15. A rectangular shape is
Chapter 4