Adding Noise and Predistortions, and Impairing the Signal
R&S
®
SMBV100B
270User Manual 1178.4460.02 ─ 03
Figure 4-4: Effect of an increased amplitude in the I/Q constellation diagram
An I gain multiplies the I amplitudes by a factor, leaving the Q amplitudes unchanged.
A Q gain has the opposite effect. Different I and Q gain factors result in an I/Q imbal-
ance, which is due to different gains of the amplifiers in the I and Q channels of the I/Q
modulator. The effect of a positive and negative gain imbalance is shown on Fig-
ure 4-5.
Figure 4-5: Negative gain imbalance (left) and positive gain imbalance (right) in the I/Q constellation
diagram
4.2.2.2 I and Q Offset
An I offset adds a constant value to all I amplitudes, leaving the Q amplitudes
unchanged. A Q offset has the opposite effect. A combination of I and Q values results
in an I/Q offset, which is due to carrier feedthrough in the I/Q modulator. Possible rea-
sons are interfering signals at the RF carrier frequency, e.g. an unsuppressed RF car-
rier subchannel. The effect of a positive I and Q offset in the I/Q constellation diagram
is shown on Figure 4-6.
Impairing the Signal
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