Instrument Function
R&S
®
SMBV100A
304Operating Manual 1176.8016.02 ─ 17
Figure 5-13: 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 5-14.
Figure 5-14: Negative gain imbalance (left) and positive gain imbalance (right) in the I/Q constellation
diagram
5.7.1.2 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 5-15.
Impairment of Digital I/Q Signal and Noise Generator - AWGN/IMP Block
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