As we have seen earlier, mixers are used in communication systems for up-converting the signal to RF for transmission, and for down-converting to baseband for reception. But for fully utilizing the bandwidth of the signal it is preferable to use quadrature mixers, also called complex mixers because they operate on the signal in the complex domain. Indeed, by using the real and imaginary parts of the signal, quadrature mixers provide the possibility to distinguish the “positive and negative” frequencies for a complex baseband spectrum centered around DC. In real implementations quadrature mixers are impaired by gain and phase mismatches which affect the signal by degrading the SNR or by generating out-of-band noise.
The normalized gain imbalance ΔG quantifies the difference between the gains GI in the real branch (I) and GQ in the imaginary branch (Q):
which is often specified as a percentage. However, sometimes the gain imbalance is specified in decibels:
The phase imbalance Δϕ is the phase error between the components I and Q ideally separated by 90°: