Due to IQ imbalance, the image falls on the same band as the wanted signal in a ZIF receiver. This impairs the sensitivity, particularly for situations where one carrier has a higher power than another in the same band. The adjacent channel selectivity (ACS) setup where a –52 dBm 20 MHz NR blocker is received at the antenna just adjacent to the operating band edge [REF 1, Section 7.4.1] is such an example. If the operating band is not perfectly centered around the DC when down-converted to the baseband, the image of the ACS blocker falls onto the wanted band, degrading the sensitivity of the receiver as shown in the following figure. From the previous calculation of keeping any in-band interferer at –109 dBm or less it means the IQ correction/rejection across the 280 MHz bandwidth must be 57 dBc or better.
Another operating scenario where the IQ image presents a problem is the ACS requirement for non-contiguous bands. In this situation, the ACS blocker is located in between the non-contiguous bands and its image falls onto one of the operating sub-bands as illustrated in the following figure. Situations like these can severely limit the designer’s frequency planning options.