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The Digital Compensation of I/Q Imbalance in Wideband Quadrature Receivers |
The development of multi-standard receiver architecture is a great challenge for the radio frequency circuit designers, whose task is to provide architectures capable of receiving various signals with different center frequency, bandwidth, guard interval, etc. Latest solutions embrace the direct conversion (Zero-IF) and Low-IF receivers, both type operating with quadrature signals. The development of this kind of architectures is impeded by system and circuit non-idealities, one of them being the imbalance between in-phase and quadrature (I/Q) signals. The I/Q imbalance is caused mainly by the mismatch in the local oscillator’s quadrature signals, gain and phase mismatched of filters on the I/Q path and electromagnetic cross-talk between the I/Q paths. Traditionally the I/Q imbalance is considered to be frequency independent in the case of narrowband signals (GSM, FM receivers). For the new wireless standards the bandwidth is often large (over 1MHz) and the frequency dependence of I/Q imbalance should be taken into account. The latest wireless communication standards adopted the OFDM scheme. The OFDM receivers uses the frame structure to compensate the I/Q imbalance and other impediments. The motivation: I/Q imbalance is due to circuit non-idealities; it would be useful to have a compensation without knowing the modulation type applied in wireless communications. This could be a leap in the development of a universal receiver.
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