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We present a single multiplier based adaptive I/Q mismatch compensation circuit for narrowband quadrature receivers. Adaptive decorrelation between I and Q channel data is used for correcting gain and phase mismatches. Adaptation step size is computed from L1-norm inverse power measurement and a gear-shifting mechanism is used that allows fast initial convergence and slow adaptation on actual burst data. Image rejection ratio in excess of 50 dB is reported for GSM receiver after compensation allowing the receiver to use IF frequencies higher than half of the channel bandwidth. The presented mismatch compensation circuit is implemented as part of a single-chip GSM wireless transceiver fabricated in a 90-nm digital CMOS process. The presented techniques are, however, equally applicable to other narrowband packet-based applications.