A notch filter alignment circuit for wireless communication FDD systems

In this paper a circuit is presented that is capable to measure the transfer function of a notch filter as it could be used in modern wireless FDD systems like LTE and UMTS. As in FDD systems the TX PLL is always running during signal reception, its output is used to downconvert the TX leakage voltage after the notch filter to DC. A logarithmic amplifier boosts the dynamic range of the system to 30 dB. The whole system consumes less than 1 mA from a 1.4 V supply which is compensated for by the selectivity gain of the better adjusted notch filter. The circuit has been simulated on a 90 nm commercial technology.

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