A 45-degree CMOS phase shifter and gain/phase imbalance compensation for multi-band transceivers

The paper describes two approaches to achieve multi-band modulators and demodulators for direct conversion architecture. One is a 45-degree phase-shifter CMOS IC, whose input frequency is 4/3 times the desired frequency. The proposed circuit achieves a lower input frequency than conventional 45-degree phase shifters that use frequency dividers. The other is a gain/phase compensation method for quadrature demodulators and modulators. The power measurements for the imbalance estimation can be taken in less than ten symbol periods of the test signals. This means that the compensation method does not need as many iterative cycles as required when using the least mean square or discrete Fourier transform used in previous methods. Compensation procedures involving an actual demodulator were successfully performed.