A 45nm low-power SAW-less WCDMA transmit modulator using direct quadrature voltage modulation

To address the low-noise low-power requirement of the WCDMA transmitter this paper describes an innovative approach using direct quadrature voltage modulation via a passive voltage mixer driven by a 25%-duty-cycle LO. As shown in Fig. 6.5.2, first the IF l/Q voltage inputs are filtered by the passive LPF to lower the far-out noise. By switching on/off the transistors M1 to M4 through the quadrature-phased LO with 25% duty cycle, the filtered IF quadrature input voltages V_l+, V_Q+, V_l-, and V_Q-. are sequentially copied to the voltage mixer output, which sees the high input impedance of the PA driver. Such operation leads to a direct quadrature voltage modulation. The output of the voltage mixer contains both the desired fundamental LO mixing product, and the unwanted odd-harmonic LO mixing products. For a packaged device the unwanted harmonics can be attenuated below -40dBc by a harmonic filter (formed by the RF output bonding wire and capacitors) after the PA driver.