A CMOS 1.5-bit ΣΔ digital IF to RF transmitter for WCDMA application

A digital intermediate frequency (IF) to RF transmitter targeted for 2GHz wideband code division multiple access (WCDMA) is presented. The proposed transmitter integrates a 3-level digital IF current-steering cell, an up-conversion mixer with a tuned load and an RF variable gain amplifier (RF VGA) with an embedded semi-digital finite impulse response (FIR) reconstruction filter. A 4^th-order 1.5-b IF bandpass sigma delta modulator (BP ΣΔM) is designed to guarantee in-band signal-to-noise ratio (SNR) and error vector magnitude (EVM) performances while the out-of-band quantization noise due to noise shaping is suppressed by the embedded FIR reconstruction filter to meet spectrum emission mask and ACPR requirements. The on-chip RF VGA provides 50dB power scaling in 10-dB steps with less than 1dB gain error. Together with the power control at the ΣΔM stage, the 74-dB dynamic range at RF frequency is achieved. The design is fabricated in a 4-metal 0.18-μm CMOS technology with a total core area of 0.8 × 1.6 mm ². The IC delivers 0dBm output power to the 50Ω load at 2GHz and it draws approximately 120mA from a 1.8V DC supply at the maximum output power.