A 1.55 GHz to 2.45 GHz center frequency continuous-time bandpass delta-sigma modulator for frequency agile transmitters

This paper presents a 4^th order continuous-time bandpass delta-sigma modulator (CT-BPDSM) with a programmable center frequency ranging from 1.55 GHz to 2.45 GHz. The modulator is suited to be applied in multistandard class-S power amplifiers. The circuit features a multifeedback architecture with return-to-zero (RZ) and half-return-to-zero (HRZ) pulses. The loop filters consist of LC-resonators with emitter degenerated input transconductors and Q-enhancement circuits. A configuration register allows to program the resonator input transconductors, the Q-enhancement circuits and the resonator capacitances with 5 bit resolution. Fine tuning of the resonator center frequency is achieved with varactors. The circuit is implemented in a 200 GHz-f_T SiGe-bipolar technology. The measured SNR at 2.2 GHz center frequency is 45.5 dB in a bandwidth of 20 MHz. At 1.55 GHz the SNR decreases to 40.7 dB. The measured uplink UMTS-FDD ACLR (adjacent channel leakage power ratio) of the modulator output is 48.4 dB in the first adjacent channel.