Self-calibrated two-point delta-sigma modulation technique for RF transmitters

A self-calibrated two-point delta-sigma modulation technique for CMOS RF transmitter is proposed. This calibration technique employs voltage-controlled oscillator (VCO) and delta-sigma modulator input ports in a frequency synthesizer. By monitoring the control voltage of a loop filter, the gain mismatch between two paths can be detected and completely calibrated by modifying the gain of the VCO path. The acceptable timing mismatch between the two paths is also investigated so that the timing can be controlled to ensure stable output performance. As a result, the phase modulation guarantees a robust performance against PVT variations without using any predistortion techniques. This technique is applied to a quad-band (850/900/1800/1900) GSM/EDGE transmitter for verification. For the amplitude modulation of the EDGE mode, a dc calibration is adopted to suppress unwanted carrier leakage tones. The measurement results show satisfactory GSM/EDGE spectrums and error vector magnitude performance at both low-and high-frequency bands.