A switching frequency modulated DC-to-DC converter with reduced spurious noise for a RF power amplifier in microwatts transmitters

Abstract

Varying a supply voltage using a DC-to-DC converter can significantly improve the efficiency of RF power amplifiers (PAs) for low-power RF transceivers. However, intrinsic switching noise from a conventional DC-to-DC converter operating in fixed Pulse-Width Modulation (PWM) can affect the system performance since the spectral components in the switching noise are confined in the fundamental switching frequency and its harmonics. This harmonic noise will have detrimental effects on the performance of the microwatt’s transmitter due to its low output power. Especially for microwatts systems, the minimized size of the inductor will increase the spurious noise due to increased peak-to-peak inductor current. By relocating the spurious noise and reducing peak noise from a switching converter, this paper proposes the technique to prevent high noise spurs in a microwatt transmitter, combining both pseudo-random frequency stepping and monotonic frequency stepping. Both modulation circuits are implemented in CMOS 0.18 µm process and incorporated to achieve the proposed DC-to-DC converter system. The measurement results indicate that the proposed PWM ramp signal modulator with random frequency modulation in a DC-to-DC converter can reduce the peak of the spurious noise-power by 22 dB. Monotonic frequency modulation with various frequency steps reduces the peak of the spurious noise-power by 18 dB.