Fundamental Study of Influence of Ripple Noise From DC–DC Converter on Spurious Noise of Wireless Portable Equipment

Power supply noise is a serious concern in noise-sensitive radio frequency or analog circuits. This paper presents an analysis of the influence of the synchronous rectification buck-type dc-dc converters for power amplifiers. An output ripple noise equation of the converter and a spurious noise equation of the carrier output voltage of the class-A amplifier powered by the converter are derived to analyze fundamental characteristics of these noise components theoretically. The amplifier's output spurious noise is calculated by these equations and measured by the evaluation circuit board. The spurious noise variation caused by the parameter variations of the amplifier or the converter is discussed based on these experimental and theoretical results. The spurious noise can be suppressed by decreasing the channel length modulation coefficient of the n-channel MOSFET constituting the amplifier and the natural frequency of the converter output LC filter. The spurious noise is reduced to low level below - 60 dB compared to the carrier signal by setting the channel length modulation coefficient below 0.1 V
^-1
and the natural frequency below 120 kHz. Furthermore, when the natural frequency is below 40 kHz, the spurious noise is - 80 dB below the carrier signal.