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Supply noise is a significant problem in RF systems where it can mix with RF signals, degrading signal/noise ratios and potentially causing violation of spectral masks. This paper presents an analysis of the supply rejection properties of RF amplifiers. We extend a conventional Volterra-series formulation to treat multiport systems and use it to describe the mixing products between power supply noise and the RF carrier. It is shown that a multiport Volterra formulation can be used to treat weak nonlinearities in the system and that the nonsymmetric cross terms accurately predict low-order mixing phenomenon. We demonstrate the validity of our hand analysis through the design and fabrication of a power amplifier in 180-nm CMOS, operating between 900 MHz-2.4 GHz with a maximum output power of 15 dBm. Spectral regrowth of single-tone and EDGE modulation waveforms is shown to match within 1-3 dB across frequency and input signal power. Importantly, this analysis provides insight into the circuit-level mechanisms for susceptibility to power supply noise and can help designers improve the power supply rejection ratio robustness of system-on-chip wireless blocks and transmitter architectures.