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Memory effects are complex phenomena that present major problems in modern high-power linear microwave PA design. Specifically, these effects have a large influence on spectral symmetry and modulation frequency sensitivity which in turn impacts overall linearity and importantly the suitability of a power amplifier (PA) to linearisation through pre-distortion. This paper presents detailed two-tone modulated measurements that clearly show how electrical memory introduced by non-ideal low-frequency base-band impedances represent the most significant contributor to overall observed memory effects in high-power LDMOS PA design. The analysis is achieved through the characterisation of a 20W LDMOS device at 2.1 GHz using two-tone excitation and a purpose built high-power measurement system that allows the collection of both RF and IF voltage and current waveforms along with all associated impedances.