This paper analyses and compares key differences between active and passive mixer structures in the context of mmWave applications. Firstly, the paper analyses mixer switching stage input impedance and its implications on frequency response. Active mixers provide isolation, whereas passive switching entails impedance transparency beneficial for filtering. Secondly, the paper analyses mixer performance with a low-amplitude sinusoidal LO signal typical for mmWave frequencies. Such an LO signal entails partly overlapping, unideal switching. However, active switching transistors commutate current more ideally during the overlap stage, demonstrating superior sinusoidal LO signal tolerance. Importantly, this work demonstrates that despite multiple passive mixer benefits, their performance is highly dependent on the LO signal. Finally, based on the analysis, the paper introduces a wideband active mmWave downconversion mixer design in 22-nm FDSOI CMOS. Measured characteristics demonstrate a particularly wide bandwidth of 55 to 100 GHz.