The effect of the diode capacitance parasitics on the performance of microwave double balanced lattice mixers, driven by a local oscillator having a large internal resistance and a current sinusoidal waveform, is examined. It is found that the current present in the diodes is modified by the diode capacitance and this in turn influences the performance of the mixers. To reduce the effect of the diode capacitance on the performance of the mixer it is necessary to reduce the internal resistance of the local oscillator. There is therefore a compromise between the need of a low noise figure, and hence a requirement of having a large internal resistance of the local oscillator, and the undesired effect on the diode current, and hence on the performance of the mixer.