Linear time-varying analysis for single-balanced diode mixers and modulators is developed to incorporate a non-linear conductance model of the non-linear elements employing the exponential i/v characteristic and series resistance of a typical Schottky-barrier diode. It is shown that over a significant range of local-oscillator drive conditions calculations based on the much simpler bilinear diode model can be expected to yield predictions of loss and matching conditions which closely approach those obtained from the more sophisticated calculations. However, in operating conditions involving the use of d.c. bias voltages approaching the peak local-oscillator voltage, as is necessary for the achievement of low loss in certain double-tuned circuits, significant departures occur resulting in failure of the ‘duality’ relationships existing between series and shunt configurations. The series circuit is shown to be theoretically capable of superior loss performance to the shunt, but under terminating conditions which will be difficult to realize in practice.