Theory and performance of high frequency lattice mixers

Abstract

Restricting the local oscillator to a sinusoidal current drive the performance of four types of lattice mixers is examined by deriving closed form equations (as compared with the usual numerical computer-aided approach), the effects of the diode parasitics being included. It is shown that the duality between Z and Y and between G and H mixers is not generally valid (as has been assumed by several workers) except when the diode is regarded as having bi-linear or exponential characteristics and in addition the effect of the diode parasitics is neglected. It is concluded that the H lattice mixer offers the best possibility of producing the lowest conversion loss in practice. The effect of the diode reactive parasitics (diode package and junction capacitances) on the performance of lattice mixers is also examined. In all the known literature, the diode capacitance parasitics are only included in small-signal analysis and their effect on the local oscillator waveform is ignored. It is shown, however, that the main effect of the diode capacitive parasitics is to modify the local oscillator current waveform present at each diode. It is further shown that this effect has a considerable influence on the performance of lattice mixers. Microstrip coupled-lines constitute a fundamental building block for the realization of filters associated with image-rejection mixers. The design information on such lines is normally presented in graphical form and only for particular values of relative permittivities of the substrate. To overcome this problem an analytical solution has been developed which relates the physical dimensions of the lines to the odd and even-mode impedances.