Sensitivity considerations in cascaded lattice-type quadrature-modulation networks

Generalised quadrature-modulation (QM) circuits, introduced by Saraga, can be used to process independently the two sidebands of a double-sideband signal. Such circuits can be realised as a cascade of `lattice-type¿ sections in which the arms of the lattice are linear, time-invariant, 2-port networks. This paper investigates the sensitivity of the transmission zeros (which exist at the frequencies of perfect quadrature) of the overall QM circuit to variations in the transfer functions of the lattice-arm networks. It is shown that variations occurring in the final section of a cascaded QM circuit can destroy the perfect quadrature, and hence the `potential¿ transmission zeros, produced by earlier perfect sections. An example is given to illustrate the effect.