Appendix D - Quadrature modulation and demodulation principles

Summary

This appendix focuses directly on the quadrature modulator (QM) and quadrature demodulator (QD) system blocks. In any real system there are errors in implementing circuitry, and modulators are no exception. Applying a linear, first-order error model to the ideal quadrature modulator equation, it is shown that there are nine error terms that the design engineer must pay attention to. Each of these error terms is examined for its effect on the final output signal. It is shown that, in general, offset errors produce signal leakages, and gain errors produce modulation distortions. Error signals that can be filtered out are identified. Non-filterable leakage and distortion terms that require nulling adjustments are also identified. A similar analysis of the quadrature demodulator shows an additional six first-order error terms. After identifying the ideal values for all fifteen error terms, it is shown that the derived equations all reduce to ideal transmission.

Ideal quadrature modulation and demodulation

Quadrature modulation is based upon the unique translation between polar and Cartesian (rectangular) coordinates. Any signal which is described by a polar magnitude (A) and phase angle (φ) can equivalently be described with rectangular (x and y) coordinates.