Digital demodulator for frequency-division-multiplexed signals

Abstract

An improved digital demodulator capable of demodulating a frequency multiplexed input signal is disclosed. The frequency division multiplexed input signal is sampled in an analog-to-digital converter. The samples are translated by mixing with base band frequency signals to yield real and imaginary values corresponding to phase information in the original modulation signals. After translation, the translated samples are filtered in real and imaginary digital filters. After filtering, the original modulation information is recovered by analysis of the position of vectors in the complex plane represented by the real and imaginary values. In the preferred embodiment, a successive approximation technique is used to determine the angle of the vectors to the real axis. Low pass output filtering may be employed after recovery of the modulation signal for improvements in signal to noise ratio. In the preferred embodiment, the translation is performed by multiplying the input samples by digital values corresponding to sine and cosine values of local oscillator signals at base band frequencies. Preselect filtering may be employed prior to translation to decimate the input samples where possible to reduce subsequent processing requirements. In the preferred embodiment, the unit is operated in a first initialization mode in which a microprocessor supplies appropriate filter coefficients corresponding to the frequencies of the channels to be demodulated. Thereafter, the microprocessor does not directly control supply of samples or coefficients to the filters for evaluation. Very high data rates are thus made possible.