An apparatus and accompanying method for demodulating with baseband Doppler frequency shift compensation. An RF section (20) down-converts received data communication signals (12) to baseband. A/D converters (24, 26) digitize I, Q quadrature baseband signal components. Phase (32) and frequency (50) tracking loops reside on a common digital ASIC substrate (28). A complex multiplier (30) rotates digitized baseband signals by a digitized oscillation signal, producing Doppler shift compensated signals. The phase tracking loop (32) estimates data and generates a pure phase error signal from which data modulation and Doppler shift compensation influences have been removed, which drives a frequency discriminator (52) that identifies either clockwise or counterclockwise phase rotation for each symbol (18). An integrator (54) combines identification results over a burst and a numerically controlled oscillator (56) adjusts the digitized oscillation signal frequency in a constant frequency step. This adjustment takes place once for each burst.

Demodulator with baseband doppler shift compensation and method

The frequency of a voltage controlled crystal oscillator can be pulled in the conventional way by means of a varicap diode forming the load capacitance of the oscillator crystal. In oder to realise a wider range of frequency pulling, the load capacitance 44 is alternately switched on and off in a known manner. According to the invention a band-switch diode 46 can be utilised as a switch element, if the hole-storage time of this diode is as long as or longer than the oscillator cycle. By means of a low-frequency control voltage on the diode it is rendered self-switching within the oscillator cycle so that proportional frequency tuning is possible.

Crystal oscillator with a wide range of frequency pulling

The frequency of a voltage controlled crystal oscillator is conventional pulled by means of a varicap diode forming the load capacitance of the oscillator crystal. In order to realize a wider range of frequency pulling, the load capacitance 44 is alternately switched on and off in a known manner. According to the invention a band-switch diode 46 is utilized as a switch element, the hole-storage time of this diode being as long as or longer than the oscillator cycle. By applying a low-frequency control voltage to the diode it is rendered self-switching within the oscillator cycle, so that proportional frequency tuning is achieved.

Clock-signal regenerator comprising a crystal oscillator incorporated in a phase-locked loop

A phase-locked oscillator has an oscillator unit switchable between two frequencies, the switches being controlled by a switching control signal in the form of pulses at a regular repetition rate, said pulses having a duration relative to the repetition period linearly proportional to the amplitude of an integrated phase error signal. The oscillator unit is thus switched regularly between its two frequencies, the relative duration of the operation on each frequency being controlled whereby the mean oscillator frequency is controlled to hold the required phase lock, a high degree of linearity between controlling voltage and average output frequency.

David Geoffrey Hughes

Papers

Phase locked oscillators