| 摘要 |
In a communications system using a synchronous digital data stream employing multiple-amplitude-level symbols, preamble acquisition and d.c. offset correction is achieved simultaneously by receiver averaging of sampled values in amplitude and in time to determine the average d.c. offset and the nominal transition boundaries between symbols. After symbol timing acquisition and d.c. offset acquisition, other circuitry is enabled for symbol tracking, and additional circuitry uses the acquired d.c. offset and symbol timing information to extract the data from the symbol stream. D.c. offset tracking involves subtracting the d.c. offset value from the output of an ADC representing the digitized baseband signal and then adjusting the d.c. offset value by a fraction of the error between the actual value and the expected value relative to the offset corrected signal. Symbol timing tracking is accomplished through examination of the d.c. offset-corrected digitized baseband signal during a symbol timing window centered on the symbol clock transition for signal amplitude near the nominal center of amplitude ranges expected for any one of the allowable symbols. Timing is corrected by adding or subtracting counts from a binary counter whose phase was previously preset during acquisition mode. The subtracting and adding may be done upon overflow or underflow of an up/down counter which tracks the coincidence between the amplitude window and the phase window, incrementing and decrementing symmetrically whenever window coincidence is exact; otherwise overflow or underflow results which causes isolated correction in the count of the binary counter.
|
