Selective call receiver with fast bit synchronizer

A radio communications receiver comprising:

• receiving means for receiving and demodulating a radio frequency signal to generate a received signal;
• and
• digital signal absence detecting means coupled to the receiving means for generating an absence signal in response to absence from within the received signal of a signal having a plurality of digital symbols received at a predetermined baud rate, the digital signal absence detecting means comprising:
• data clock means for generating a timing signal;
• phase generating means, responsive to the timing signal, for generating a reference signal having a frequency corresponding to the baud rate, the reference signal comprising a plurality of independent phase signals generated relative to the received signal;
• transition detecting means responsive to the timing signal for detecting transitions which may occur between symbols;
• counting means responsive to said transition detecting means and said phase generating means, and having a plurality of phase accumulators, each responsive to the detected transitions and one of the phase signals which is selectively coupled thereto for accumulating a transition count when the detected transition coincides with the phase signal selectively coupled thereto;
• said counting means further including totalizing means for accumulating a total count of the number of transitions detected within a fixed time interval;
• controlling means for resetting said counting means and for operating said counting means of the fixed time interval and for generating a completion signal at the end of the fixed time interval;
• and
• analyzing means, including memory means for storing a table of predetermined threshold values corresponding to the total transitions detected, said analyzing means being coupled to said counting means and responsive to the completion signal, for selecting from said memory means a threshold value corresponding to the total transitions detected, and for generating the absence signal in response to the threshold value selected not being exceeded by a sum of values within any predetermined number of said phase accumulators which correspond to contiguous phases of the reference signal.

A method in a radio communications receiver of detecting a signal having a plurality of digital symbols received at a predetermined baud rate, the method comprising the steps of:

• receiving a radio frequency signal;
• demodulating the radio frequency signal to derive the signal;
• generating a reference signal having a frequency corresponding to the baud rate, the reference signal comprising a plurality of independent phase signals having substantially unvarying phases generated relative to the received signal;
• detecting transitions which may occur between the digital symbols to derive transition detections;
• accumulating counts of said transition detections in a plurality of phase accumulators, wherein each phase accumulator accumulates a count in response to the transition detection being detected in coincidence with the phase signal corresponding to a particular phase accumulator;
• performing said steps of generating, detecting and accumulating for a fixed time interval;
• accumulating a total number of transition detections occurring during the fixed time interval;
• selecting a threshold value from a table of values stored in a memory in response to the total number of transition detections at the end of the fixed time interval;
• and
• producing a detect signal in response to the threshold value selected being exceeded by a value within a phase accumulator.