Radio communication receiving device detecting a frequency modulation preamble signal

DETAILED DESCRIPTION The object of the present invention is to provide a radio communication receiving device that has a preamble detection function that is simple and highly reliable, at the time of preamble detection is able to activate only the circuits required for that preamble detection, and provides a high level of performance.
To attain the above object, the radio communication receiving device of this invention handles digital signals but is based on a concept whereby the parts that are needed are not involved with digital processing (the usual concept, as illustrated by the prior art example in FIG.
5, has been to use digital signal processing throughout, starting with receiver detector output) and, in view of the special nature possessed by a preamble defined by a series of alternating reversals of logical 1s and 0s, is comprised as follows.
Connected to the output of a receiver that detects frequency-modulated signals and outputs analog base band signals is a low-pass filter having a numerical cutoff frequency value that does not exceed half the bit rate of the preamble to be detected.
The output of the low-pass filter is connected to a comparison circuit provided for comparing the output voltage of the low-pass filter with a prescribed threshold value.
A determining circuit is provided that determines a preamble has been detected when the comparison by the comparison circuit shows that at absolute value the low-pass filter output voltage has not exceeded the prescribed threshold value for at least a prescribed time period.
The above pager can be provided with an internal microcomputer that forms the main control circuit and can also be used for the determining circuit.
However, for reasons relating to the battery conservation described below it is more advantageous to have the low-pass filter and comparison circuit provided separately from the microcomputer as a preamble detection circuit module.
As one embodiment of this invention that satisfies the basic configuration described above, there has been proposed a radio paging receiver having a pair of threshold values that, when the analog base band signal that is the receiver detection output oscillates equally positively and negatively relative to a reference level, at their absolute value the prescribed threshold values are mutually equal relative to the reference potential, and in the positive and negative polarity signs are reversed