Received signal detector for digital demodulator

A received signal detector, comprising:

• a wide-band filter for suppressing signal components in a frequency band higher than allowed in a sampling of a received signal modulated by digital modulation, said wide-band filter having output signals;
• an A/D converter for sampling the output signals of said wide-band filter and converting said sampled output signals into digital signals;
• first averaged power measuring means for measuring an averaged power of said digital signals having passed through said A/D converter, said first averaged power measuring means having a first output signal;
• a threshold setting circuit for setting a threshold in accordance with the first output signal measured by said first averaged power measuring means;
• a frequency converter for converting a first frequency of said digital signals having passed through said A/D converter to a second frequency and for adjusting said second frequency into a frequency range of a narrow-band filter, said frequency converter inputting a frequency conversion signal and outputting converted output signals;
• said narrow-bend filter filtering the converted output signals of said frequency converted, said narrow-bend filter having narrow-filtered output signals;
• second average power measuring means for measuring the average power of the narrow-filtered output signals, said second average power measuring means having a second output signal;
• a comparator circuit for comparing said threshold and said second output signal and judging whether said received signal is present or absent, said comparator circuit having a comparator output signal;
• a voltage-controlled oscillator providing as an output said frequency conversion signal;
• and
• a frequency sweep control circuit for controlling said voltage control oscillator in accordance with said comparator output signal, such the said frequency conversion signal has a periodically changing oscillation frequency.

A received signal detector, comprising:

• a wide-band filter for suppressing signal components in a frequency band higher than allowed in a sampling of a received signal modulated by digital modulation, said wide-band filter having output signals;
• an A/D converter for sampling the output signals of said wide-band filter and converting said sampled output signals into digital signals;
• first averaged power measuring means for measuring an averaged power of said digital signals having passed through said A/D converter, said first averaged power measuring mean having a first output signal;
• a threshold setting circuit for setting a threshold in accordance with the first output signal measured by said first averaged power measuring means;
• a frequency converter for converting a first frequency of said digital signals having passed through said A/D converter to a second frequency and for adjusting said second frequency into a frequency range of a narrow-band filter, said frequency converter outputting converted output signals;
• said narrow-band filter filtering the converted output signals of said frequency converter, said narrow-band filter having narrow-filtered output signals;
• a demodulator that receives and demodulates the narrow-filtered output signals;
• second averaged power measuring means for measuring the averaged power of the narrow-filtered output signals, said second averaged power measuring means having a second output signal;
• a comparator circuit for comparing said threshold and said second output signal and judging whether said received signal is present or absent, said comparator circuit having a comparator output signal;
• bit synchronization means for controlling a clock phase control operation from a signal demodulated by said demodulator, and for demodulating said narrow-filtered output signals according to said comparator output signal;
• loop-band switching means for switching a loop band of said demodulator between a narrow band and a wide band;
• and
• automatic frequency control (AFC) means for controlling an AFC operation when said comparator circuit detects a presence said received signal and for holding said AFC control operation when said comparator circuit detects an absence of said received signal.

A received signal detector, comprising:

• a wide-band filter for suppressing signal components in a frequency band higher than allowed in a sampling of a received signal modulated by digital modulation, said wide-band filter having output signals;
• an A/D converter for sampling the output signals of said wide-band filter and converting said sampled output signals into digital signals;
• first averaged power measuring means for measuring an averaged power of said digital signals having passed through said A/D converter, said first averaged power measuring means having a first output signal;
• a threshold setting circuit for setting a threshold in accordance with the first output signal measured by said first averaged power measuring means;
• a frequency converter for converting a first frequency of said digital signals having passed through said A/D converter to a second frequency and for adjusting said second frequency into a frequency range of a narrow-band filter, said frequency converter outputting converted output signals;
• said narrow-band filter filtering the converted output signals of said frequency converter, said narrow-band filter having narrow-filtered output signals;
• a demodulator that receives and demodulates the narrow-filtered output signals;
• second averaged power measuring means for measuring the averaged power of the narrow-filtered output signals, said second averaged power measuring means having a second output signal;
• a comparator circuit for comparing said threshold and said second output signal and judging whether said received signal is present or absent;
• wherein said threshold is set so that a voltage divided from said first output signal is a voltage between said second output signal at the time of a presence of said received signal and said second output signal at a time of an absence of said received signal.