Received signal detector for digital demodulator

• US 5,974,098 A
• Filed: 10/21/1997
• Issued: 10/26/1999
• Est. Priority Date: 09/12/1994
• Status: Expired due to Fees

• Current Assignee: NEC Corporation
• Original Assignee: NEC Corporation
• Inventors: Tsuda, Hiroki
• Primary Examiner(s): Pham, Chi H.
• Assistant Examiner(s): Corrielus, Jean B
• Application Number:
  US08/955,239
• Time in Patent Office: 735 Days
• Field of Search: 375/316, 375/317, 375/324, 375/340, 375/325, 375/326, 375/328, 375/339, 375/344, 329/304, 329/306-308, 455/227, 455/255-260, 455/264-266, 455/316, 455/315
• US Class Current: 375/340
• CPC Class Codes: H03D 3/004 wherein the demodulated signal is used for controlling an oscillator, e.g. the local oscillator
• CPC Class Codes: H03D 3/006 by sampling the oscillations and further processing the samples, e.g. by computing techniques H03D3/007 takes precedence
• CPC Class Codes: H04L 27/22 Demodulator circuits; Receiver circuits

Abstract

The present invention refers to a received signal detector for detecting and demodulating a digital modulated signal wherein a signal detection in a short time can be implemented independently of the signal frame length. The wide-band filter is a filter having a pass frequency band set to the frequency range of received signals to be demodulated. The A/D convertor, first power detector and first averaging circuit measure the average power of a signal having passed through the wide-band filter. The threshold setting circuit sets a threshold from the averaged power. The narrow-band filter filters an output signal of the frequency convertor in a narrower frequency band than the pass frequency band of the wide-band filter. The comparator circuit compares the threshold and the average power of a signal taken out from the second averaging circuit and passing through the narrow-band filter and judges whether the received signal is taken out from the wide-band filter.

Claims

1. 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 having 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;

a carrier frequency estimator for sampling received signals taken out from said wide-band filter and estimating a carrier frequency for said demodulator and said frequency converter when said comparator circuit judges that said received signal is present, said carrier frequency estimator having an output control signal; and

voltage control frequency generation means for generating said frequency conversion signal to variably control said frequency converter in accordance with said output control signal of said carrier frequency estimator, wherein the narrow-filtered output signals are provided to said demodulator in accordance with said output control signal of said carrier frequency estimator.

2. A received signal detector as claimed in claim 1 wherein said received signal is a signal received in a satellite communications.

3. A received signal detector as claimed in claim 1 wherein said wide-band filter, said first average power measurement means, said threshold setting circuit, said frequency converter, said narrow-band filter, said second average power measurement means and said comparator circuit are all operated in digital signal processing.

4. A received signal detector as claimed in claim 1 wherein said wide-band filter and said narrow-band filter are an anti-aliasing filter and a Nyquist filter, respectively.

5. 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.

6. A received signal detector as claimed in claim 5 wherein said received signal is a signal received in a satellite communications.

7. A received signal detector as claimed in claim 5 wherein said wide-band filter, said first average power measurement means, said threshold setting circuit, said frequency converter, said narrow-band filter, said second average power measurement means and said comparator circuit are all operated in digital signal processing.

8. A received signal detector as claimed in claim 5 wherein said wide-band filter and said narrow-band filter are an anti-aliasing filter and a Nyquist filter, respectively.

9. 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.

10. A received signal detector as claimed in claim 9 wherein said received signal is a signal received in a satellite communications.

11. A received signal detector as claimed in claim 9 wherein said wide-band filter, said first average power measurement means, said threshold setting circuit, said frequency converter, said narrow-band filter, said second average power measurement means and said comparator circuit are all operated in digital signal processing.

12. A received signal detector as claimed in claim 9 wherein said wide-band filter and said narrow-band filter are an anti-aliasing filter and a Nyquist filter, respectively.

13. 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.

14. A received signal detector as claimed in claim 13 wherein said received signal is a signal received in a satellite communications.

16. A received signal detector as claimed in claim 13 wherein said wide-band filter and said narrow-band filter are an anti-aliasing filter and a Nyquist filter, respectively.

15. A received signal detector as claimed in clam 13 wherein said wide-band filter, said first average power measurement means, said threshold setting circuit, said frequency converter, said narrow-band filter, said second average power measurement means and said comparator circuit are all operated in digital signal processing.