Frequency shift keying signal demodulator method and apparatus
View Patent ↗A method and apparatus for a frequency shift keying (FSK) demodulator use a configuration to improve the autocorrelation for better receiver performance. The demodulator uses parallel first and second lines connected to the same input signal, the first line having a delay element to provide an integer-delay of M, the second parallel line having a filter for causing a group delay of τ+M where τ is fractional, and a multiplier for receiving the signals from said first and second lines and generating a resultant signal from which a base band signal can be recovered. The resultant signal is passed through a low pass base band filter to recover the base band signal. τ may have a value of 3.25 and M may be 6. The demodulator may selectively be implemented in caller ID service and in low end modems chosen from a group comprising V.21, Bell 103, V.23 and Bell 202A modems.
1. A frequency shift keying (FSK) demodulation method using improved correlation, comprising:
using a demodulator wherein autocorrelation between a signal and a time-shifted version of the signal is used, said modulator having a base band signal as an output;
configuring the demodulator to include first and second parallel connected lines, said first line having a first signal with a group delay M for a signal coming into the demodulator, said second line having a second signal passing through a delay filter with a group delay of τ+M where τ is fractional,
the method including the step of multiplying said first and second signals to obtain a resultant signal from which said base band signal may be obtained.
2. The method as in claim 1 , including the step of passing said resultant signal through a low pass base band filter to obtain said base band signal.
3. The method as in claim 2 , wherein said delay filter is derived from one polyphase component of a SINC interpolating filter giving a fractional delay close to a fractional delay τ+M.
4. The method as in claim 3 , where M depends on a length of the SINC interpolating filter.
5. The method as in claim 2 , where said delay filter is configured using a filter design tool that can design filters with an arbitrary amplitude and group delay response.
6. The method as in claim 1 where τ is approximately 3.
7. The method as in claim 1 where τ is 3.25 and M is approximately 6.
8. The method as in claim 1 , implemented in a caller ID service unit.
9. The method as in claim 1 implemented in low end modems.
10. The method as in claim 9 , wherein the modems are chosen from a group comprising V.21, Bell 103, V.23 and Bell 202A modems.
11. A frequency shift keying demodulation apparatus, comprising:
an automatic gain control (AGC) block connected to receive an analog front end signal;
a demodulator connected to a signal from said AGC block and configured to provide an output into a slicer which provides a digital symbol after comparing said output against a threshold;
a baud recovery clock which is connected to recover a corrected sample sliced from said demodulator output;
said demodulator including first and second parallel connected lines, said first parallel connected line having a first signal with a group delay M for said signal from the AGC block, said second parallel connected line having a delay filter with a second signal having a delay of τ+M where τ is a fraction.
12. The apparatus as in claim 11 , including a band pass filter interposed between said AGC block and said analog front end signal.
13. The apparatus as in claim 11 , wherein said demodulator includes a multiplier for multiplying signals in said first and second parallel connected lines.
14. The apparatus as in claim 13 , wherein said demodulator includes a low pass base band filter through which signals in said first and second parallel connected lines after multiplication are passed.
15. The apparatus as in claim 11 , wherein said delay filter in said second parallel connected line has a unity gain.
16. The apparatus as in claim 12 , wherein said delay filter is configured using one polyphase component of a SINC interpolating filter giving a fractional delay close to a fractional delay τ+M.
17. The apparatus as in claim 16 , where M depends on a length of the SINC interpolating filter.
18. The apparatus as in claim 12 , where said delay filter is configured using a filter design tool that can design filters with an arbitrary amplitude and group delay response.
19. The apparatus as in claim 11 , where τ is approximately 3.
20. The apparatus as in claim 19 , where τ is 3.25 and M is approximately 6.
21. The apparatus as in claim 11 , implemented in a caller ID service unit.
22. The apparatus as in claim 11 implemented in low end modems.
23. The apparatus as in claim 22 , wherein the modems are chosen from a group comprising V.21, Bell 103, V.23 and Bell 202A modems.
24. An article comprising a storage medium having instructions thereon which when executed by a computing platform will result in execution of a frequency shift keying (FSK) demodulation method using improved correlation, comprising:
using a demodulator wherein autocorrelation between a signal and a time-shifted version of the signal is used, said modulator having a base band signal as an output;
configuring the demodulator to include first and second parallel connected lines, said first line having a first signal with a group delay m for a signal coming into the demodulator, said second line having a second signal passing through a delay filter with a delay of τ+M where τ is fractional,
the method including the step of multiplying said first and second signals to obtain a resultant signal from which said base band signal may be obtained.
25. The article as in claim 24 , the method including the step of passing said resultant signal through a low pass base band filter to obtain said base band signal.
26. A frequency shift keying (FSK) demodulation apparatus using autocorrelation and having an input end for receiving an analog input signal and an output end for delivering a demodulated signal, comprising first and second lines connected in parallel, said first line having a delay element to provide an integer-delay of M, said second parallel line having a delay filter for causing a delay of τ+M where τ is fractional, and an integrator for receiving the signals from said first and second lines and generating a resultant signal from which a base band signal can be recovered.
27. The FSK apparatus as in claim 26 , wherein said integrator comprises a multiplier.
28. The FSK apparatus as in claim 27 , including a slicer connected to said output end which with assistance from a baud clock recovery, produces a digital symbol after comparing said base band signal with a fixed threshold.
29. The FSK apparatus as in claim 27 , selectively implemented caller ID service and in low end modems chosen from a group comprising V.21, Bell 103, V.23 and Bell 202A modems.
30. The FSK apparatus as in claim 26 , wherein τ is 3.25 and M is 6.