Patent ID: 6738435
Filing Date: 2004-05-18
Classification: H03D

Abstract:
A method for demodulating frequency shift keyed (FSK) signals comprising pulses of different frequencies each. spaced by a deviation from a center frequency in an input waveform, comprising the steps of:providing a local oscillator signal; mixing the input waveform with said local oscillator signal, resulting in an intermediate-frequency (IF) signal at a frequency FIF; employing an analog-to-digital converter to provide digital samples of said IF signal at a sampling rate Fs, wherein FIF=(N+0.25) Fs, where N is an integer, such that the samples of the FSK signal are centered at Fs/4; providing a second local oscillator signal having a frequency of Fs/4; mixing the second local oscillator signal with the sampled signal, thus heterodyning the sampled signal to dc, and providing complex in-phase and quadrature phase signals I and Q corresponding thereto; wherein said mixing step is performed by evaluating said second local oscillator signal at sample intervals S0, S1, S2, S3, S0, . . . , such that the value of said second local oscillator signal is either âˆ’1, 0, or 1, and two's complementing, setting to zero, or passing unaltered the corresponding sample, respectively; low-pass filtering and decimating said I and Q signals, such that a second set of samples is produced at a second sampling rate Fsâ€², wherein Fsâ€²=Fs/M, and wherein Fsâ€²/4 is approximately equal to the deviation of said FSK pulses; wherein said steps of low-pass filtering and decimating said I and Q signals are performed by evaluating said second set of samples at sample intervals S0, S1, S2, S3, S0, . . . , such that the values of the filter and decimating coefficients are either âˆ’1, 0, or 1, and two's complementing, setting to zero, or passing unaltered the corresponding sample, respectively; processing the decimated signals in two parallel channels, each deriving values for the power of one of the high or low FSK tones; wherein the steps performed in said parallel channels include: performing complex multiplication of the I and Q signals for each channel by cos(Fsâ€²/4) and sin(Fsâ€²/4) signals, wherein said step of performing complex multiplication are performed by evaluating said cos(Fsâ€²/4) and sin(Fsâ€²/4) signals at sample intervals S0â€², S1â€², S2â€², S3â€², S0â€², . . . , such that the values thereof are either âˆ’1, 0, or 1, and two's complementing, setting to zero, or passing unaltered the corresponding sample, respectively; low-pass filtering the signals resulting from said complex multiplication, by evaluating said signals resulting from said complex multiplication at sample intervals S0â€², S1â€², S2â€², S3â€², S0â€², . . . , such that the coefficients of the filter are either âˆ’1, 0, or 1, and two's complementing, setting to zero, or passing unaltered the corresponding sample, respectively; and evaluating the power of the signals in each of said channels; comparing the values of the power of the signals in each of said channels; and providing an output series of bits responsive to the result of said comparison step.