MSK digital demodulator for burst communications

A demodulator for sampled digital minimum-shaft keyed signals having a nominal carrier frequency, and comprising a plurality of sets of sidebands, said sampled digital minimum-shift keyed signals being arranged in a format including a preamble consisting of a predetermined arrangement of Mark and Space, each said Mark being represented by one of a first frequency and a second frequency, and each said Space being represented by the other of said first frequency and said second frequency, said first frequency being represented by the sum of said carrier frequency and an incremental frequency, and each said second frequency being represented by said carrier frequency minus said incremental frequency, said demodulator comprising:

• a first mixer including a first input terminal and also including a second input terminal coupled to receive said digital minimum-shift keyed signals;
• a second mixer including a first input terminal and also including a second input terminal coupled to receive said digital minimum-shift keyed signals;
• a first controllable source of reference signals representative of a sinusoid having a frequency near said nominal carrier frequency, said first controllable source of reference signals being coupled to said first terminal of said first mixer for generating from said first mixer a first component of a heterodyned complex signal, said first component of said heterodyned complex signal having a second plurality of sets of spectral components corresponding to said plurality of sets of sidebands of said minimum-shift keyed signals, one set of said second plurality of sets of spectral components of said first component of said heterodyned complex signal being a first baseband component of said complex signal;
• a second controllable source of reference signals representative of a sinusoid having a frequency near said nominal carrier frequency, said second controllable source of reference signals being coupled to said first terminal of said second mixer for generating from said second mixer a second component of said heterodyned complex signal, said second component of said heterodyned complex signal having a third plurality of sets of spectral components corresponding to said plurality of sets of sidebands of said minimum-shift keyed signals, one set of said third plurality of sets of spectral components of said second component of said heterodyned complex signal being a second baseband component of said complex signal;
• first filter means coupled to said first and second mixers for passing said first and second baseband components of said complex signal and for removing other sidebands to produce first and second filtered baseband components of said complex signal;
• matched filter means coupled to said first filter means for receiving said first and second filtered baseband components of said complex signal therefrom, for generating time-delayed samples of said first and second filtered baseband components of said complex signal, for weighting said time-delayed samples to produce weighted time-delayed samples, and for adding said weighted time-delayed samples together in such a fashion that first, second, third and fourth impulse signals may be generated at the instant at which a complete preamble is received by said matched filter means, the magnitudes of said first and second impulse signals generated at said instant being inversely related so that said first impulse signal has a maximum magnitude when said second impulse signal has a minimum magnitude, and said second impulse signal has a maximum magnitude when said first impulse signal has a minimum magnitude, the magnitudes of said third and fourth impulses signals generated at said instant being inversely related so that said third impulse signal has a maximum magnitude when said fourth impulse signal has a minimum magnitude, and said fourth impulse signal has a maximum magnitude when said third impulse signal has a minimum magnitude;
• summing means coupled to said matched filter means for receiving said first, second, third and fourth impulse signals for summing together said first and third impulse signal to produce a first summed impulse signal, and for taking the difference between said second and fourth impulse signals to produce a second summed impulse signal, the magnitude of said first summed impulse signal being a maximum and that of said second summed impulse signal being a minimum when said second reference signal is in-phase with said carrier and said first reference signal is in phase-quadrature with said carrier, and the magnitude of said second summed impulse signal being a maximum and that of said first summed impulse signal being a minimum when said second reference signal is in phase-quadrature with said carrier and said first reference signal is in-phase with said carrier;
• first signal processing means coupled to said summing means for squaring each of said first and second summed impulse signals to produce first and second squared impulse signals, and for adding together said squared impulse signals to generate a time-of-arrival signal indicative of said instant at which said preamble is received;
• aquisition-mode arctangent signal generating means coupled to said summing means for generating an arctangent signal representing the angle whose tangent is the ratio of said second summed impulse signal and said first summed impulse signal, and also coupled to said first signal processing means for producing said arctangent signal in response to said time-of-arrival signal, said arctangent signal being an estimate of the phase difference between said carrier and said second reference signal;
• and
• control means coupled to said arctangent signal generator and to said first and second controllable sources of reference signals for changing the phase of said first and second reference signals in response to said arctangent signal in a polarity which reduces the phase difference between said second reference signal and said carrier.

A method for demodulating sampled digital minimum-shift keyed signals having a nominal carrier frequency, and comprising a plurality of sets of sidebands, said sampled digital minimum-shift keyed signals being arranged in a format including a preamble consisting of a predetermined arrangement of Mark and Space, each said Mark being represented by one of a first frequency and a second frequency, and each said Space being represented by the other of said first frequency and said second frequency, said first frequency being represented by the sum of said carrier frequency and an incremental frequency, and each said a second frequency being represented by said carrier frequency minus said incremental frequency, said method comprising the steps of:

• applying said digital minimum-shift keyed signals to a first input terminal of a first mixer and to a first input terminal of a second mixer;
• controllably generating reference signals representative of a sinusoid having a frequency near said nominal carrier frequency and coupling them to a second terminal of said first mixer for generating from said first mixer a first component of a heterodyned complex signal, said first component of said heterodyned complex signal having a second plurality of sets of spectral components corresponding to said plurality of sets of sidebands of said minimum-shift keyed signals, one set of said second plurality of sets of spectral components of said first component of said heterodyned complex signal being a first baseband component of said complex signal;
• controllably generating reference signals representative of a sinusoid having a frequency near said nominal carrier frequency and coupling them to a second terminal of said second mixer for generating from said second mixer a second component of said heterodyned complex signal, said second component of said heterodyned complex signal having a third plurality of sets of spectral components corresponding to said plurality of sets of sidebands of said minimum-shift keyed signals, one set of said third plurality of sets of spectral components of said second component of said heterodyned complex signal being a second baseband component of said complex signal;
• filtering said first and second baseband components of said complex signal for removing other sidebands to produce first and second filtered baseband components of said complex signal;
• matched-filtering said first and second filtered baseband components of said complex signal by generating time-delayed samples of said first and second filtered baseband components of said complex signal, weighting said time-delayed samples to produce weighted time-delayed samples, and adding said weighted time-delayed samples together in such a fashion that first, second, third and fourth impulse signals may be generated at the instant at which a complete preamble is match-filtered, the magnitudes of said first and second impulse signals generated at said instant being inversely related so that said first impulse signal has a maximum magnitude when said second impulse signal has a minimum magnitude, and said second impulse signal has a maximum magnitude when said first impulse signal has a minimum magnitude, the magnitudes of said third and fourth impulse signals generated at said instant being inversely related so that said third impulse signal has a maximum magnitude when said fourth impulse signal has a minimum magnitude, and said fourth impulse signal has a maximum magnitude when said third impulse signal has a minimum magnitude;
• summing together said first and third impulse signal to produce a first summed impulse signal, taking the difference between said second and fourth impulse signals to produce a second summed impulse signal, the magnitude of said first summed impulse signal being a maximum and that of said second summed impulse signal being a minimum when said second reference signal is in-phase with said carrier and said first reference signal is in phase-quadrature with said carrier, and the magnitude of said second summed impulse signal being a maximum and that of said first summed impulse signal being a minimum when said second reference signal is in phase-quadrature with said carrier and said first reference signal is in-phase with said carrier;
• squaring each of said first and second summed impulse signals to produce first and second squared impulse signals, and adding together said squared impulse signals to generate a time-of-arrival signal indicative of said instant at which said preamble is match-filtered;
• generating an arctangent signal representing the angle whose tangent is the ratio of said second summed impulse signal and said first summed impulse signal, and producing said arctangent signal in response to said time-of-arrival signal, said arctangent signal being an estimate of the phase difference between said carrier and said second reference signal;
• and
• changing the phase of said first and second reference signals in response to said arctangent signal in a polarity which reduces the phase difference between said second reference signal and said carrier.