Demodulators – Amplitude modulation demodulator – Having specific distortion – noise or other interference...
Patent
1985-09-16
1987-07-21
LaRoche, Eugene R.
Demodulators
Amplitude modulation demodulator
Having specific distortion, noise or other interference...
331 17, 455265, H03D 300
Patent
active
046821180
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a phase-shift-keying (P.S.K.) and phase modulation transmission system. In particular the invention concerns such a system which employs a P.S.K. modulation and a P.S.K. demodulation, the latter of which incorporates a novel phase-locked loop (P.L.L.).
BACKGROUND OF THE INVENTION
Phase-shift-keying for the transmission of binary or other signals has been shown to have advantages over other modulation techniques including a lower error rate in the presence of Gaussian noise. Its use in practice has been limited owing to the problems associated with the regeneration of the carrier at the point of reception and problems with phase-locked loop based demodulators. The latter problem has been resolved somewhat using a tan-locked loop. The former problem is related to the use of a 180 degree phase-shift for modulation to ensure waveform continuity in transmission.
Digitally controlled phase shifters usually rely on a P.L.L. technique in which various harmonics are generated. This allows phase-shifts to have values of m360
degrees to be implemented (m is an integer and n is the harmonic number). While such techniques are adequate for P.S.K. generation, the frequency range of operation is limited by the lock-in range of the P.L.L. and the phase steps are large and discrete. It is more difficult to obtain values for n greater than 8 which results in a comparatively large modulation bandwidth.
A prime requirement of a P.S.K. demodulator is the regeneration of the carrier so that phase changes can be recorded without loss or ambiguity. This is particularly important when the P.S.K. transmission involves phase-shifts of 180 degrees. For any other phase-shift, one can make a decision about whether the shifted phase leads or lags the carrier from the magnitude of the phase-shift.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a transmission system including a modulator and demodulator which at least minimize the disadvantages of prior P.S.K. transmission systems including such effects as lock, ambiguity, false lock and relatively large transmission bandwidth.
The invention will be described in relation to its application for the transmission of binary digital data. It should be appreciated that this is by way of example only and that the invention may also be employed for the transmission of analog data and in combinations of digital modulation methods (e.g. multilevel phase-shift-keying). Also it should be appreciated that the invention may be employed both for the transmission of low and high frequency data, voice or other information.
For example, the invention may be employed for the transmission of data over at least a band of frequency extending from the audible end of the frequency spectrum to the radio frequency or microwave frequency end of the spectrum. The invention will be described by way of example with reference to the low frequency end of the spectrum.
The technique of the invention avoids potential ambiguity and preferably employs a modulation phase-shift of less that 180 degrees. The phase-shifted wave form may be generated digitally so that conversion to a sinusoidal form may incur a transition time. This transition time will be reduced by decreasing the value of the phase-shift. The demodulation technique of the invention functions well with existing P.S.K. generators.
The invention provides a phase demodulator including:
receiver means for receiving a modulated signal transmitted by a transmitter; and
a modified phase locked loop adapted to receive the modulated signal from the receiver means, said phase locked loop including a phase detector for receiving the modulated signal, a low pass filter receiving an output from the phase detector, a voltage controlled oscillator providing a controlled input to the phase detector, a sample and hold circuit for receiving the output from the filter and providing a control signal for the voltage controlled oscillator and lock-in control means deriving a lock control signal from either the modulated
REFERENCES:
patent: 3454904 (1969-07-01), Clites et al.
patent: 3761820 (1973-09-01), Nieson et al.
patent: 3946337 (1976-03-01), Philips et al.
patent: 4099125 (1978-07-01), Bennett, Jr. et al.
patent: 4345219 (1982-08-01), Jackson
Griffith University
LaRoche Eugene R.
Pascal Robert J.
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