Pulse or digital communications – Receivers – Amplitude modulation
Patent
1994-03-29
1998-04-21
Tse, Young T.
Pulse or digital communications
Receivers
Amplitude modulation
3649228, 3649234, 455 46, 329356, 332167, H03D 300, H04L 2714
Patent
active
057426457
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to devices which make it possible to demodulate a carrier signal of composite modulation, with digital processing. It relates more particularly to the signals transmitted by radio from a marine vehicle transporting several measurement signals in the form of a set of modulations suitably multiplexed together.
2. Discussion of the Background
In order to identify various objects situated within the sea, buoys released from a helicopter are currently used, which carry out sonar soundings. The results of these soundings are transmitted by a VHF radio set carried by the buoy to a receiver situated in the releasing helicopter, or possibly in another aircraft or vessel.
The information thus transmitted essentially comprises omnidirectional and directional detection signals and indications on the orientation of the buoy making it possible to give a reference to the directional signals. These signals modulate the HF carrier of the transmission radio set according to a defined and known modulation plan. On reception, the demodulation of the radio signal restores the measurement signals, which make it possible to know the location of the objects to be identified.
The technique universally used up to the present is to demodulate this signal by conventional analogue means, including for example filters, multipliers, etc.
A method was proposed in the Patent U.S. Pat. No. 4,356,562 for digital demodulation of these signals. However, the system proposed uses a variable sampling frequency slaved to the fluctuation of the carrier frequency, which very greatly complicates the digital processing of the signals thus sampled.
SUMMARY OF THE INVENTION
In order to alleviate these drawbacks, the invention proposes a method of digital demodulation of a composite signal comprising a first omnidirectional baseband signal, a first subcarrier at a first frequency Fp, a second subcarrier at a frequency 2Fp exactly equal to double the frequency Fp, a first dipolar signal modulating the second subcarrier in double sideband mode with carrier suppression, and a second dipolar signal modulating the second subcarrier in double sideband mode with carrier suppression in quadrature with the first signal, mainly characterized in that the following steps are performed: in order to obtain a first signal of the frequency difference between F0 and Fp; error signal in order to obtain a first frequency-correction signal; equal to twice the frequency F0 in order to obtain a first approximation to the two dipolar signals; the first frequency-correction signal in order to obtain frequency-corrected dipolar signals; signals in order to isolate the DC component corresponding to the second subcarrier brought back to the frequency 0; frequency 0; phase-error signal in order to obtain the frequency- and phase-corrected dipolar signals.
BRIEF DESCRIPTION OF THE DRAWINGS
Other features and advantages of the invention will appear clearly in the following description, given by way of a non-limiting example, with regard to the attached figures which represent:
FIG. 1, the frequency plan of the signal to be demodulated; and
FIG. 2, the diagram of a demodulation device according to the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The composite modulation signal of the HF carrier represented in FIG. 1 corresponds to a set of signals and of modulations generally used in sonar buoys. It comprises: omnidirectional reception signals of the sonar. These signals are known as "omni signal". The bandwidth of this signal is 2400 Hz, for example. serves as reference. Having regard to the rudimentary means available in the buoy allowing this frequency to be generated, the latter varies randomly, for example by .+-.25 Hz. It is this variation which poses the main problems for the subsequent processing. that of the first subcarrier. For that purpose, the second subcarrier is generated simply by frequency doubling of the first, which allows a perfect correspondence between the frequen
REFERENCES:
patent: 4316058 (1982-02-01), Christensen
patent: 4799212 (1989-01-01), Mehrgardt
patent: 4868792 (1989-09-01), Grado
patent: 4962488 (1990-10-01), Dell-Imagine
Calvano Philippe
Nicolas Jean-Luc
Riccardi Guy
Thomson - CSF
Tse Young T.
Webster Bryan
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