Demodulators – Phase shift keying or quadrature amplitude demodulator
Patent
1997-12-18
1999-09-14
Mis, David
Demodulators
Phase shift keying or quadrature amplitude demodulator
329306, 375324, 375329, H03D 300, H04L 2722
Patent
active
059528796
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
1. Technical Field
The present invention relates to a device for the simultaneous demodulation of a multifrequency signal, particularly for a measurement by eddy currents.
2. Prior Art
The mimic diagram of an eddy current measuring apparatus, illustrated in FIG. 1, comprises a balancing oscillator 10, an injector 11 and a demodulator 12 supplying a digital output SN.
The oscillator has the function of producing a multifrequency signal on the basis of a sum of 1 to N pure sinusoids. The function of the balancer is to reduce the influence of carriers in order to relatively amplify the modulating signal (eddy current signal). The function of the injector 11 is to supply a sensor 13 in which the eddy current modulation takes place. The function of the demodulator 12 is to extract the modulating signal (eddy current modulation) from the carriers used.
In such an apparatus, the technical data are generally as follows: simultaneously; each of the carriers and extracts in cartesian form the signal modulated in the sensor having the following equation: Xcos(wt)+Ysin(wt). The demodulator extracting X and Y.
Existing apparatuses inter alia use two demodulation methods.
Synchronous sinusoidal demodulation
The mimic diagram used is shown in FIG. 2.
The signal coming from the injector 11 is distributed in two identical channels. In one it is multiplied by a pure sinusoid S1 at the frequency of one of the carriers used. In the other it is multiplied by a sinusoidal signal S2 at the same frequency, but in phase quadrature with the first. The result of these multiplications has the effect of transposing the signal of the injector in base band. A low-pass filtering (filters 20, 21) at the maximum frequency of the eddy currents (e.g. fc=500 Hz) makes it possible to extract the pairs X,Y.
This demodulation method makes it necessary to reproduce the diagram of FIG. 2 with the same number of copies as there are simultaneously used frequencies: f1, f2, fn, where signals S1 and S2 have frequency f1, signals S1' and S2' have frequency f2, and so forth, as shown in FIG. 3.
Demodulation by quadrature sampling
This method uses the mimic diagram illustrated in FIG. 4.
The signal coming from the injector 11 is distributed on two identical channels. On one, it is connected to the filter 20 for a brief instant of the period of the sinusoid of the carrier which it is wished to demodulate. For the remainder of the time, the thus sampled voltage is maintained at the input of the filter. On the other channel, sampling takes place at the same frequency, but with a 90.degree. phase shift (in quadrature).
In the same way as hereinbefore, this demodulation method makes it necessary to reproduce the mimic diagram illustrated in FIG. 4 with the same number of copies as there are simultaneously used frequencies. It also fails to efficiently reject any harmonics of the carrier signal.
DESCRIPTION OF THE INVENTION
The invention relates to a device for the simultaneous demodulation of a multifrequency signal, which comprise a processor in which demodulation takes place by calculation. Advantageously, said device comprises an injector, whose output signal is multiplied by a complex signal, which can have the following form: ##EQU1## .epsilon..sub.1 being a small frequency deviation and .alpha..sub.1 a phase, the frequencies of the N sinusoids being the frequencies of the frequency-shifted carriers, so as to transpose the signal of the injection on the intermediate frequencies.
Advantageously, the device comprises an injector followed by a multiplier and a low-pass filter. This filter, having an antialiasing function, is followed by a digital converter.
Advantageously, said device can be used for a measurement by eddy currents.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a prior art, eddy current measuring apparatus.
FIGS. 2 and 3 illustrate prior art, synchronous, sinusoidal demodulation devices.
FIG. 4 illustrates a prior art, quadrature sampling demodulation device.
FIG. 5 shows a demodulation device for illustrating the p
REFERENCES:
patent: 4303885 (1981-12-01), Davis et al.
patent: 4467281 (1984-08-01), Davis et al.
patent: 5805642 (1998-09-01), Wei et al.
Intercontrole
Mis David
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