1. Field of the Invention
The field of the invention is that of transmitters of signals with 2.sup.n phase states obtained by phase shift keying with M states or by amplitude and phase modulation with M states.
2. Description of the Prior Art
The invention is more precisely concerned with a malfunction estimator for a quadrature modulator including two mixers to which two carrier signals are applied and to each of which a modulating signal is applied, the output signals of the mixers being applied to a combiner to constitute a modulated signal. The carriers signals are in principle in phase quadrature and are either intermediate frequency signals or carrier frequency signals.
The invention concerns an estimator that can be used in a control loop to modify the phase difference between the carrier signals (quadrature estimator) and/or to eliminate residual unmodulated carrier signals in the modulated signal (residual carrier estimator) and/or to modify the amplitude of the output signals of the mixers (balance estimator for estimating the relative amplitude of the two channels). The invention is of particular benefit in monitoring a modulator stage operating in a wide band of frequencies and for high order modulation (high values of n).
In the case of modulation with 2.sup.n phase states, if a Cartesian coordinate type representation in a signal space is required, the phase states can be uniformly distributed relative to axes corresponding to orthogonal carrier signals X and Y. There is obtained in this way a constellation that corresponds to the vector representation of the carrier signal states whose projections on the X and Y axes give the coordinates of the end of the vector represented.
However, a constellation can be off-center and/or phase-shifted relative to these axes, which degrades transmitter performance.
FIG. 1 shows a modulator stage of a prior art transmitter of signals with 2.sup.n phase states. A stage of this type is described in French patent No. 2 641 923 (U.S. Pat. No. 5,054,037).
Two modulating signals X and Y to be transmitted to a receiver are applied to digital/analog converters 10 and 11 the outputs of which are respectively connected to inputs of variable gain amplifiers 12 and 13 that receive on another input two control voltages V1 and V2. The output signals X, Y of the amplifiers 12 and 13 constitute signals that are applied to two mixers 14 and 15 each receiving on their other port a carrier signal P1, P2, these carrier signals in principle having a relative phase of 90.degree.. This relative phase angle can be adjusted by means of a voltage V3 applied to a variable 90.degree. phase-shifter 16 receiving a carrier signal P from a local oscillator, like the mixer 15. The output signals of the mixers 14 and 15 are applied to a combiner 17 supplying a modulated signal SM to be transmitted with 2.sup.n phase states. The signal SM can be an intermediate frequency signal or a microwave frequency signal.
FIG. 2 shows the constellation of the signal SM. This signal corresponds to that of an ideal QPSK (Quaternary Phase Shift Keying) signal.
The crosses, corresponding to transmitted symbols, represent the various phase states of the signal SM. The constellation is centered, i.e. the projections of the vectors corresponding to the various states onto the X and Y axes are in pairs with the same amplitude and opposite phase.
However, the phase quadrature relationship of the carrier signals applied to the FIG. 1 mixers may not be strictly complied with, these carrier signals then having a relative phase of 90.degree..+-..alpha., where .alpha. corresponds to an unwanted angular offset. If any such angular offset exists, the constellation of the signal SM is as shown in FIG. 3, for example.
A quadrature error is due to a time-delay either between the carrier signal access paths or between the two modulated carrier signals. The quadrature error depends on the frequency of the carrier signal. The quadrature error renders the constellation asymmetrical which degrades the qualities of the transmission link between the transmitter equipped with the modulator stage and a receiver receiving the signal SM. The voltage V3 is used to compensate the quadrature error.
FIG. 4 shows a QPSK constellation of a signal SM from a modulator stage suffering from asymmetry of its mixers. Asymmetry of the mixers causes residual unmodulated carrier signals in the modulated signal and the energy is unequally distributed. The voltages V1 and V2 are modified to remedy this.
The function of a residual carrier signal estimator is to provide a signal for eliminating these carrier signals in the modulated signal. Conventionally, if the carrier signals are microwave (direct modulation) signals, the cooperation of the estimator and of the modulator eliminates the carrier in the transmitted signal. The device is then a carrier elimination device.
Generally speaking, residual carrier signals in the modulated signal gives rise to a line at the frequency of the carrier signals at the center of the modulated spectrum. This can be caused by:
an imbalance of the mixers, allowing a fraction of the carrier signals to pass for a modulating signal of 0 volt; PA1 defective isolation of the mixer outputs from the local oscillator. PA1 as the processing is digital, a limitation is imposed by the maximal operating frequency of the microprocessor; correction of the constellation therefore takes some time; PA1 the quantizing of the modulated signal SM by the analog-digital converter leads to a lack of precision and it is not possible to effect a very fine adjustment of the quadrature, for example; PA1 the cost of analog-digital and digital-analog converters increases with the digital bit rate; PA1 its implementation is complex; PA1 it is designed to operate at a given digital bit rate, at a given carrier frequency and for a given type of generator. PA1 means for detecting the instantaneous power of the modulated signal; PA1 means for multiplying the detected instantaneous power by at least one of the modulating signals; PA1 means for integrating the result of this multiplication, the integrator means supplying a signal indicative of the malfunction of the modulator. PA1 means for detecting a first signal representative of the instantaneous power of said modulated signal; PA1 means for generating a second signal representative of a reference envelope obtained from said modulating signals; PA1 means for correcting the amplitude of one of said first and second signals in accordance with the mean amplitude difference observed between this amplitude corrected signal and the other signal; and PA1 processing means supplying, from the observed amplitude difference between said amplitude corrected signal and said other signal, at least one signal relating to said defect.
The patent mentioned above describes an estimator for preserving a constellation of the SM signal like that shown in FIG. 2.
The estimator described in the above patent application is of the digital type and takes account of the modulating signals X and Y and the modulated signal SM. The modulated signal SM is subjected to amplitude detection and is digitized for comparison with the signals X and Y. A microprocessor controls counters-downcounters followed by digital-analog converters the outputs of which constitute the control voltages V1 through V3.
However, the above estimator has a number of drawbacks:
French patent No. 2 729 261 (U.S. Pat. No. 5,663,691) describes a quadrature modulator malfunction estimator including two mixers to which two carrier signals are applied and to each of which a modulating signal is applied, the output signals of the mixers being applied to a combiner to constitute a modulated signal. This estimator includes:
If the modulator malfunction is residual carrier signals in the modulated signal, the multiplier means multiply the instantaneous power by each of the modulating signals to supply, after integration, two signals representative of the residual carrier signals in the modulated signal.
If the modulator malfunction is a quadrature error between the signals from the mixers, the multiplier means multiply the detected instantaneous power by the modulating signals to supply, after integration, a signal representative of the relative phase of the signals from the mixers.
One of the problems with the estimators described in the above patent application is that there convergence times to obtain an accurate estimate are long. In other words, the integration times are long.
Nor is there any provision for estimating the imbalance of the modulator. Accordingly, a constellation like that shown in FIG. 5, which is a constellation of a QPSK modulated signal from a modulator with imbalance (amplitude difference between the X and Y channels) cannot be corrected.
One object of the present invention is to remedy these drawbacks.
To be more precise, one object of the invention is to provide a fast estimator for estimating a malfunction of a quadrature modulator, the malfunction being residual carrier signals in the modulated signal or a quadrature error between the signals from the mixers or an imbalance. It must be possible to use the estimator in a quadrature modulator operating in a wide band of frequencies.