Demodulators – Phase shift keying or quadrature amplitude demodulator – Input signal combined with local oscillator or carrier...
Reexamination Certificate
1998-11-19
2001-01-16
Lee, Benny (Department: 2817)
Demodulators
Phase shift keying or quadrature amplitude demodulator
Input signal combined with local oscillator or carrier...
C329S346000, C331S012000
Reexamination Certificate
active
06175269
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a demodulation unit for demodulating a quadrature input signal having at least two components, comprising a controlled oscillator for supplying a quadrature oscillator signal in dependence upon a control signal, and a mixer for mixing the quadrature input signal with the quadrature oscillator signal from the controlled oscillator and for supplying at least two mixed components, the mixer and the oscillator being incorporated in a PLL.
The invention also relates to a divider and a mixer for use in such a demodulation unit and to a receiver provided with such a demodulation unit.
The invention further relates to a method of demodulating a quadrature input signal having at least two components, wherein a quadrature oscillator signal is supplied in dependence upon a control signal and the quadrature input signal is mixed with the quadrature oscillator signal, and wherein at least two mixed components are supplied.
Such a demodulation unit and method may be used for, for example demodulating both AM and FM-modulated signals for radio, television, and communication receivers, etc., and for, for example “zero-IF” generation for QAM/QPSK signals and VSB signals, in which the PLL then comprises, for example, a COSTAS loop.
In addition to a synchronous demodulation function, such a demodulation unit also has a frequency-shift function.
In the present invention, a quadrature signal is also understood to be a real signal, which is a simplification of the signal to one of the two axes (X,Y) of the quadrature signal, in which both components are either equal or one of the components is zero.
A demodulation unit of this type is known from European patent application EP-A 0 579 100. This application describes a demodulation unit which comprises a PLL with a quadrature mixer and a controlled oscillator. This demodulation unit is intended to regain the phase and amplitude information of the carrier from the incoming signal.
A drawback of such a demodulation unit is that it is not very well possible to demodulate both FM and AM signals without taking extra measures as regards filtering, etc. Such a demodulation unit may also have acquisition problems, inter alia, due to amplitude variations.
To eliminate amplitude variations of the input signal, an extra bandpass filter and a limiter will have to be used for generating the carrier signal by means of the PLL. When the amplitude of the incoming signal varies, the lock-in range of the PLL will vary. Moreover, an amplitude variation of the control signal results in phase modulation of the carrier signal, which will distort the demodulated output signal.
If the PLL is used as an FM demodulator (for example, for TV sound), the amplitude variation of the input signal will result in a distortion of the demodulated output signal.
In a known digital or analog PLL, the limiter and filtering function must be incorporated in the carrier-generating circuit for demodulating AM signals. When demodulating FM signals, the limiter precedes the PLL so as to eliminate unwanted amplitude variations.
SUMMARY OF THE INVENTION
It is an object of the invention to obviate the above-mentioned drawbacks. To this end, a demodulation unit according to the invention is characterized in that the PLL also comprises a divider for dividing the at least two mixed components by each other and for supplying the output signal as a control signal to the oscillator, and a filtering unit for filtering one or more components and/or signals.
By making use of a divider in the PLL for dividing the two components by each other, amplitude variations of the input signal will be prevented from causing demodulation distortion or acquisition problems. By eliminating the amplitude variation, the loop gain and, hence, the lock-in range remain constant. Moreover, by performing the division in the PLL, it is no longer necessary to use a limiter and/or extra filtering function for demodulating AM or FM signals.
The invention may be used in both digital and analog complex demodulation units.
In one embodiment, the divider may be implemented as a linear CORDIC preventing a division by zero. In one embodiment, the mixer may be implemented as a circular CORDIC. A CORDIC (CO-ordinate Rotation Digital Computer) is known per se from, for example U.S. Pat. No. 5,230,011 (PHN 13.500) and U.S. patent application Ser. No. 08/704200 (PHN 15.428). In these implementations, a very efficient digital embodiment of a demodulation unit is obtained.
A embodiment in which the filtering unit has first and second low pass filters between the mixer and the divider.
The advantage that each mixed component is separately filtered for filtering the sum frequency(ies) which have been added by mixing for the case where the input signals are real signals. If the input signals are complex signals, but not ideal signals, the components should also be filtered before the division takes place. Furthermore, the filters have the customary loop filter function of a PLL.
According to the invention, a receiver comprises such a demodulation unit.
A method according to the invention is characterized by dividing the at least two mixed components by each other and supplying the output signal as a control signal for generating the oscillator signal, and by filtering one or more components and/or signals. In an integrated solution, the different functions may be incorporated in a CPU, wherein the division in the control loop prevents amplitude variations of the input signal from influencing the demodulated signal.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.
REFERENCES:
patent: 5230011 (1993-07-01), Gielis et al.
patent: 5696797 (1997-12-01), Bucher et al.
patent: 5784414 (1998-07-01), Bruekers et al.
patent: 0579100A1 (1994-01-01), None
PHN 15,248, U.S. Serial No. 08/704,200, Filed: Aug. 28, 1996.
Gielis Gerardus C. M.
van de Plassche Rudy J.
Glenn Kimberly
Lee Benny
U.S. Philips Corporation
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