Modulators – Frequency shift keying modulator or minimum shift keying...
Patent
1996-08-07
1998-03-17
Mis, David
Modulators
Frequency shift keying modulator or minimum shift keying...
331 17, 331 23, 375274, 375278, 375305, H03C 300, H03L 7085, H03L 716, H03L 2712
Patent
active
057291829
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
Continuous phase modulation by frequency synthesizer with phase-locked loop is known; the modulating signals are signals exhibiting continuous transitions in phase and may be of analogue or digital type according to circumstances. Among the digital signals used there are, among others, the signals known as MSK signals from their name in the literature, which is the acronym for Minimum Shift Keying, and which translates into French as minimum-displacement modulation or, better, as modulation with minimum phase gradient; a variant of the MSK signals which can also be used consists of signals known as GMSK signals, from their name in the literature: Gaussian Minimum Shift Keying.
A frequency synthesizer with a phase-locked loop essentially includes a controlled-frequency oscillator and a phase-locked loop. The oscillator, which will be called variable oscillator in the remainder of this document, is called voltage controlled oscillator or VCO in the literature. The slaving loop includes, successively between the output and the input of the variable oscillator, a frequency division circuit, a phase comparator and a low-pass filter; in the comparator, the synthesizer with phase-locked loop output signal of the frequency division circuit is compared with the signal supplied by a reference oscillator. In order to perform continuous phase modulation of the output signal of the variable oscillator, it is known to apply the modulation signal either by addition to the signal supplied by the filter to the variable oscillator, or by subtraction from the signal supplied by the phase comparator to the low-pass filter, or as a control of the rate of division of the division circuit; these three ways of working exhibit various drawbacks which will be seen further on and, among these drawbacks, one common drawback: the modulation frequency band is affected by the frequency band of the loop.
It is known to reduce these drawbacks by applying the modulation signal not only by addition to the signal supplied by the filter to the oscillator but also, in order to compensate for the effects of the modulation in the loop, by applying it either by subtraction from the signal supplied by the phase comparator to the low-pass filter, or as a control of the rate of division of the division circuit. Despite everything, there still remains a modulation residue in the control signal derived by the loop, in particular due to the drift, in temperature and over time, of the components.
SUMMARY OF THE INVENTION
The object of the present invention is to reduce, or even to cancel out, this residue.
This is obtained, in particular, by deriving a correction signal in a continuous way and thus differently from what is proposed in the patent application UK 2,228,840, where the operation of a synthesizer is stopped in order to be able to apply a specific test signal to it, in place of the modulation signal, and to adjust the loop; it results therefrom not only that the operation of the synthesizer has to be interrupted but that, after the adjustment, the synthesizer loses its setting depending on the drift of its components.
According to the invention, there is provided a modulation device for continuous phase modulation by frequency synthesizer, including an adder circuit with a modulation input, a control input and an output, a variable oscillator with a control input coupled to the output of the adder circuit to receive a control voltage, and an output, a slaving loop with an input coupled to the output of the oscillator and an output coupled to the control input of the adder circuit, the slaving loop including a reference oscillator and, successively between the output and the input of the loop, a frequency division circuit, a phase comparator and a loop filter of low-pass type, having an output, the comparator comparing the output signals of the division circuit and of the reference oscillator, and the loop including one of two modulation accesses which consist respectively of an input coupled to the input of the
REFERENCES:
patent: 5144260 (1992-09-01), Stribling et al.
patent: 5150082 (1992-09-01), Grimmett et al.
patent: 5281930 (1994-01-01), Taromaru et al.
patent: 5412353 (1995-05-01), Chaplik et al.
patent: 5424688 (1995-06-01), Phillips
Chelouche Marc
Collin Laurent
de Gouy Jean-Luc
Fousset Lionel
"Thomson-CSF"
Mis David
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