Modulators – Frequency modulator
Reexamination Certificate
1998-12-10
2001-06-26
Kinkead, Arnold (Department: 2817)
Modulators
Frequency modulator
C332S123000, C455S112000, C455S118000, C455S323000, C455S086000
Reexamination Certificate
active
06252472
ABSTRACT:
DESCRIPTION
1. Field of the Invention
The present invention relates to a modulator comprising:
an oscillator intended to generate a signal having a frequency called oscillation frequency, and
a mixer intended to receive a modulating signal and a carrier signal, and to supply a modulated signal with a given output frequency.
2. Backgroung of the Invention
Such modulators are currently manufactured in the integrated circuit industry and used, for example, in magnetoscopes or decoders of digital television signals. The applicants market such an integrated circuit which is referenced TDA 8722. In this modulator, the carrier signal is formed by the signal generated by the oscillator and the modulated signal has a frequency equal to the oscillation frequency because the oscillator is directly connected to the mixer. It has been found that the oscillator and the output of the mixer interact by way of connections to energy sources shared by these two elements, on the one hand, and via electromagnetic radiation phenomena, on the other hand. This interaction has undesired effects. The oscillator induces in the modulated signal a parasitic signal, having the oscillation frequency, which is, due to the structure of the modulator, equal to the frequency of the carrier signal. The variations of the parasitic signal thus coincide with those of the modulated signal and modify the amplitude of the latter. The result is a modification of the envelope of the modulated signal which is equivalent to an alteration of the information transported by said signal.
An object of the present invention is to remedy this drawback by proposing a modulator whose structure is such that the effects of interaction between the oscillator and the output of the mixer may easily be eliminated.
SUMMARY OF THE INVENTION
Indeed, a modulator as defined in the opening paragraph is characterized according to the invention in that the oscillator is arranged so that its nominal oscillation frequency is N times higher than the output frequency, and in that the modulator further includes a divide-by-N frequency divider intended to receive the output signal of the oscillator and to apply the carrier signal to the mixer.
In such a modulator, the carrier signal is no longer the output signal of the oscillator, but the output signal of the frequency divider. As the frequency of the carrier signal is different from the oscillation frequency, the parasitic signal induced by the oscillator in the modulated signal has a frequency that is different from that of the modulated signal. The parasitic signal may thus easily be identified and filtered, for example, by means of a pass-band filter.
The modulator according to the invention further enables a modulated signal that features an improved spectral purity compared with that featured by the modulated signal generated by the known modulator. Indeed, the assembly of the oscillator as well as its connections to the rest of the circuit is to be perfectly symmetrical, so that it can deliver a signal which has a cycle ratio of ½. This condition of symmetry is very hard to realize in practice. The result is that the cycle ratio of the output signal of the oscillator cannot be exactly equal to ½, which notably causes the presence in this signal of a signal having a frequency of twice the oscillation frequency which is commonly called second harmonic. In the known modulator, this second harmonic appears in the modulated signal, since the output signal of the oscillator forms the carrier signal. In the modulator according to the invention, the carrier signal is formed by the output signal of a frequency divider whose precision of the cycle ratio may easily be mastered. Various embodiments may be considered for this object.
A particular embodiment of the invention features a modulator as described above, characterized in that, while the ratio N of the frequency division effected by the divider is equal to 2
P
, where P is an integer, the divider is formed by P divide-by-two dividers arranged in cascade, each of them including a first and a second memory flipflop which have each a control input as well as a data input and output, the control input of the first memory flipflop forming the input of the divide-by-two divider and being connected, via an inversion, to the control input of the second memory flipflop, the data output of the second memory flipflop being connected, via an inversion, to the data input of the first memory flipflop, the data output of the first memory flipflop being connected to the data input of the second memory flipflop, one of the data outputs of the first and second memory flipflops forming the output of the divide-by-two divider.
As stated above, a modulator in accordance with the invention may be advantageously used in various devices such as magnetoscopes or decoders of digital television signals. The invention thus also relates to a device used for the conversion of data into signals having a form that is compatible with television standards, comprising:
a unit called input unit intended to ensure the reception of the data and to extract therefrom signals of two different types, one called audio signal and the other called video signal,
a first and a second processing module intended to reconstitute the audio and video signals, respectively,
an adder intended to supply on an output a signal resulting from a superpositioning of signals present on the outputs of the first and second processing modules, and
a modulator intended to perform a conversion of the frequency of the output signal of the adder into a given frequency, which device is characterized in that the modulator is such as described above, while its modulating signal is formed by the output signal of the adder.
REFERENCES:
patent: 5097230 (1992-03-01), Lautzenhiser
patent: 5463356 (1995-10-01), Palmer
patent: 5568205 (1996-10-01), Hurwitz
Multifunction Silicon MMIC's for Frequency Conversion Applications by Kevin Negus et al. IEEE MTT, vol. 38, #9, pp. 1191-1198, 1986.*
Block Diagram TDA 8722, Philips Semiconductors,p. 3768, Mar. 21, 1995.
Kinkead Arnold
Piotrowski Daniel J.
U.S. Philips Corporation
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