Demodulators – Frequency modulation demodulator – Input signal converted to and processed in pulse form
Reexamination Certificate
2000-06-08
2001-07-24
Mis, David (Department: 2817)
Demodulators
Frequency modulation demodulator
Input signal converted to and processed in pulse form
C329S315000, C455S214000, C455S337000
Reexamination Certificate
active
06265933
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to an FM (Frequency Modulation) demodulator, and more particularly to an arctangent type FM demodulator using a digital signal processing technique.
2. Description of the Related Art
A digital signal processing technique is applied to a construction of an FM demodulator, so as to realize a miniaturization and a high reliability of the apparatus. As an FM demodulator using such a digital signal processing technique, there is an arctangent type FM demodulator.
In the arctangent type FM demodulator, an AGC (Automatic Gain Control) circuit is equipped at an input stage thereof. An FM signal is inputted to this AGC circuit from a tuner or the like. The AGC circuit performs a variable-controls of the gain to amplify the FM signal in an analog form so that the FM signal has a constant amplitude. Namely, the function of the AGC circuit is to remove the amplitude fluctuation included in the FM signal. Then, the FM signal outputted from the AGC circuit is inputted to an A/D (Analog to Digital) convertor. The A/D convertor samples the FM signal, in which the amplitude level is made constant by the AGC circuit, and converts it into a digital signal. A sampling pulse generator supplies a sampling pulse to the A/D convertor.
When the digital signal corresponding to the FM signal is obtained in this manner, the processes after that is mainly performed by using the digital signal processing technique, so as to generate a digital FM demodulated signal, which corresponds to the FM signal in the analog form, through an arctangent operator and a differentiator for performing a predetermined digital signal process for example.
In this arctangent type FM demodulator, if the AGC circuit is not equipped at the input or prior stage of the A/D convertor, since the amplitude of the FM carrier wave received at a tuner etc., drastically changes depending upon the receiving condition, the value of the digital signal outputted from the A/D convertor drastically changes. Then, an error is generated in the arctangent operator or the differentiator for performing the predetermined digital signal process at the later stage of the A/D convertor. This results in a difficulty in obtaining a digital FM demodulated signal in a good condition.
In this manner, in the arctangent type FM demodulator, it is necessary to equip the AGC circuit at the input or prior stage of the A/D convertor.
However, the AGC circuit has a rather complicated structure and is rather expensive and brings about a complexity and a high cost of the apparatus as a whole, which is a problem.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an arctangent type FM demodulator, which can avoid a complexity and a high cost and can sufficiently get a merit of the digital signal processing technique.
The above object of the present invention can be achieved by an FM demodulator, into which an FM signal is inputted, for demodulating the inputted FM signal and outputting a digital FM demodulated signal. The FM demodulator of the present invention is provided with: a limiter amplifier for amplifying and converting the inputted FM signal to a rectangular wave having a predetermined amplitude; a reference signal generator for generating a reference signal, which is a sine wave having a constant frequency; an A/D convertor for sampling the reference signal outputted from the reference signal generator by the rectangular wave outputted from the limiter amplifier to thereby output a digital signal; an orthogonalizing device for converting the digital signal outputted from the A/D convertor to two signals orthogonal to each other; an arctangent operator for calculating an arctangent value on the basis of a ratio of the two signals outputted from the orthogonalizing device; and a differentiation operator for calculating a differentiation of the arctangent value outputted from the arctangent operator, to thereby output the differentiation as the digital FM modulated signal.
According to the FM demodulator of the present invention, the FM signal inputted to the FM demodulator is amplified, is clipped by a constant level, and is converted into a rectangular wave having a predetermined amplitude, by the limiter amplifier. On the other hand, the sine wave having a constant frequency outputted from the reference signal generator is inputted as the reference signal into the A/D convertor, so that the A/D conversion is performed by using the rectangular wave from the limiter amplifier as the sampling pulse, to thereby output the digital signal. This digital signal is converted into two signals orthogonal to each other by the orthogonalizing device. The arctangent value is calculated on the basis of the ratio of these two signals by the arctangent operator. Then, the differentiation of the arctangent value is obtained by the differentiation operator. This calculated differentiation becomes the digital FM demodulated signal.
Accordingly, since the FM signal is converted into the rectangular wave, it is possible to stabilize the amplitude level without equipping the AGC circuit at the input or prior stage of the A/D convertor. By A/D-converting the sine wave having a constant frequency by using this rectangular wave as the sampling pulse, the digital signal including the FM modulation component is obtained. Thus, the processes at the later stage can be performed by using this digital signal. Therefore, the FM demodulator can be realized, in which the arctangent calculation is performed by the digital signal process, which can be miniaturized, which can have a high reliability, whose structure can be simplified, and whose cost can be reduced without employing the AGC or the like.
In one aspect of the FM demodulator of the present invention, the rectangular wave outputted from the limiter amplifier is used as a reference clock for the orthogonalizing device, the arctangent operator and the differentiation operator.
According to this aspect of the present invention, since it is sufficient for the block of the FM demodulator, where the digital signal process is performed, to operate when the FM signal is inputted, the sampling pulse obtained from the FM signal is used also as the reference clock for this block. Therefore, since the clock generation circuit or the like is not necessary, the structure of the FM demodulator can be even more simplified and the cost thereof can be even more reduced.
The nature, utility, and further features of this invention will be more clearly apparent from the following detailed description with respect to preferred embodiments of the invention when read in conjunction with the accompanying drawings briefly described below.
REFERENCES:
patent: 4620160 (1986-10-01), Waggener
patent: 4906942 (1990-03-01), Nakai et al.
patent: 5293247 (1994-03-01), Ganse
patent: 5481368 (1996-01-01), Ozaki
patent: 5694079 (1997-12-01), Allpress et al.
Mis David
Nixon & Vanderhye
Pioneer Corporation
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