Modulators – Frequency modulator – Including stabilization or alternatively distortion – noise...
Reexamination Certificate
2000-02-15
2001-04-03
Mis, David (Department: 2817)
Modulators
Frequency modulator
Including stabilization or alternatively distortion, noise...
C381S003000
Reexamination Certificate
active
06211748
ABSTRACT:
TECHNICAL FIELD
This invention concerns a frequency modulation circuit.
BACKGROUND ARTS
A method for rendering a signal line that connects a headphone and an audio set wirelessly using an FM wave has been made practical.
In that case, processing to be executed in the audio set, comprises:
(1) forming the sum signal (L+R) and the difference signal (L−R) from the left-channel audio signal L and the right-channel audio signal R;
(2) performing frequency modulation of the sub-carrier signal by the difference signal (L−R), in order to obtain the FM signal S
SUB
;
(3) performing frequency modulation of the main carrier signal by the addition signal S
MPX
of the sum signal (L+R) and the FM signal S
SUB
; and
(4) transmitting the FM signal that is obtained in step (3) to the headphone.
That is, frequency multiplexing of the sum signal (L+R) and the difference signal (L−R) into a signal S
MPX
is performed, and the multiplexed-frequency signal S
MPX
is transmitted.
As an example,
sub-carrier frequency f
SUB
=50 kHz
main-carrier frequency f
FM
=915 MHz.
By the way, as a method for performing frequency modulation by the use of PLL, a method is known in which a modulation signal is inputted as a disturbance of the loop, that is, such a method that the modulation signal is added to the output signal of the low-pass filter that is a component of the loop. Therefore, when the FM signal according to the term (3) is to be formed, it is possible to add the addition signal S
MPX
to the output signal of the low-pass filter.
However, with modulation by disturbance, it is not possible to obtain sufficient characteristics of distortion factor, frequency response and others, at a lower frequency than the natural frequency &ohgr;n (which is determined on the basis of the loop gain and the time constant of the low-pass filter). And, in the case where the main-carrier frequency f
FM
is as high as 915 MHz as shown above, the frequency-division ratio N of the frequency divider circuit in PLL becomes as large as about 9000, hence, the natural frequency &ohgr;n becomes 100 Hz or so. Therefore, modulation by disturbance is not suitable for modulation based on the addition signal S
MPX
having frequency components lower than 100 Hz such as a musical signal.
Another method is obtain an FM signal by performing frequency modulation of the reference signal of PLL; however, in the case of this method, at a higher frequency than the natural frequency &ohgr;n, it is impossible to perform modulation having sufficient characteristics, and so it can not be used as the addition signal S
MPX
as well.
That is, performing frequency modulation in PLL conflicts with stabilizing the frequency that is the original object of PLL, and so, when modulation is performed in PLL, sufficient characteristics can not be obtained. Therefore, a method of performing modulation in PLL is only practical in mobile telephones, transceivers and other devices in which tone quality is not much of a question.
DISCLOSURE OF INVENTION
This invention solves such problems, and enables suitable frequency modulation by the use of a frequency-multiplexed signal S
MPX
in PLL.
Therefore, the present invention offers a frequency modulation circuit, which comprises
a first VCO;
a frequency divider circuit for dividing an oscillated signal that is generated by the first VCO;
an oscillation circuit for outputting an oscillation signal having the reference frequency;
a phase comparison circuit for comparing the phases of the divided signal that is outputted from the frequency divider circuit and the oscillated signal of the oscillation circuit; and
a low-pass filter for taking a direct-current voltage having a level that corresponds to the phase difference between the divided signal and the oscillation signal of the oscillation circuit, out of the phase comparison output of the phase comparison circuit, and
the direct-current voltage that is outputted from the low-pass filter is supplied to the first VCO as a control signal for its oscillation frequency, so that a PLL is composed;
the oscillation circuit is composed of a second VCO;
a first audio signal is supplied to the second VCO, and the oscillation frequency is controlled hereby;
a sub-carrier signal is modulated by a second audio signal, so that the modulated signal is formed whose occupied band is outside the band of the first audio signal; and
this modulated signal is added to the direct-current voltage that is outputted from the low-pass filter, and the obtained result is supplied to the first VCO.
In this way, frequency modulation is performed by the first audio signal, and frequency modulation is also performed by a signal modulated by the second audio signal, then an FM signal is outputted which is equivalent to that in the case of frequency modulation being performed by the frequency-multiplexed signal of the both signals.
REFERENCES:
patent: 4704727 (1987-11-01), Beard
Abe Kensaku
Kito Kazuhisa
Kobayashi Nobuo
Sugimoto Yoshihiro
Maioli Jay H.
Mis David
Sony Corporation
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