Telecommunications – Transmitter – Angle modulation
Reexamination Certificate
2000-02-02
2003-03-18
Le, Thanh Cong (Department: 2684)
Telecommunications
Transmitter
Angle modulation
C455S076000, C455S118000
Reexamination Certificate
active
06535719
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a frequency modulation (FM) transmitter for radio transmission of frequency modulated stereo audio signals.
BACKGROUND OF THE INVENTION
FM transmitters for radio transmission of frequency modulated stereo audio signals have been used. A typical FM transmitter has an arrangement as shown in FIG.
2
.
As seen in
FIG. 2
, the transmitter includes:
a right audio section
10
having a pre-emphasis circuit
11
, a volume
12
, a limiter
13
, a low-pass filter
14
, and a muting circuit
15
;
a left audio section
20
having a pre-emphasis circuit
21
, a volume
22
, a limiter
23
, a low-pass filter
24
, and a muting circuit
25
;
a stereo modulator section
30
having an audio amplifier
31
for amplifying audio signals received from the audio sections
10
and
20
, an oscillator circuit
32
coupled with an external quartz oscillator Xosc
1
(having a fundamental frequency of 38 kHz) to generate 38 kHz signals, a multiplexer
33
for switching between right and left amplified 38 kHz audio signals received from the oscillator circuit
32
, and a radio frequency (RF) amplifier circuit
34
;
a modulation level adjust circuit
41
for regulating the output level of the multiplexer
33
;
a pilot level adjust
42
for regulating the level of 19 kHz pilot signal obtained by frequency dividing the 38 kHz output of the oscillator circuit
32
;
a mixer circuit
43
for mixing the regulated output signals of the modulation level adjust circuit
41
with the output of the pilot level adjust circuit
42
to generate a composite signal;
a PLL frequency synthesizer
44
coupled to an external quartz oscillator Xosc
2
(having a fundamental frequency of 7.2 MHz) and with the RF amplifier circuit
34
to receive the RF output thereof to generate a frequency control signal;
an oscillation control section
47
having a low-pass filter
45
for filtering the low frequency components of the output of the PLL frequency synthesizer
44
;
a mixing circuit
46
for mixing the output of the PLL frequency synthesizer
44
with the composite signal received from the mixer circuit
43
to generate a frequency modulation signal;
a frequency modulation circuit
48
controlled by the output signal of the frequency control section
47
; and
an RF output level regulation circuit
49
for regulating the RF output of the RF amplifier circuit
34
.
It is noted that the stereo modulator section
30
and the PLL synthesizer
44
are themselves provided in the form of integrated circuits. The PLL synthesizer
44
is represented by a single block in
FIG. 2
, but it actually comprises several components such as a frequency divider, a phase comparator, and program counter.
The PLL synthesizer
44
frequency divides the signal generated by the quartz oscillator Xosc
2
(having 7.2 MHz) into several different frequencies and supplies its output to an input end of a phase comparator for use as reference frequency signals. The RF frequency signal generated by the oscillation modulator circuit
48
is also frequency divided by a program counter, which are supplied to the other input end of the phase comparator for comparison with the reference frequency signals. Upon comparison of the input signals, the phase comparator provides its outputs to the oscillation control section
47
, which determines RF frequencies based on the frequency divided frequencies and the reference frequencies.
The PLL frequency synthesizer
44
is used to generate reference frequencies, for example, 100 kHz, 50 kHz, 25 kHz, 10 kHz, 9 kHz, 5 kHz, and 1 kHz for radio broadcasting. In this case, an appropriate external quartz oscillator Xosc
2
is one having a fundamental frequency of 7.2 MHz.
As discussed above, prior art FM transmitters have been constructed as a composite of different types of elements such as audio sections
10
and
20
, a stereo modulation section
30
, a PLL frequency synthesizer
44
, an oscillation controller
47
, and a frequency modulation circuit
48
. Of these, the stereo modulator
30
and the PLL frequency synthesizer
44
are formed into integrated circuits.
As a result, these components are arranged in a complex configuration and require complex wiring thereof when these elements are integrated to form an FM transmitter.
In addition, the stereo modulator section
30
and the PLL frequency synthesizer
44
, built in the integrated circuit configurations, utilize different quartz oscillators Xosc
1
and Xosc
2
having different fundamental frequencies (38 kHz and 7.2 MHz), respectively, for their intended purposes. The use of such different quartz oscillators in one transmitter inevitably results in an extra manufacturing cost.
SUMMARY OF THE INVENTION
In accordance with one aspect of the invention, a frequency modulating (FM) transmitter includes:
a reference frequency generator for generating a reference frequency;
a reference frequency divider for frequency dividing the reference frequency;
a stereo modulation circuit for frequency modulating a right audio signal and a left audio signal by using one output of the reference frequency divider to supply resultant stereo modulated signals as FM radio signals;
an oscillator circuit for generating carrier waves to transmit the FM signals received from the stereo modulation circuit;
a program counter for frequency dividing the carrier waves into variable frequency components; and
a PLL frequency synthesizer which has a phase comparator circuit for comparing the variable frequency components output from the program counter with another output of the reference frequency divider to provide at an output end of the PLL frequency synthesizer a control signal for controlling the oscillator circuit.
In this arrangement, since the FM transmitter may generate various frequency signals for both the stereo modulation and frequency comparison in the PLL frequency synthesizer by means of a single oscillator, the resulting FM transmitter has most of the elements integrated in one chip, which implies that the transmitter has far smaller dimensions and a simpler structure in number and arrangement of elements than conventional transmitters, and hence offers increased reliability and reduced manufacturing cost. Further, the program counter may be constructed such that the frequency division ratios of the program counter and the modulation level of the stereo modulator circuit are controllable by external means. It is then possible to vary the carrier frequency of the FM transmission wave as needed and to set the FM modulation at a characteristically optimum level.
The frequency of the reference frequency generator may be chosen to be 7.6 MHz or an integral multiple or fractional frequencies of 7.6 MHz obtained by dividing 7.6 MHz by integers (hereinafter referred to as integer fractions). It should be noted that this choice of the fundamental frequency allows provision of not only commonly used 38 kHz and 19 kHz Stereo modulation frequencies through the frequency divisions but also FM radio frequencies which are close to conventional frequencies by the same quartz oscillator.
In accordance with another aspect of the invention, a frequency modulating (FM) transmitter includes:
a reference frequency generator for generating a reference frequency;
a reference frequency divider for frequency dividing the reference frequency;
a stereo modulation circuit for frequency modulating audio signals by using one output of the reference frequency divider to supply resultant stereo modulated signals as FM signals;
an oscillator circuit for generating carrier waves to transmit the FM signals;
a program counter for frequency dividing the carrier waves into variable frequency components; and
a PLL frequency synthesizer which has a phase comparator circuit for comparing the variable frequency components output from the program counter with another output of the reference frequency divider.
In accordance with another aspect of the invention, a method of generating an FM signal includes:
generating a reference frequency;
dividing the reference frequency using a r
Ashida Hiroyuki
Suzuki Tamotsu
Tsuji Masatoshi
Cong Le Thanh
Hogan & Hartson LLP
Rohm & Co., Ltd.
Tran Pablo
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