Pulse or digital communications – Systems using alternating or pulsating current – Angle modulation
Reexamination Certificate
1999-03-16
2002-04-23
Pham, Chi (Department: 2631)
Pulse or digital communications
Systems using alternating or pulsating current
Angle modulation
C375S296000, C375S297000, C455S127500
Reexamination Certificate
active
06377630
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a transmission circuit of a type wherein turning on/off a transmission state is repeated and in particular to a transmission circuit for reducing occurrence of spurious modulation noises at the beginning of the transmission.
A voltage controlled oscillator (VCO) whose oscillation frequency can be controlled according to voltage is used as an intermediate-frequency signal source of a transmission circuit. The VCO is not used singly and is built in a part of a phase locked loop (PLL) for use. The VCO used as an intermediate-frequency signal source of a transmission circuit has an oscillation frequency modulated by a modulation signal. The frequency shift keying (FSK) is one of preferred modulation techniques.
FIG. 3
is a block diagram to show an example of a transmission circuit using a VCO in a related art. In the figure, numeral
1
denotes a fixed-frequency oscillator for transmission, numeral
2
denotes a VCO for outputting a signal containing a modulation signal component, numeral
3
denotes a frequency mixer for mixing (for example, upconverting) an output signal of the fixed-frequency oscillator
1
and an output signal of the VCO
2
, and numeral
4
denotes a high-frequency amplifier for amplifying output of the frequency mixer
3
and supplying the amplified signal to a transmission antenna
5
. The VCO
2
is built in a PLL
6
.
Output of the fixed-frequency oscillator
1
is supplied to the frequency mixer
3
through a narrow-band-pass filter
7
. Output of the high-frequency amplifier
4
is supplied to the transmission antenna
5
through a wide-band-pass filter
8
. Output of the mixer
3
is supplied to the amplifier
4
through a band-pass filter
9
used as required. An external crystal oscillator and an external loop filter connected to the PLL
6
and an external circuit for inputting a modulation signal to the VCO
2
are not shown.
The transmission circuit in
FIG. 3
controls turning on/off power supply +B in two blocks. A first power control block
11
is controlled by a switch SW
1
and contains the fixed-frequency oscillator
1
, the filter
7
, the mixer
3
, and the filter
9
. A second power control block
12
is controlled by a switch SW
2
and contains the amplifier
4
.
It is assumed that the transmission circuit in
FIG. 3
is not held in a transmission state all the time and that turning on/off the transmission circuit is repeated frequently. At the beginning of the transmission, the switch SW
1
is first turned on for first operating the fixed-frequency oscillator
1
. Then, oscillation of the oscillator
1
becomes stable before the switch SW
2
is turned on for operating the amplifier
4
. Therefore, output of the amplifier
4
reaches the maximum amplitude at the same time as the switch SW
2
is turned on as shown in a waveform chart in FIG.
3
.
If power control is performed in the first and second power control blocks
11
and
12
for sequentially turning on the SW
1
and SW
2
as in the transmission circuit in the related art shown in
FIG. 3
, a high-level signal is oscillated suddenly, thus spurious modulation noise occurs.
FIG. 4
is a spectrum characteristic chart to show the fact. In the figure, waveform S indicated by the solid line shows the spectrum of a transmission signal of center frequency f
0
. In contrast, waveform N which is hatched denotes the spurious modulation noise.
The configuration of the transmission circuit in
FIG. 3
does not involve any problem if the spurious modulation noise N can be removed through the filter
8
placed at the stage following the amplifier
4
. However, since the filter
8
has the bandwidth set wide so as to cover a plurality of channels, noise removal with a limited band cannot be expected and after all, output having a characteristic similar to that in
FIG. 4
is transmitted from the antenna
5
. This point is the problem to be solved by the present invention.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a transmission circuit wherein the spurious modulation noise occurrence level can be remarkably decreased by changing the power control order.
In order to achieve the above object, there is provided a transmission circuit comprising: a fixed-frequency oscillator for oscillating a frequency for the transmission; a voltage controlled oscillator for outputting a signal containing a modulation signal component; a frequency mixer for mixing an output signal of the fixed-frequency oscillator and an output signal of the voltage controlled oscillator; a high-frequency amplifier for amplifying output of the frequency mixer and supplying the amplified signal to a transmission antenna; and a power control circuit for first turning on power of the voltage controlled oscillator and the high-frequency amplifier at the beginning of the transmission and after expiration of a predetermined time, turning on power of the fixed-frequency oscillator.
In the transmission circuit, the power control circuit turns on the power of the fixed-frequency oscillator after the oscillation of the voltage controlled oscillator becomes stable.
In the transmission circuit, the voltage controlled oscillator is built in a phase-controlled loop. Output of the fixed-frequency oscillator is supplied to the frequency mixer through a narrow-band-pass filter. Output of the high-frequency amplifier is supplied to the transmission antenna through a wide-band-pass filter.
REFERENCES:
patent: 4731870 (1988-03-01), Black et al.
patent: 4859967 (1989-08-01), Swanson
patent: 4932073 (1990-06-01), Ueda
patent: 5033111 (1991-07-01), Marui
patent: 5506546 (1996-04-01), Kowaguchi
patent: 5883927 (1999-03-01), Madsen et al.
patent: 5995853 (1999-11-01), Park
Mitsumi Electric Co. Ltd.
Pham Chi
Phu Phuong
Whitham, Curtis & Christofferson
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