Amplifiers – Modulator-demodulator-type amplifier
Reexamination Certificate
2001-01-25
2002-05-07
Pascal, Robert (Department: 2817)
Amplifiers
Modulator-demodulator-type amplifier
C330S107000, C332S103000, C455S126000
Reexamination Certificate
active
06384677
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATION
The present application relates to subject matters described in co-pending application Ser. No. 09/672,688 filed on Sep. 29, 2000.
BACKGROUND OF THE INVENTION
The present invention generally relates to a transmitter. More particularly, the present invention is concerned with a negative feedback amplifier circuit employed in the transmitter of digital modulation type for compensating for nonlinear distortions. Further, the invention is also concerned with a method of controlling the phase of the negative feedback amplifier circuit.
In radio systems in which a linear digital modulation system such as, for example, 16QAM (Quadrature Amplitude Modulation), &pgr;/4 shift QPSK (Quadrature Phase Shift Keying) or the like is employed, it is indispensably required to compensate for nonlinear distortion of a power amplifier. To this end, a variety of nonlinear distortion compensating systems (linearizers) are proposed for practical application. Among them, a Cartesian loop negative feedback type linearizer has been conventionally employed long since. For having better understanding of the background techniques of the present invention, description will first be made in some detail of the conventional linear feedback amplifier known heretofore by reference to
FIG. 2
which is a block diagram showing an arrangement of a transmitting section of a digital radio system provided with a Cartesian loop negative feedback type linearizer.
Referring to
FIG. 2
, reference numeral
1
denotes a baseband signal generator which is designed to output an in-phase component (hereinafter referred to simply as the I-component) and a quadrature component (hereinafter referred to simply as the Q-component) of a baseband signal. The I-component is added with a corresponding feedback signal by an adder
2
-
1
, the output of which is supplied to a loop filter
3
-
1
. Likewise, the Q-component is added with a corresponding feedback signal by an adder
2
-
2
, the output signal of which is applied to a loop filter
3
-
2
. The loop filers
3
-
1
and
3
-
2
operate to limit the bandwidths of the inputted I-component and the inputted Q-component, respectively. The I- and Q-components undergone the bandwidth limitation are then inputted to a quadrature modulator
4
, as indicated by I′ and Q′, respectively.
A numeral
11
denotes a reference signal generator which is designed to generate a reference frequency signal which is then supplied to the first and second PLL frequency synthesizers
12
and
13
, respectively. The PLL frequency synthesizer
12
is designed to generate a first local oscillation signal (hereinafter referred to as the first LO
1
signal) on the basis of the reference frequency signal. The first LO
1
signal is then supplied to a quadrature modulator
4
and a phase shifter
18
. On the other hand, the PLL frequency synthesizer
13
generates a second local oscillation signal (hereinafter referred to as the second LO
2
signal) on the basis of the reference signal. The second LO
2
signal is supplied to mixers
6
and
15
, respectively. The phase shifter
18
controls the phase of the first LO
1
signal in conformance with a control signal supplied from a phase controller
19
. The first LO
1
signal undergone the phase control is then supplied to a quadrature demodulator
16
.
The quadrature modulator
4
serves to orthogonally modulate the first LO
1
signal (a carrier signal) into a signal of an intermediate frequency band (hereinafter referred to as the IF frequency band) with the I-component I′ and the Q-component Q′ of the baseband signal inputted to the quadrature modulator
4
. Then, the modulated signal is applied to a bandpass filter (BPF)
5
. The bandpass filter
5
operates to eliminate unnecessary components from the modulated signal. The output signal of the bandpass filter
5
is then applied to the mixer
6
. The mixer
6
operates to convert the modulated signal applied therein into a signal of a desired frequency by making use of the second LO
2
signal outputted from the PLL frequency synthesizer
13
. The output signal of the mixer
6
is then applied to a bandpass filter (BPF)
7
. The bandpass filter
7
serves to eliminate unnecessary spurious components from the signal inputted. The output of the bandpass filter
7
is then inputted to the amplifying circuit (PA)
8
which operates to amplify the input signal to a specified or rated output level for transmission by way of an antenna
9
.
Since the negative feedback amplifier described above is implemented in the form of the negative feedback linearizer based on the Cartesian loop, a part of the output signal of the amplifying circuit
8
is fed back to the input-side circuitry to be supplied to an attenuator (ATT)
14
through the medium of a directivity coupler
10
. In response, the attenuator
14
operates to regulate the power level of the input signal to a proper value. The output of the attenuator
14
is supplied to the mixer
15
. The mixer
15
then converts the frequency of the signal inputted from the attenuator
14
to an IF frequency by using the second LO
2
signal. The IF frequency signal is then supplied to the quadrature demodulator
16
.
The quadrature demodulator
16
operates to divide the inputted IF signal into two IF signals having 90°-phase shifted from each other and produce baseband signals i and q of the I-component and the Q-component, respectively, by making frequency conversion of two IF signals with the first LO
1
signal inputted from the phase shifter
18
which has undergone the phase control. The I-component is applied to a subtracting or minus input terminal of the adder
2
-
1
as the I-component baseband signal i for feedback by way of a switch
20
-
1
, while the Q-component is applied to a subtracting or minus input terminal of the adder
2
-
2
as the Q-component baseband signal q for feedback by way of a switch
20
-
2
. In this manner, feedback operation is performed on the I-component and the Q-component, respectively. At this time, the input terminals a and b of the switches
20
-
1
and
20
-
2
are connected to the first and second adders
2
-
1
and
2
-
2
, respectively.
In this negative feedback amplifier circuit, it is required for the purpose of circuit stabilization that the input signals I and Q on one hand and the feedback signals i and q on the other hand are in phase, respectively, (i.e., phase difference=0°) on the input side of the adders
2
-
1
and
2
-
2
. To say in another way, in case phase difference takes place between the input signals and the feedback signals, it is required that such control be carried out that the phase of the first LO signal inputted to the quadrature demodulator
16
can be adjusted by 180° (&pgr; radian) at maximum by means of the phase shifter
18
in order to make the phases of the input signals coincide with those of the feedback signals in the adders
2
-
1
and
2
-
2
, respectively.
At this juncture, a phase control method will be described below. Initially, the switches
20
-
1
and
20
-
2
shown in
FIG. 2
are set to such positions that the outputs q and i of the quadrature demodulator
16
are supplied to the phase controller
19
with the feedback loop being in the opened state.
In this state, in the baseband signal generator
1
, a predetermined DC voltage is given to only the I-component for the purpose of phase adjustment while the Q-component being held zero (Q=0), whereon the quadrature modulation is carried out straightforwardly for signal transmission by way of the antenna
9
in accordance with the procedure described previously. In that case, the output waveform of the amplifying circuit
8
assumes the waveform of the non-modulated carrier signal. Such being the circumstances, when a part of the output of the amplifying circuit
8
is fed back by way of the directivity coupler
10
, then the DC voltage makes appearance only for the I-component of the feedback signal outputted from the quadrature demodulator
16
while no DC volt
Hitachi Kokusai Electric Inc.
Nguyen Khanh Van
Pascal Robert
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