Modulators – Phase shift keying modulator or quadrature amplitude modulator
Reexamination Certificate
2000-06-28
2002-03-19
Grimm, Siegfried H. (Department: 2817)
Modulators
Phase shift keying modulator or quadrature amplitude modulator
C332S105000, C375S261000, C375S285000, C375S298000
Reexamination Certificate
active
06359523
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The present invention relates to an orthogonal modulator for removing a DC offset component of each of a plurality of base band signals, a mobile terminal comprising the orthogonal modulator, and a communication system including such mobile terminals.
2. Description of the Prior Art
Conventionally, a mobile terminal such as a cellular phone has employed the orthogonal modulation by which a digital voice signal (base band signal) is transmitted, for example, by two carrier waves having different phases by 90°.
More specifically, the conventional mobile terminal amplifies and modulates an I channel base band signal (I signal), a Q channel base band signal (Q signal) with a carrier wave having an orthogonal relationship by means of the orthogonal modulator and then adds them, thereby obtaining a modulated output signal.
However, in the case in which the orthogonal modulator or the base band signal input thereto has DC offset, the carrier leak appears due to the DC offset over a modulated wave obtained by the orthogonal modulator. Therefore, there have been developed several techniques for suppressing the carrier leak.
The conventional orthogonal modulator as shown in
FIG. 5
comprises base band LSI
81
for generating an I signal and a Q signal and an IB signal and a QB signal of which phases are inverted thereto by 90° phase shifter
83
for modulating the phase of the output carrier wave source
84
, and I/Q mixer
82
such as a Gilbert multiplier for mixing the output signal of the base band LSI
81
and the output signal of the 90° phase shifter
83
through filter
85
.
The operation of the orthogonal modulator shown in
FIG. 5
is explained below. The base band LSI
81
inputs an I/Q DC level offset setting signal from non-shown ROM, and outputs an I signal, a Q signal, an IB signal and a QB signal. The IB signal and the QB signal are generated in order to operate the I/Q mixer
82
in good balance.
Moreover, the I/Q DC level offset setting signal is decided so as to minimize the carrier leak of an MOD signal (modulated wave) outputted from an MOD terminal of the I/Q mixer
82
. In other words, the carrier leak is suppressed in response to the I/Q DC level offset setting signal.
A pair of I and IB base band signals and a pair of Q and QB base band signals are inputted into transistors Q
1
to Q
4
of the I/Q mixer
82
through the filter
85
.
Moreover, transistors Q
5
to Q
12
of the I/Q mixer
82
input, in a predetermined configuration, a 0° carrier wave, a 90° carrier wave, a 180° carrier wave and a 270° carrier wave which are outputted from the carrier wave source
84
through the 90° phase shifter
83
.
These carrier waves and each base band signal are mixed and are transmitted from MOD terminal to the air. Thus, the carrier leak of the MOD signal is minimized.
FIG. 6
is a block diagram of a carrier leak suppressing circuit as an orthogonal modulator disclosed in JP 6-303145 A, 1994. Orthogonal modulation unit
100
inputs the I and Q signals having phases different from each other by 90° and outputs an orthogonal modulating signal. A part of the orthogonal modulating signal is inputted into demodulation unit
101
. The demodulation unit
101
inputs a carrier Lo outputted from the orthogonal modulation unit
100
.
The demodulation unit
101
demodulates the inputted signal into the I and Q signals. The DC offset component of the I signal and the DC offset component of the Q signal thus demodulated are fed back to the orthogonal modulation unit
100
in order to remove the DC offset components from the I and Q signals. Thus, the carrier leak of the modulated wave is suppressed.
A specific structure of carrier leak suppressing circuit as shown in
FIG. 6
is shown in FIG.
7
.
The orthogonal modulation unit
100
′ modulates the I signal, while the orthogonal modulation unit
100
″ modulates the Q signal. Further, demodulation unit
101
′ demodulates the I signal, while demodulation unit
101
″ demodulates the Q signal.
Each of the I and Q signals is converted from a digital signal into an analog signal by D/A converters
111
and
121
, is subjected to a predetermined processing in operational amplifiers
112
and
122
and roll-off filters
113
and
123
, and is outputted into mixers
114
and
124
together with carrier waves 0° and 90° which are the outputs of a local oscillator
120
.
Then, the I and Q signals and the orthogonal carrier wave are mixed and amplitude-modulated. Thus, an orthogonal modulating signal is outputted from the antenna. A part of the orthogonal modulating signal is inputted into demodulation units
101
′ and
101
″. The offset components are extracted by band pass filters
101
A and
102
A, tuned to the timing of the carrier on the modulation side by means of delay elements
101
L and
102
L and are outputted into demodulating mixers
101
X and
102
X.
The demodulating mixers
101
X and
102
X also input the orthogonal carrier waves 0° and 90°, mix the output signals of the delay elements
101
L and
102
L and the orthogonal phase carrier waves, thereby detecting DC offset components. The DC offset component is integrated by low-pass filters
101
F and
102
F and is fed back to the operational amplifiers
112
and
122
through amplifiers
101
a
and
102
a.
Each of the operational amplifiers
112
and
122
calculates a difference between the output signal of the D/A converter and the output signals
101
a
and
102
a
and remove the DC offset components. A base band signal from which the DC offset component is removed is outputted into composite hybrid
130
through the roll-off filters
113
and
123
and the mixers
114
and
124
. The output from the mixer
114
and the output signal from the mixer
124
are mixed and the mixed signal is outputted to the outside of the orthogonal modulator through the band pass filter
140
.
In the conventional orthogonal modulator as shown in
FIG. 5
, the base band LSI generates each base band signal in response to the I/Q DC level offset setting signal output from the ROM. However, the carrier leak of the modulated wave obtained from the base band signal cannot be always decreased. The DC offset between the I and Q signals of the base band LSI and mixer circuit does not always have a mutual relationship, due to variations in manufacturing processes and environmental conditions such as an ambient temperature, or light.
Moreover, the set value of the I/Q DC level offset cannot be changed because it is stored in the ROM. For this reason, it is impossible to carry out correction corresponding to a variation in the DC offset between the I and Q signals of the base band LSI and mixer circuit and the like.
Furthermore, in the conventional orthogonal modulator as shown in
FIG. 5
, when the DC levels of the I and Q signals outputted from the base band LSI are lowered, the voltage between the collector and the emitter of the transistor of the I/Q mixer is also lowered, so that a constant current does not flow to the I/Q mixer. Consequently, the modulating signal loses its linearity with respect to the I/Q signal and an I/Q tertiary modulation distortion is increased.
In the conventional orthogonal modulator as shown in
FIG. 6
, the modulated signal is demodulated into the I and Q signals and the DC offset component of the I signal and the DC offset component of the Q signal are fed back to the orthogonal modulation unit. However, there is no feedback loop for removing the DC offset component between the I signal and the Q signal. For this reason, it is impossible to remove the carrier leak by the DC offset component between the I signal and the Q signal.
Moreover, the power consumption of the demodulation unit is almost equivalent to that of the orthogonal modulation unit. Accordingly, the power consumption as a whole circuit as shown in
FIG. 6
is increased to almost a double as compared with the orthogonal modulator according to the circuit as shown in FIG.
5
.
Further, in the conventional orthogonal
Grimm Siegfried H.
NEC Corporation
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