Modulators – Phase shift keying modulator or quadrature amplitude modulator
Reexamination Certificate
2000-02-29
2004-04-13
Kinkead, Arnold (Department: 2817)
Modulators
Phase shift keying modulator or quadrature amplitude modulator
C332S170000, C375S296000, C455S126000, C455S118000, C455S260000, C455S296000, C455S295000
Reexamination Certificate
active
06720839
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to quadrature modulators, and, more specifically, to reducing or eliminating the unwanted sideband in a signal derived from the output of a quadrature modulator.
2. Background
In a quadrature modulator, identified with numeral
30
in
FIG. 1A
, a complex baseband signal, i.e., a baseband signal having I and Q components, BBI and BBQ, assumed to be in quadrature (out of phase by 90°), is mixed with a complex local oscillator signal, also having I and Q components, LOI and LOQ, and also assumed to be in quadrature, to form an output signal. The output signal may be at the desired RF transmit frequency in the case of a direct conversion transceiver, or it may be at an intermediate frequency which must in subsequent stages be unconverted to the desired RF transmit frequency.
The I component of the baseband signal, BBI, is mixed, through mixer
31
, with the I component of the local oscillator signal, LOI, and the Q component of the baseband signal, BBQ, is mixed, through mixer
32
, with the Q component of the local oscillator signal, LOQ. The outputs of mixers
31
and
32
are then combined, through combiner
33
, to form the output signal of the quadrature modulator.
If the I and Q components of the baseband signal, and the I and Q components of the local oscillator signal, are perfectly in quadrature, i.e., out of phase by 90° exactly, and the components of the quadrature modulator are perfectly accurate, only the “wanted” sideband will appear at the output of the quadrature modulator, and no energy will appear in the “unwanted” sideband. This follows mathematically as follows: assuming BBI can be represented as A cos(&ohgr;
BB
t+&pgr;/2), BBQ as A cos(&ohgr;
BB
t), LOI as B cos(&ohgr;
LO
t+&pgr;/2) , and LOQ as B cos(&ohgr;
LO
t) , then the output of the quadrature modulator, (BBI×LOI)+(BBQ×LOQ), reduces, through known mathematical identities, to A×B cos([&ohgr;
BB
+&ohgr;
LO
]t). This is the “wanted” sideband. As can be seen, there is no component at the “unwanted” sideband, i.e., at the frequency &ohgr;
LO
−&ohgr;
BB
.
However, in the real word, there will be some inaccuracy in the quadrature of the baseband or local oscillator signals or in the components of the quadrature modulator. That will result in some energy at the unwanted sideband frequency. This situation is illustrated in
FIG. 2B
, which is a frequency domain representation of the output of the quadrature modulator in the case in which there is some inaccuracy in the quadrature of the baseband or local oscillator signals or in the components of the quadrature modulator. Representing baseband as a single frequency, the wanted sideband is identified with numeral
50
, while the unwanted sideband is identified with numeral
51
. (Representing baseband as a range of frequencies, the wanted sideband is identified with numeral
52
, while the unwanted sideband is identified with numeral
53
).
Typically, as illustrated, the amplitude of the unwanted sideband is less than that of the wanted sideband. However, in extreme cases, when the quadrature inaccuracy is large, the amplitude of the unwanted sideband can approach that of the wanted sideband.
In the case in which the transmitted signal is a phase-modulated signal, the presence of the unwanted sideband in the output of the quadrature modulator translates into phase error in the transmitted signal. However, current GSM standards impose tight limits on the phase error of the transmitted signal.
In “An ISM band Transceiver Chip for Digital Spread Spectrum Communication”, ESSCIRC 97, a circuit for generating the LOI and LOQ inputs is described. The circuit is illustrated in FIG.
1
B. The LOI and LOQ signals are provided by a divide by two circuit comprising two D-type flip-flops
2
and
3
driven by VCO
1
. These signals are input to phase detector
4
, which outputs a current proportional to any deviation from quadrature in the LOI and LOQ signals. This current is integrated by integrator
5
to produce an error voltage. The error voltage is input to comparator
6
along with the output from VCO
1
. The error voltage is used to modify the mark-space ratio of the VCO output in order to correct for inaccuracies in the VCO and divide by two circuit.
There are several problems with this approach. First, it does not correct for intrinsic errors in the phase detector and comparator (see FIG.
1
B).
Second, it does not correct for any inaccuracies in the components of the quadrature modulator (mixers
31
and
32
, and combiner
33
, in FIG.
1
A).
Third, it does not correct for inaccuracies in the quadrature of the baseband signal.
Fourth, it requires a highly accurate phase detector in order to be effective.
Fifth, since it involves making corrections to the LO signal, a high frequency signal, it is difficult to achieve satisfactory results with this approach.
Accordingly, there is a need for a system for and method of reducing or eliminating the unwanted sideband in the output of a quadrature modulator which overcomes one or more of the disadvantages of the prior art.
SUMMARY OF THE INVENTION
In accordance with the purpose of the invention as broadly described herein, there is provided a system of and method for reducing or eliminating the unwanted sideband in the output of a quadrature modulator in which the presence of the unwanted sideband is detected through an unwanted sideband detector applied to the output of the quadrature modulator. In one embodiment, the system comprises a baseband correction circuit, a quadrature modulator, and an envelope detector. The I and Q components of the baseband signal, BBI and BBQ, are input to the baseband correction circuit. The outputs of the baseband correction circuit, BBI′ and BBQ′, are input to the quadrature modulator as are the I and Q components of the local oscillator signal. The output of the quadrature modulator is input to the unwanted sideband detector. The output of the unwanted sideband detector is input to the baseband correction circuit.
Any inaccuracy in the quadrature of the baseband or local oscillator signals, or in the components of the quadrature modulator, results in an unwanted sideband in the output of the quadrature modulator. The unwanted sideband detector detects the presence of an unwanted sideband detector in the output of the quadrature modulator, and provides a signal representative thereof to the baseband correction circuit. In one embodiment, the baseband correction circuit iteratively corrects the phase relationship of the I and Q components of the baseband signal responsive to the signal provided by the unwanted sideband detector until the relationship is about a quadrature relationship. In another embodiment, the process iterates one or more times until the unwanted sideband in the output of the quadrature modulator has been reduced to an acceptable level.
In one implementation, the unwanted sideband detector is an envelope detector. This implementation of the invention exploits the property that the output of the quadrature modulator can be expressed as the superposition of an AM signal, and an FM signal whose carrier frequency &ohgr;
c
is the frequency of the wanted sideband, &ohgr;
BB
+&ohgr;
LO
, and whose modulation frequency &ohgr;
m
is the frequency offset between wanted and unwanted sidebands, 2&ohgr;
BB
. Mathemetically, this property can be expressed as follows:
S
(
t
)
=
s
fm
(
t
)
+
s
am
(
t
)
=
sin
(
ω
c
t
)
+
(
B
/
2
)
(
sin
{
[
ω
c
+
ω
m
]
t
}
-
sin
{
[
ω
c
-
ω
m
]
t
}
)
+
sin
(
ω
c
t
)
+
(
M
/
2
)
(
sin
{
[
ω
c
+
ω
m
]
t
}
+
sin
{
[
ω
c
-
ω
m
]
t
}
)
Because of this property, the amplitude of the envelope of the output of the quadrature modulator at a given instant in time is proportional to or representative of the amplitude of the unwan
Clark Ricke W.
Damgaard Morten
Della Torre Valentina
Stubbe Frederic M.
Kinkead Arnold
Mintz Levin Cohn Ferris Glovsky & Popeo P.C.
Skyworks Solutions Inc.
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