Pulse or digital communications – Transmitters – Antinoise or distortion
Reexamination Certificate
2000-02-29
2003-12-02
Bayard, Emmanuel (Department: 2631)
Pulse or digital communications
Transmitters
Antinoise or distortion
C375S327000
Reexamination Certificate
active
06658065
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 the output of a transmitter comprising a quadrature modulator followed by a translational loop.
2. Related Applications
This application is related to U.S. patent application Ser. No. 09/515,633, entitled “SYSTEM OF AND METHOD FOR REDUCING OR ELIMINATING THE UNWANTED SIDEBAND IN A SIGNAL DERIVED FROM THE OUTPUT OF A QUADRATURE MODULATOR,” and U.S. patent application Ser. No. 09/515,538, entitled “SYSTEM OF AND METHOD FOR COMPENSATING A BASEBAND SIGNAL TO REDUCE THIRD ORDER MODULATION DISTORTION,” both of which are filed on even date herewith, both of which are owned in common by the assignee hereof, and both of which are hereby fully incorporated by reference herein as though set forth in full.
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
1
and LOQ
1
, and also assumed to be in quadrature, to form an output signal. The output signal is typically at an intermediate frequency and may be upconverted to the desired RF transmit frequency through various means such translational loop
120
, which upconverts the output of the quadrature modulator by the frequency of a second local oscillator signal, LO
2
. The frequency of the signal is thus placed at the desired transmit frequency TX, where TX=LO
1
+LO
2
.
In the quadrature modulator
30
, 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
1
as B cos (&ohgr;
LO1
t+&pgr;/2), and LOQ as B cos (&ohgr;
LO1
t), then the output of the quadrature modulator, (BBI×LOI
1
)+(BBQ×LOQ
1
), reduces, through known mathematical identities, to A×B cos ([&ohgr;
BB
+&ohgr;
LO1
]t). This is the “wanted” sideband. As can be seen, there is no component at the “unwanted” sideband, i.e., at the frequency &ohgr;
LO1
−&ohgr;
BB
.
However, in the real world, 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. This follows from the fact that the spectrum of the transmitted signal will simply be that of the signal output from the quadrature modulator, but translated upwards in frequency by the frequency of LO
2
. Thus, the frequency spectrum of the transmitted signal can be represented as shown in FIG.
2
C. As can be seen, the spectrum is identical to that shown in
FIG. 2B
, except that all components thereof have been translated upwards by LO
2
.
Current GSM standards impose tight limits on the phase error of the transmitted signal. For example, under current GSM standards, the energy of the unwanted sideband should be less than that of the energy of the wanted sideband by 40 dB or more. Such limits are difficult if not impossible to meet with current quadrature modulators.
In “An ISM band Transceiver Chip for Digital Spread Spectrum Communication”, ESSCIRC 97, a circuit for generating the LOI and LOQ inputs to a quadrature modulator 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 both detecting quadrature inaccuracy from and 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 followed by a translational loop which overcomes 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 transmitter comprising a quadrature modulator followed by a translational loop in which the presence of the unwanted sideband is detected through an unwanted sideband detector coupled to the translational loop. In one embodiment, the system comprises a baseband correction circuit, a quadrature modulator, a translational loop, and an unwanted sideband 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, LOI
1
and LOQ
1
. The output of the quadrature modulator is input to the translational loop. The signal for transmission is derived from the output of the translational loop. The unwanted sideband detector is coupled to the translational loop. The output of the unwanted sideband detector is input to the baseband correction circuit.
In one implementation, the unwanted sideband detector is coupled to a low frequency s
Ahmed Kashif A.
Clark Ricke W.
Della Torre Valentina
Stubbe Frederic M.
Bayard Emmanuel
Mintz Levin Cohn Ferris Clovsky & Popeo, P.C.
Skyworks Solutions Inc.
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