Amplifiers – With pilot frequency control means
Reexamination Certificate
2002-06-11
2004-06-15
Mottola, Steven (Department: 2817)
Amplifiers
With pilot frequency control means
C330S151000
Reexamination Certificate
active
06750706
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an FF distortion compensation amplifier comprising a feedforward (hereinafter referred to as “FF”) loop that compensates for distortion occurring in a main amplifier, for example, intermodulation distortion, and in particular, to a control circuit and method that optimizes the FF loop.
2. Related Art of the Invention
Mobile communications base stations or the like transmit by radio multicarrier signals which have predetermined frequency intervals and which are each appropriately modulated, after radio frequency amplification. If an amplifier for use in radio-frequency amplification has insufficient linearity, various types of distortion for example, intermodulation distortion may occur, Such distortion hinders realization of normal and high-quality communications. Thus, for amplification of a multicarrier signal, the entire frequency band to which the multicarrier signal belongs must be appropriately linear, and a strict tolerance is specified for the distortion.
As an approach to implementation of a distortion compensation amplifier suitable for amplification of a multicarrier signal, an FF amplifying method is described in Japanese Patent Laid-Open No. 2000-196366 or the like.
With this FF amplifying method, if on a signal path from a signal input end through a main amplifier to a signal output end, i.e. a signal path over which signals to be amplified or amplified signals are transmitted (this signal path is hereinafter referred to as a “main line”), a signal branched from a point located after the main amplifier and a signal branched from a point located before the main amplifier on the main line travel an equal electric length and have the same amplitude and opposite phases, then these signals can be coupled together to cause their carrier components to cancel each other to take out a signal corresponding to distortion caused by the main amplifier and its peripheral circuit.
The thus taken-out signal, which corresponds to the distortion, is passed through a distortion compensation loop and recoupled to the signal on the main line. When adjustment of amplitude or phase is appropriately carried out in the distortion compensation loop or main line so that a signal delay occurring on the main line is compensated for by a signal delay in the distortion compensation loop or that a distortion component contained in the signal on the main line and the signal obtained from the distortion compensation loop have the same amplitude and opposite phases, the above described signal recoupling operation compensates for the distortion occurring in the main amplifier.
FIG. 8
shows an example of a configuration of a conventional FF amplifier. In this figure, for example, a multicarrier signal input through a signal input end IN is branched into two by a hybrid HYB
1
. One of the branched signals is amplified by the main amplifier A
1
and then reaches a hybrid HYB
2
. The other signal is supplied to the hybrid HYB
2
via a delay line D
1
. The delay line D
1
compensates for a signal delay that may occur in the main amplifier A
1
, and a signal delayed via the delay line D
1
is coupled by the hybrid HYB
2
to a signal containing distortion occurring in the main amplifier A
1
.
As described above, carrier components are mutually cancelled to take out (detect) distortion occurring in the main amplifier A
1
by coupling a signal branched from an output signal from the main amplifier A
1
to a signal obtained via the delay line D
1
as described above. To achieve this, upon the coupling at the hybrid HYB
2
, the carrier components of the two signals must have opposite phases and the same amplitude and follow the same timing. The delay line D
1
is means of allowing carrier components to follow the same timing, and a variable attenuator ATT
1
, a variable phase shifter PS
1
, and a control circuit
110
that adjusts and controls a signal attenuation G
1
and a phase shift &thgr;
1
in the variable attenuator ATT
1
and the variable phase shifter PS
1
, respectively, to optimum values are means of allowing carrier components to have opposite phases and the same amplitude.
Next, in the FF amplifier shown in
FIG. 8
, carrier components amplified by the main amplifier A
1
and containing distortion components are delivered to the hybrid HYB
2
. Then, in a distortion compensation loop L
2
, a signal containing no carrier signals but only distortion components is supplied to a hybrid HYB
3
via the delay line D
2
. Simultaneously, the same signal is amplified by an auxiliary amplifier A
2
and supplied to a hybrid. In the distortion compensation loop L
2
, the two signals have opposite phases and the same amplitude and follow the same timing upon coupling at HYB
3
in order to compensate for (cancel) the distortion by coupling the signal from the delay line D
2
and the signal from the auxiliary amplifier A
2
together. The delay line D
2
is means of allowing distortion components to follow the same timing, and the control circuit
110
that adjusts and controls a signal attenuation G
2
in a variable attenuator ATT
2
and a phase shift &thgr;
2
in a variable phase shifter PS
2
to optimum values is means of allowing distortion components to have opposite phases and the same amplitude.
In the FF amplifier shown in
FIG. 8
, an optimization process in the distortion compensation loop L
2
is executed by inserting and detecting a pilot signal as described below. The control circuit
110
comprises a synchronous detector
138
, an oscillator OSC
2
that serves to generate a pilot signal, and an in-phase divider
128
that divides the signal from the oscillator OSC
2
into two: a pilot signal and a reference signal REF. In the thus constructed distortion compensation loop L
2
, to cancel distortion by coupling a pilot signal from the delay line D
2
and a pilot signal from the auxiliary amplifier A
2
together, an output signal from the synchronous detector
138
adjusts and controls the amplitude attenuation G
2
in the variable attenuator ATT
2
and the phase shift &thgr;
2
in the variable phase shifter PS
2
to optimum values.
With the circuit constructed as described above, an FF amplifier can be actualized which is suitable for amplification of a multicarrier signal.
However, in the conventional example shown in
FIG. 8
, only one pilot signal is used, which has a frequency located a certain distance above or below the band in which the amplifier is operated. Accordingly, if the pilot signal has a frequency located above the operating band, the capability of removing or suppressing distortion in this frequency or frequencies located close thereto is optimized, whereas for a frequency band located below the band in which the amplifier is actually operated, the capability of removing or suppressing distortion is not always optimized. Thus, it has been desired to use both a pilot signal having a frequency located below the operating band and a pilot signal having a frequency located above the operating band.
Furthermore, according to an example of a conventional configuration such as the one shown in
FIG. 8
, an output signal from the distortion compensation loop L
2
is supplied from a directional coupler DC
4
to a band-pass filter BPF
3
to extract only a pilot signal therefrom, which is then fed to the synchronous detector
38
as an error signal ERR. However, the output signal taken out from the directional coupler DC
4
contains an amplified carrier signal in spite of the cancellation in hybrid HYB
2
. A filter with a very steep characteristic is required to remove this carrier signal component to extract a weak pilot signal. However, implementation of such a filter requires the physical size thereof to be increased, thereby making it difficult to miniaturize the circuit.
Further, to avoid this problem, it is contemplated that an output signal from the distortion compensation loop L
2
may be down-converted so as to have a frequency in an IF band before filtering. However, this requires extra oscillators
Ishida Kaoru
Matsuyoshi Toshimitsu
Saito Yuji
Takachi Naoki
Matsushita Electric - Industrial Co., Ltd.
Mottola Steven
RatnerPrestia
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