Multichannel amplification system using mask detection

Details Multichannel amplification system using mask detection Multichannel amplification system using mask detection

1998-10-19

2002-12-10

Hunter, Daniel (Department: 2684)

Telecommunications

Transmitter

With feedback of modulated output signal

C330S149000, C330S151000, C327S359000

Reexamination Certificate

active

06493543

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates generally to amplification systems and more particularly to methods and apparatus for reducing distortion in amplifiers used in such systems.
As is known in the art, amplifiers have a wide variety of applications. Amplifiers can be biased to operate in one of a number of so-called Classes. When biased to operate in Class A, the amplifier provides a linear relationship between input voltage and output voltage. While operation in Class A has a wide range of applications, when higher power output and efficiency are required or desired, the amplifier is sometimes biased to operate in Class A/B. When biased to operate in Class A/B, however, the Class A/B amplifier power transfer curve
10
is less linear than for Class A amplifiers, illustrated in
FIG. 1
by trace
14
. To increase efficiency, communication systems often operate amplifiers in the non-linear region
12
. This practice, however, does introduce amplitude and phase distortion components into the output signal produced by the amplifier.
As is also known in the art, most communication systems have FCC allocated frequency bandwidths
18
(that is, in-band frequencies) centered about a carrier frequency
20
as shown in FIG.
2
A. For example, a CDMA (Code Division Multiple Access) communication system signal has a predefined bandwidth of 1.25 MHz. Different CDMA communication channels are allocated different bands of the frequency spectrum. Amplifiers are used in such systems, and are frequently biased to operate in Class A/B. Referring to
FIG. 2B
, signal processing such as amplification by an amplifier operating in the non-linear region
12
(
FIG. 1
) can produce distortion frequency “shoulders”
22
a
-
22
b
outside a signal's allocated bandwidth
18
. (These are called out-of-band frequencies.) These distortion frequency components
22
a
-
22
b
can interfere with bandwidths allocated to other communication signals. Thus, the FCC imposes strict limitations on out-of-band frequency components.
Many techniques exist to reduce out-of-band distortion. One such technique is shown in
FIG. 3
where a predistortion unit
24
is fed by a signal
25
to be amplified. The predistortion unit
24
has a power transfer characteristic
24
a
(
FIG. 1
) and compensates for distortion introduced by subsequent amplification in Class A/B amplifier
26
. More particularly, the predistortion unit
24
transforms electrical characteristics (for example, gain and phase) of the input signal such that subsequent amplification provides linear amplification to the phase and frequency characteristics of the input signal. The predistortion unit
24
is configured with a priori measurements of the non-linear characteristics of the Class A/B amplifier. Unfortunately, the amplifier characteristics (amplification curve
10
with region
12
of
FIG. 1
) change over time and temperature making effective predistortion more difficult. For example, as the temperature of the amplifier increases, its non-linear region
12
may become more or less linear, requiring a compensating change in the transform performed by a predistortion unit
24
. Some adaptive predistortion systems use look-up tables to alter predistorter characteristics based on environmental factors such as temperature. These look-up tables include predetermined predistorter control settings for use in predetermined situations. However, environmental factors alone do not determine the alterations in an amplifier's characteristics. Thus, over time, amplifier characteristics vary unpredictably due to aging of amplifier components.
Another approach to reduce amplifier distortion is to use feedforward compensation, as shown in FIG.
4
. Here, a feedforward network
31
is included for reducing out-of-band distortion. The feedforward network
31
includes a differencing network or combiner
30
, a main amplifier
33
operating as a Class A/B amplifier, an error amplifier
32
, delay circuits
28
and
28
a,
and a combiner
29
. The differencing network
30
produces an output signal representative of the difference between a portion of the signal fed to the amplifier
33
operated Class A/B and the signal fed to the amplifier
33
prior to such amplification. The frequency components in the differencing network
30
output signal are, therefore, the out-of-band frequency components
22
a
-
22
b
introduced by amplifier
33
. Amplifying and inverting the output produced by the differencing network
30
, by error amplifier
32
, produces an out-of-band correcting signal. More particularly, the combiner
29
combines the correcting signal produced by differencing network
30
and amplifier
32
, with the delayed signal output of amplifier
31
thus reducing the energy in the out-of-band frequencies
22
a
-
22
b
(
FIG. 2B
) of the signal output by amplifier
33
. Feedforward network
31
includes delay line
28
to compensate for the delay in error amplifier
32
. It should be noted that minute differences in timing between these elements can impair the effectiveness of a feedforward system. While a manufacturer can carefully match components prior to shipment, as feedforward components age, the correcting signal and processed signal can become mistimed if not properly compensated.
SUMMARY OF THE INVENTION
The invention relates to an apparatus and method for reducing out-of-band frequency components of an amplified, multichannel RF signal able to have at least two frequency bandwidth limited signal channels, each channel having a carrier frequency which is not known in advance. Each channel of the amplified RF signal has both in-band frequency components and out-of-band frequency components. The apparatus features a network for amplifying an input signal for producing the RF signal, the network having adjustable electrical characteristics, and a control system connected to the network for locating a frequency within the bandwidth of one of the channels of the RF signal and for detecting energy in the out-of-band frequency components in the one located channel for producing a control signal related to the energy in the out-of-band frequency components of that one channel. The control signal is coupled to the network to adjust the electrical characteristics of the network to reduce the energy in the out-of-band frequencies of all the channels.
In a preferred embodiment of the invention, the network has a predistorter having adjustable characteristics controlled by the control signal. In another aspect, the network can include a power amplifier having adjustable characteristics controlled by the control signal. In this instance, at least one adjustable characteristic is a bias point parameter of the amplifier. The network, in other aspects, can include a feed forward network and the signal can be, for example, a multichannel CDMA signal, having well known characteristics.
The method of the invention features locating an in-band frequency component of one of the channel signals having out-of-band frequency components of the RF signal, detecting energy at frequencies which are at a predetermined offset from the located in-band frequency component, and adjusting network electrical characteristics to reduce the out-of-band frequency energy of the located channel. Thereby, out-of-band frequency components of the other out-of-band channel signals are reduced.
In particular aspects, the method features measuring the energy at a first frequency, measuring the energy at a second frequency, and determining whether the energy measured at the second frequency exceeds the energy measured at the first frequency by more than a selected threshold. In particular aspects, the method heterodynes the multichannel signal having out-of-band frequencies to baseband.
In one particular aspect of the invention, the apparatus includes a predistorter connected to receive an amplified, multichannel input signal and having its output coupled to a power amplifier. The predistorter has a nonlinear output signal versus input signal transfer level characteristic which ca

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