WO2003017478A2

WO2003017478A2 - Feed forward compensation circuit

Info

Publication number: WO2003017478A2
Application number: PCT/CA2002/001254
Authority: WO
Inventors: Calin Moldoveanu
Original assignee: Redline Communications Inc.
Priority date: 2001-08-14
Filing date: 2002-08-13
Publication date: 2003-02-27
Also published as: WO2003017478A3; EP1425848A2; AU2002322892A1

Abstract

A feed forward compensation circuit for a radio frequency power amplifier includes a nulling loop and a compensation loop. The nulling loop includes active components that provide feedback to the amplitude and phase adjusters. This feedback compensates for component drift and ensures stable operation. In a preferred embodiment the active components include operational amplifiers that compare input and output signals to determine when adjustments are needed.

Description

FEED FORWARD COMPENSATION CIRCUIT

Field of the Invention

The present invention relates to feed forward compensation circuits.

Background of the Invention

It is well known to use feed forward compensation circuits to compensate for the non-linearity of radio frequency (RF) power amplifiers. Typically such circuits included components that can be trimmed to tune the circuit for operation. One of the main drawbacks of the feed forward scheme is the difficulty in maintaining an optimum operating state due to the drift of parameters of the components.

Summary of the Invention

An object of the present invention is to provide an improved feed forward compensation circuit.

In accordance with an aspect of the present invention there is provided a feed forward compensating circuit for coupling to a radio frequency power amplifier having an input and an output comprising: a first loop for generating an error signal representation of signal distortion due to the power amplifier; and a second loop for the applying a compensating signal derived from the error signal to the output. The first loop including a first comparator for controlling amplitude adjustment therein and a second comparator for controlling phase adjustment therein.

In accordance with another aspect of the present invention there is provided A method of compensating for radio frequency power amplifier distortion comprising the steps of: determining a difference between an input signal to the RF power amplifier and an output from the RF power amplifier.

Brief Description of the Drawings

Fig. 1 illustrates a known feed forward compensation circuit; Fig. 2 illustrates a feed forward compensation circuit in accordance with an embodiment of the present invention; and

Fig. 3 graphically illustrates various signals within the circuit of Fig. 2 when operational.

Detailed Description of the Preferred Embodiment

Referring to Fig. 1, there is illustrated a known feed forward compensation circuit. The feed forward compensation circuit 10 includes an input 12, and output 14, a nulling loop 16 and a compensation loop 18. The nulling loop 16 includes a first splitter 20, in front of a power amplifier 22, a second splitter 24 after the power amplifier 22, an amplitude adjuster 26 a phase adjuster 28 and a comparator 30. The compensation loop 18 includes an amplitude adjuster 32, a phase adjuster 34, and an amplifier 36 and a combiner 38.

In operation, the nulling loop samples an RF signal at the input 12 and at an output of amplifier 22 via splitters 20 and 24. Because of non-linearity of the power amplifier 22, distortions are introduced in the RF amplification chain between the input 12 and the output 14. The nulling loop 16, by comparing the input signal to the output of the power amplifier, generates an error signal, which when added with the correct phase and amplitude to the signal as output by the power amplifier 22, can theoretically, cancel out the distortion introduced by the power amplifier.

In the compensation loop the error signal is controlled in amplitude by the amplitude adjuster 32 and by the phase adjuster 34, amplified by the error amplifier

(EA) and summed up with the signal generated at the output of the power amplifier PA in combiner 38. With a proper adjustment of the amplitude and phase, the error signal is injected with equal amplitude and opposite phase, compared with the distortions generated during the amplification process, thus resulting in a distortion cancellation effect. One of the main drawbacks of the feed forward scheme is the difficulty in maintaining an optimum operation state due the drift of the parameters of the various components. Also there is no straightforward manner to devise proper quantitative optimization criteria.

This problem is especially critical in the nulling loop where the error signal results from a small difference between two much larger signals (typically, both the reference and the output PA samples are 15dB to 20dB higher than the resulting error signal). Even a minor drift of the loop components can result in significant change of the error signal that could dramatically alter the balance of the loop or may even damage the error amplifier.

Referring to Fig. 2, there is illustrated a feed forward compensation circuit in accordance with an embodiment of the present invention. The feed forward compensation circuit 50 includes an input 52, an output 54, a nulling loop 56 and a compensation loop 58. The nulling loop 56 is established by a first splitter 60 before a power amplifier 62 and a second splitter 64 after the power amplifier 62.

A first path from the first splitter 60 includes a first amplitude adjuster 66, a first phase adjuster 67 and a third splitter 70. A second path from the second splitter

64 includes a splitter 72 and a first detector 74. A second detector 76 is connected to the third splitter 70. Detectors 76 and 74 are coupled to the differential inputs of an operational amplifier 78 whose output controls the first amplitude adjuster 66.

A ninety-degree combiner 80 has its outputs S2, S3 coupled to third and fourth detectors 82 and 84 whose outputs are coupled to a second operational amplifier 86 whose output controls the phase controller 68. Input to the 90° combiner 80 is provided by fifth and sixth splitters 88 and 90 connected to splitters 70 and 72, respectively. The other outputs of splitters 88 and 90 are connected to 3dB combiner

92. The compensation loop 58 includes the 3dB combiner 92, amplitude adjuster 94, phase adjuster 96 and an error amplifier 98, series connected. The output of error amplifier 98 is added to the output of splitter 64 via a combiner 100. In operation, the third and fourth detectors 82 and 84 are configured to maintain an equal level of signals R and D applied to comparison network made of the 3dB splitters 88, 90 and 92 and the 3dB 90° combiner 80. Operation of this combination can be further understood from Figure 3 which shows the change of amplitude of the signals at the connections when the phase between signals R and D is modified between 0° and 360°. As can be seen the signal at the output of 3dB combiner 92 is minimum when signals output from comparator 80 are equal. The slope of the variation of the signals output from comparator 80 are opposite which make it possible to generate a differential DC signal which is 0 when the error signal is minimum.

Numerous modifications, variations and adaptations may be made to the particular embodiments of the invention described above without departing from the scope of the claims, which is defined in the claims.

Claims

What is claimed is:

1. A feed forward compensating circuit for coupling to a radio frequency power amplifier having an input and an output comprising:

a first loop for generating an error signal representation of signal distortion due to the power amplifier; and

a second loop for the applying a compensating signal derived from the error signal to the output;

the first loop including a first comparator for controlling amplitude adjustment therein and a second comparator for controlling phase adjustment therein.

2. A feed forward compensating circuit as claimed in Claim 1 wherein the first loop includes first and second splitters for sampling an input signal and an output signal, respectively.

3. A feed forward compensating circuit as claimed in Claim 1 wherein the first comparator includes a first operational amplifier.

4. A feed forward compensating circuit as claimed in Claim 1 wherein the second comparator includes a second operational amplifier.

5. A feed forward compensating circuit as claimed in Claim 1 wherein the first loop includes third and fourth splitters for providing samples of the input and output signals to the second comparator and the second loop, respectively.

6. A feed forward compensating circuit as claimed in Claim 5 wherein the second comparator is coupled to the third and fourth splitters via a hybrid circuit.

7. A feed forward compensating circuit as claimed in Claim 6 wherein the hybrid circuit has output for orthogonal signals.

8. A method of compensating for radio frequency power amplifier distortion comprising the steps of:

determining a difference between an input signal to the RF power amplifier and an output from the RF power amplifier, to derive an error signal, monitoring the error signal to detect changes in amplitude and phase due to component drift and adjuster amplified and phase of the error signal in dependence upon detected changes.