Pipelined digital predistorter for a wideband amplifier

• US 5,650,758 A
• Filed: 11/28/1995
• Issued: 07/22/1997
• Est. Priority Date: 11/28/1995
• Status: Expired due to Fees

• Current Assignee: Radio Frequency Systems Incorporated (Amphenol Corporation)
• Original Assignee: Radio Frequency Systems Incorporated (Amphenol Corporation)
• Inventors: Xu, Xiangqing, Herzog, James H., Larkin, Robert S.
• Primary Examiner(s): MOTTOLA, STEVEN J
• Application Number:
  US08/563,613
• Time in Patent Office: 602 Days
• Field of Search: 330/107, 330/129, 330/149, 330/151, 332/123, 332/124, 332/159, 332/160
• US Class Current: 330/149
• CPC Class Codes: H03F 1/3247 using feedback acting on predistortion circuits H03F1/3264 takes precedence
• CPC Class Codes: H03F 1/3258 based on polynomial terms
• CPC Class Codes: H03F 1/3294 Acting on the real and imaginary components of the input signal
• CPC Class Codes: H03F 2200/438 Separate feedback of amplitude and phase signals being present
• CPC Class Codes: H03F 2201/3233 Adaptive predistortion using lookup table, e.g. memory, RAM, ROM, LUT, to generate the predistortion

Abstract

An apparatus for linearizing the output of a wideband amplifier by digitally predistorting the input to the amplifier in a way that just compensates for the distortion in the amplified signal, comprising a pipeline controller (12), for providing pipeline controller signals, and a pipelined predistorter (11), consisting of several stations, serially connected to each other, wherein each station performs one or more tasks concurrently, under the direction of the pipeline controller (12). The result is a digital predistorter with a bandwidth large enough to linearize, by itself, the entire output of a wideband amplifier. More generally, an apparatus for linearizing a wideband amplifier by using digital signal processing performed using a pipeline architecture, as opposed to using a sequential processing architecture, under the supervision of a pipeline controller.

Claims

1. An apparatus for predistorting analog input signals (S) to be subsequently amplified by an amplifier (16) that otherwise distorts the input signals (S) at the amplifier output, such that the predistorting of the analog input signals reduces the distortion of the amplifier output, comprising:

a) a pipeline controller (12), for providing pipeline controller signals (C_p), and

b) a pipelined predistorter (11), responsive to the analog input signals (S), and further responsive to the pipeline controller signals (C_p), for generating, using digital signal processing, predistorted analog input signals (S_d) that compensate for the distortion of the amplifier, wherein the pipelined predistorter includes two or more stations serially connected, each station performing a portion of the digital signal processing used to predistort the input signals (S), and each station acting under the direction of the pipeline controller, concurrently with all the other stations,

whereby digital signal processing is achievable for analog input signals (S) having a bandwidth significantly greater than otherwise achievable in a non-pipelined predistorter.

2. An apparatus, as claimed in claim 1, wherein the analog input signals (S) are an analog I-channel input (I) and an analog Q-channel input (Q), and further wherein the predistorted analog input signals (S_d) are a predistorted analog I-channel input (I_d) and a predistorted analog Q-channel input (Q_d).

3. An apparatus as claimed in claim 2, further comprising:

a) a quadrature demodulator (14) responsive to a wideband input signal (X), for providing a baseband analog Q-channel input (Q) and a baseband analog I-channel input (I),

b) a quadrature modulator (15) responsive to the predistorted analog Q-channel input (Q_d) and the predistorted analog I-channel input (I_d), for providing a predistorted wideband input signal (X_d), and

c) a wideband amplifier (16) responsive to the predistorted wideband input signal (X_d), for providing an amplified wideband signal (Y).

4. An apparatus as claimed in claim 3, further comprising an adaptive controller (13) responsive to a feedback (Y_b) of the amplified signal (Y), and responsive to a feedforward (X_f) of the input signal (X), for providing predistortion values (D) adjusted to account for changes in how the wideband amplifier (16) responds to an input, and for providing adaptive controller signals (C_a), and further wherein the pipelined predistorter (11) is responsive to the predistortion values (D) and, indirectly, through the pipeline controller (12), to the adaptive controller signals (C_a), and utilizes the predistortion values (D) as commanded by the adaptive controller signals (C_a), thereby adapting to changes in how the wideband amplifier (16) responds to an input.

5. An apparatus, as claimed in claim 4, wherein the pipelined predistorter (11) comprises:

a) a first station (31), responsive to the pipeline controller signals (C_p), including an A/D converter (17,

     18) for each of the analog input signals (I, Q), each A/D converter further responsive to one of the analog input signals, and each A/D converter providing a sequence of digital input signals (i or q) corresponding to the analog input signal it converts,

b) a second station (32), responsive to the pipeline controller signals (C_p), including an R/P converter (20), responsive to pairs of digital input signals (i, q) associated with the pair of analog input signals (I, Q), for providing, from each pair of digital signals (i, q), which are a rectangular coordinate representation of a signal, a polar coordinate representation in terms of an amplitude signal (ρ

) and a phase signal (θ

),

c) a third station (33), responsive to the pipeline controller signals (C_p), including

1. a phase difference predistorter (21), responsive to the amplitude signal (ρ

), for providing a predistorted phase difference signal (Δ

θ

_d), and

2. an amplitude predistorter (22), also responsive to the amplitude signal (ρ

), for providing a predistorted amplitude signal (ρ

_d),

d) a fourth station (34), responsive to the pipeline controller signals (C_p), including a phase predistorter (23), responsive to the predistorted phase difference signal (Δ

θ

_d), and responsive to the phase signal (θ

), for providing a predistorted phase signal (θ

_d),

e) a fifth station (35), responsive to the pipeline controller signals (C_p), including a P/R converter (24), responsive to the predistorted phase signal (θ

_d), further responsive to the predistorted amplitude signal (ρ

_d), for providing a predistorted I-channel digital signal (i_d) and a predistorted Q-channel digital signal (q_d), the I-channel and Q-channel signals being a rectangular representation of the digital signal represented by the predistorted phase signal (θ

_d) and the predistorted amplitude signal (ρ

_d), as a pair, and

f) a sixth station (36), responsive to the pipeline controller signals (C_p), including a D/A converter (25,

     26) for each predistorted digital signal (i_d, q_d), each responsive to one of the predistorted digital signals (i_d, q_d), and each providing a predistorted analog input (I_d, Q_d) corresponding to the predistorted digital signal it converts.

6. An apparatus as claimed in claim 5, wherein the predistortion values (D) occur in two versions, one in use by the pipelined predistorter (11), and one in a state of adjustment by the adaptive controller (13).

7. An apparatus as claimed in claim 6, wherein each set of predistortion values (D) is stored in two tables, one for predistorting the phase signal (θ

) and one for predistorting the amplitude signal (ρ

).

8. An apparatus as claimed in claim 7, wherein the adaptive controller (13) continually adjusts one of the sets of tables of predistortion values (D), and, upon completing adjustment of a set of tables, issues a command to the pipeline controller (12) to signal the pipelined predistorter (11) to stop using the active set and to start using the just-adjusted set.

9. An apparatus used for predistorting a wideband analog input signal (S) to be subsequently amplified by an amplifier (16) that otherwise distorts the input signal (S) at the amplifier output, such that the predistorting of the analog input signal reduces the distortion of the amplifier output, the apparatus comprising:

a) means (14, 17-20), responsive to the pipeline controller signals (C_p) and to the analog input signal (S), for generating a phase signal (θ

) and an amplitude signal (ρ

) representing the phase and amplitude of the analog input signal (S),

b) means (21,

     23), responsive to the pipeline controller signals (C_p) and to the amplitude signal (ρ

) and to the phase signal (θ

), for generating a predistorted phase signal (θ

_d),

c) means (22), responsive to the pipeline controller signals (C_p) and to the amplitude signal (ρ

), for generating a predistorted amplitude signal (ρ

_d),

d) means (24, 25, 26

     15), responsive to the pipeline controller signals (C_p) and to the predistorted phase signal (θ

_d) and to the predistorted amplitude signal (ρ

_d), for generating a predistorted analog input signal (S_d), and

e) means (12), communicating with the means of elements a-d, for generating the pipeline controller signals (C_p) to cause the concurrent operation of the means of elements a-d.

10. An apparatus, as claimed in claim 9, wherein the means of element a) of claim 9 further comprises:

a) means (17,

     18), responsive to the pipeline controller signals (C_p) and to the analog input signal (S), for converting the analog input signal (S) into a first digital signal (i) that represents a first quality of the analog input signal, and a second digital signal (q) that represents a second quality of the analog input signal, and

b) means (20), responsive to the pipeline controller signals (C_p) and to the first and second digital signals (i, q), for generating a phase signal (θ

) and an amplitude signal (ρ

) representing the phase and amplitude of the analog input signal (S).

11. An apparatus for linearizing a wideband amplifier (16), comprising:

a) a pipeline controller (12), for providing pipeline controller signals (Cp), and

b) a pipelined predistorter (11), consisting of a plurality of stations (31, 32, 33, 34, 35,

     36), serially connected to one another, wherein each station performs one or more digital signal processing tasks, wherein the pipelined predistorter (11) is responsive to an analog input signal (S), and further responsive to the pipeline controller signals (C_p) that orchestrate the concurrent operation of each station, for providing predistorted analog input signals (S_d), so that the output of the wideband amplifier (16) is approximately a constant factor of the analog input signal (S) and in phase with it.