Digital branch calibrator for an RF transmitter

• US 7,280,612 B2
• Filed: 07/25/2003
• Issued: 10/09/2007
• Est. Priority Date: 07/25/2003
• Status: Expired due to Fees

• Current Assignee: Zarbana Digital Fund LLC (Intellectual Ventures LLC)
• Original Assignee: Zarbana Digital Fund LLC (Intellectual Ventures LLC)
• Inventors: Saed, Aryan
• Primary Examiner(s): Fan; Chieh M.
• Assistant Examiner(s): Lee; Siu M
• Application Number:
  US10/627,881
• Publication Number:
  US 20050018787A1
• Time in Patent Office: 1,537 Days
• Field of Search: 375/146, 375/221, 375/222, 375/226, 375/268, 375/271, 375/295, 375/300, 375/302, 375/296, 375/232, 375/233, 330/149, 330/151, 330/52, 330/124.R, 330/295, 330/136, 330/2, 455/114.3, 455/163.1, 455/125
• US Class Current: 375/296
• CPC Class Codes: H04L 27/2626 Arrangements specific to the transmitter only
• CPC Class Codes: H04L 27/368 adaptive predistortion

Abstract

The present invention provides a digital (computational) branch calibrator which uses a feedback signal sensed from an RF transmit signal path following the combining stage of LINC circuitry of a transmitter to compensate for gain and phase imbalances occurring between branch fragment signals leading to the combiner. The calibrator feeds a quiet (zero) base band signal through the transmit path during the calibration sequence (i.e. a period when data is not transmitted) and adjusts the phase and gain of the phasor fragment signals input thereto by driving the sensed output power to zero. The calibration is performed by alternating phase and gain adjustments with predetermined (programmable) and multiple update parameters stages (speeds). A baseband modulation is preferably used to distinguish false leakage (e.g. due to local oscillator, LO, feed through and DC offset in the base band Tx) from imbalance leakage.

Claims

1. A digital branch calibrator for use in an RF transmitter to compensate for phase and/or gain imbalances between two phasor fragment signals in a transmit path from a phasor fragmenter in a digital front end of said transmitter to a power amplification and combining component in an analog front end of said transmitter outputting an RF transmit signal based on a sum of said fragment signals, said calibrator comprising:

(a) a closed loop controller operable during a calibration sequence, defined by a predetermined number of control loop iterations, and comprising transmit and feedback signal paths, said transmit signal path to transmit a zero base band transmit signal and said feedback signal path to receive a feedback signal correlated with a power level of said output RF transmit signal, said transmit signal path comprising a phase and/or gain adjusting component to adjust the phase and/or gain of said phasor fragment signals to minimize said power level, said adjusting being performed iteratively to the end of said calibration sequence and resulting in sequence phase and/or gain update signals, wherein said sequence update signals are provided for updating the phase and/or gain of data signals transmitted through said transmit signal path; and

(b) digital signal processing means to operate said controller and to control said transmission of said zero base band transmit signal for processing by said calibrator;

wherein said closed loop controller further comprises false imbalance removal means to remove from said feedback signal any portion thereof correlating to local oscillator and/or other non-imbalance feed through energy at the carrier frequency, said false imbalance removal means comprising a digital modulator/demodulator to modulate said zero base band signal by a sub-carrier frequency signal and to demodulate said feedback signal; and

wherein said phase and/or gain adjusting component comprises a complex accumulator to decimate said feedback signal.

2. A calibrator according to claim 1 wherein said phase and/or gain adjusting component is adapted to alternate iterations of phase adjustments and iterations of gain adjustments.

3. A calibrator according to claim 1 wherein said phase adjusting produces a phase gradient calculated from a magnitude of said feedback signal, a sign of a differential of a phase adjustment from one iteration to a next iteration and a sign of a differential of the magnitude of said feedback signal from one iteration to the next iteration and said gain adjusting produces a gain gradient calculated from the magnitude of said feedback signal, the sign of the differential of a gain adjustment from one iteration to the next iteration and the sign of the differential of the magnitude of said feedback signal from one iteration to the next iteration.

4. A calibrator according to claim 1 wherein said gain gradient(s) are calculated so as to limit the magnitudes of said phasor fragment signals to a predetermined maximum value L and so that the magnitude of at least one of said phasor fragment signals has the value L.

5. An RE transmitter having a Linear amplification with Nonlinear Components (LINC) architecture comprising;

a digital front end with a fragmenter to fragment an input signal into a plurality of output fragment signals which sum to said input signal; and

an analog front end of to amplify and combine said fragment signals;

an in-phase (I), quadrature-phase (Q) signal pre-balancing component (IQPBAL) in the digital front end transmit path to mitigate I/Q phase and/or gain imbalances on each said fragment signal;

a digital branch calibrator for use in an RF transmitter to compensate for phase and/or gain imbalances between two phasor fragment signals in a transmit path from a phasor fragmenter in a digital front end of said transmitter to a power amplification and combining component in an analog front end of said transmitter to output an RE transmit signal based on a sum of said fragment signals, the calibrator including;

(a) a closed loop controller operable during a calibration secluence, defined by a predetermined number of control loop iterations, and comprising transmit and feedback signal paths, said transmit signal path to transmit a zero base band transmit signal and said feedback signal path to receive a feedback signal correlated with a power level of said output RF transmit signal, said transmit signal path comprising a phase and/or gain adjusting component to adjust the phase and/or gain of said phasor fragment signals to minimize said power level, said adiusting being performed iteratively to the end of said calibration sequence and resulting in sequence phase and/or gain update signals, where said sequence update signals arc provided to update the phase and/or gain of data signals transmitted through said transmit signal path; and

(b) digital signal processing means to operate said controller and to control said transmission of said zero base band transmit signal for processing by said calibrator;

wherein said closed loop controller further comprises false imbalance removal means to remove from said feedback signal any portion thereof correlating to local oscillator and/or other non-imbalance feed through energy at the carrier frequency, said false imbalance removal means comprising a digital modulator/demodulator to modulate said zero base band signal by a sub-carrier frequency signal and to demodulate said feedback signal; and

a DC removal component configured to remove DC signal components of said feedback signal, said DC removal component comprising means for estimating the DC signal level and means for removing said DC estimation from said feedback signal.

6. A method for compensating for phase and/or gain imbalances between two phasor fragment signals in a transmit path of an RF transmitter outputting an RF transmit signal based on a stun of said fragment signals, said method comprising:

(a) transmitting a zero base band transmit signal along a transmit path during a calibration sequence defined by a predetermined number of iterations;

(b) receiving a feedback signal correlated with a power level of said output RF transmit signal; and

,

(c) adjusting phase and/or gain of said phasor fragment signals to minimize said power level, said adjusting being performed iteratively to an end of said calibration sequence and resulting in sequence phase and/or gain update signals, where said sequence update signals are provided to update the phase and/or gain of data signals transmitted through said transmit signal path; and

(d) removing from said feedback signal any portion thereof correlating to local oscillator and/or other non-imbalance feed through energy at the carrier frequency, said removing including modulating said zero base band signal by a sub-carrier frequency signal and demodulating said feedback signal;

where said phase and/or gain adjusting includes alternating iterations of phase adjustments and iterations of gain adjustments.

7. A method to compensate for phase and/or gain imbalances between two phasor fragment signals in a transmit path of an RF transmitter outputting an RE transmit signal based on a sum of said fragment signals, said method comprising:

(a) transmitting a zero base band transmit signal along a transmit path during a calibration sequence defined by a predetermined number of iterations;

(b) receiving a feedback signal correlated with a power level of said output RE transmit signal; and

,

(c) adjusting a phase and/or gain of said phasor fragment signals to minimize said power level, said adjusting being performed iteratively to an end of said calibration sequence and resulting in sequence phase and/or gain update signals, where said sequence update signals are provided to update the phase and/or gain of data signals transmitted through said transmit signal path; and

(d) removing from said feedback signal any portion thereof correlating to local oscillator and/or other non-imbalance feed through energy at the carrier frequency, said removing including modulating said zero base band signal by a sub-carrier frequency signal and demodulating said feedback signal;

where said adjusting includes producing a phase gradient calculated from a magnitude of said feedback signal, a sign of a differential of a phase adjustment from one iteration to a next iteration and a sign of a differential of the magnitude of said feedback signal from one iteration to the next iteration and where said adjusting includes producing a gain gradient calculated from the magnitude of said feedback signal, a sign of a differential of the a gain adjustment from one iteration to the next iteration and the sign of the differential of the magnitude of said feedback signal from one iteration to the next iteration.

8. A method to compensate for phase and/or gain imbalances between two phasor fragment signals in a transmit path of an RF transmitter outputting an RF transmit signal based on a sum of said fragment signals. said method comprising:

(a) transmitting a zero base band transmit signal along a transmit path during a calibration sequence defined by a predetermined number of iterations;

(b) receiving a feedback signal correlated with a power level of said output RF transmit signal; and

,

(c) adiusting a phase and/or gain of said phasor fragment signals to minimize said power level, said adiusting being performed iteratively to an end of said calibration sequence and resulting in sequence phase and/or gain update signals, where said sequence update signals are provided to update the phase and/or gain of data signals transmitted through said transmit signal path and

(d) removing from said feedback signal any portion thereof correlating to local oscillator and/or other non-imbalance feed through energy at the carrier frequency, said removing including modulating said zero base band signal by a sub-carrier frequency signal and demodulating said feedback signal

where said gain update signal(s) are calculated so as to limit magnitudes of said phasor fragment signals to a predetermined maximum value L and so that a magnitude of at least one of said phasor fragment signals has the value L.