D.C. offset estimation

Patent metadata

Field	Value
Publication number	US-8976692-B2
Application number	US-201113522662-A
Country	US
Kind code	B2
Filing date	Jan 20, 2011
Priority date	Jan 20, 2010
Publication date	Mar 10, 2015
Grant date	Mar 10, 2015

Abstract

A combination of a phase shifter, a measurement receiver, and an offset estimator enable the d.c. offset in the transmit path of a quadrature transmitter to be distinguished from the d.c. offset in the measurement receiver. The measurement receiver performs a first measurement on the transmit path output with a “normal” phase shift of 0 degrees and 90 degrees for in-phase (I) and quadrature (Q) components, and a second measurement with a “special” phase shift for the I and Q components. In one embodiment, the “special” phase shift for the I and Q components is 180 degrees and 270 degrees, respectively.

First claim

What is claimed is:

1. An apparatus for estimating a direct-current (d.c.) offset in a transmitter having a transmit path for quadrature modulating a carrier with input in-phase (I) and quadrature (Q) component signals and generating a transmit signal, the apparatus comprising:
a measurement receiver, wherein the measurement receiver is configured to demodulate a portion of the transmit signal to generate an I component measurement signal and a Q component measurement signal;
a phase shifter, wherein the phase shifter is configured to generate a first pair of oscillator signals having a relative phase shift of about 90 degrees for quadrature modulation in the transmit path, to generate a second pair of oscillator signals having a relative phase shift of about 90 degrees for demodulation in the measurement receiver, and to selectively generate a third pair of oscillator signals having a relative phase shift of about 90 degrees and a phase shift with respect to the second pair of oscillator signals for demodulation in the measurement receiver; and
an offset estimator, wherein the offset estimator is configured to compute at least one of a d.c. offset of the transmit path and a d.c. offset of the measurement receiver based on the input I and Q component signals and on measurement I and Q component signals generated with the pairs of oscillator signals.

2. The apparatus of claim 1 , wherein the phase shifter is configured for at least one of a phase shift of about 180 degrees between the third pair of oscillator signals and the second pair of oscillator signals and a phase shift of about 0 degrees between the first pair of oscillator signals and the second pair of oscillator signals.

3. The apparatus of claim 1 , wherein the phase shifter is configured to generate the second and third pairs of oscillator signals by alternating a phase of a controllable oscillator signal.

4. The apparatus of claim 2 , wherein the offset estimator is configured to compute an I component d.c. offset of the measurement receiver according to:

i

DC
,
MRX

=

i
meas

+

i
⋒

meas

2

in which i DC,MRX is the I component of the d.c. offset of the measurement receiver, i meas is the measurement I component signal generated with one of the second pair of oscillator signals, and î eas is the measurement I component signal generated with one of the third pair of oscillator signals having a 180-degree phase shift with respect to the one of the second pair of oscillator signals; and the offset estimator is configured to compute a Q component d.c. offset of the measurement receiver according to:

q

DC
,
MRX

=

q
meas

+

q
⋒

meas

2

in which q DC,MRX is the Q component of the d.c. offset of the measurement receiver, meas is the measurement Q component signal generated with the other one of the second pair of oscillator signals, and {circumflex over (q)} meas is the measurement Q component signal generated with the other one of the third pair of oscillator signals.

5. The apparatus of claim 1 , wherein the offset estimator is configured to compute an I component d.c. offset of the transmit path according to:

i

DC
,
TX

=

i
meas

-

i
⋒

meas

-

2
⁢

⁢

i
ref

2

in which i DC,TX is the I component of the d.c. offset of the transmit path, i meas is the measurement I component signal generated with one of the first pair of oscillator signals, î meas is the measurement I component signal generated with one of the second pair of oscillator signals having a 180-degree phase shift with respect to the one of the first pair of oscillator signals, and i ref is the I component signal; and the offset estimator is configured to compute a Q component d.c. offset of the transmit path according to:

q

DC
,
TX

=

q
meas

-

q
⋒

meas

-

2
⁢

⁢

q
ref

2

in which q DC,TX is the Q component of the d.c. offset of the transmit path, q meas is the Q component measurement signal generated with the other one of the first pair of oscillator signals, {circumflex over (q)} meas is the measurement Q component signal generated with the other one of the second pair of oscillator signals, and q ref is the Q component signal.

6. A method of estimating a direct-current (d.c.) offset in a transmitter having a transmit signal generated by mixing input in-phase (I) and quadrature (Q) component signals with respective ones of a transmit pair of oscillator signals having a relative phase shift of about 90 degrees, the method comprising:
generating the transmit pair of oscillator signals having the relative phase shift of about 90 degrees;
generating a first pair of measurement I component and Q component measurement signals by demodulating a portion of the transmit signal with a first pair of oscillator signals having a relative phase shift of about 90 degrees;
generating a second pair of measurement I component and Q component signals by demodulating a portion of the transmit signal

Assignees

• Persson Jonas
• Mattisson Sven
• St Ericsson Sa

Inventors

• Persson Jonas
• Mattisson Sven

Classifications

• H04L2027/0016

  Stabilisation of local oscillators · CPC title

• H04B1/0475 Primary

  with means for limiting noise, interference or distortion (H04B1/0483 takes precedence) · CPC title

• H04L27/364

  Arrangements for overcoming imperfections in the modulator, e.g. quadrature error or unbalanced I and Q levels · CPC title

• H04L27/2647

  Arrangements specific to the receiver only (equalisation H04L27/01) · CPC title

• H04B1/302

  for single sideband receivers (demodulator circuits H03D1/24) · CPC title