Pulse or digital communications – Receivers
Reexamination Certificate
1999-12-29
2001-12-04
Chin, Stephen (Department: 2734)
Pulse or digital communications
Receivers
Reexamination Certificate
active
06327313
ABSTRACT:
TECHNICAL FIELD
This invention relates to techniques and apparatus for minimizing DC offsets in electronic circuits.
BACKGROUND
Traditional superheterodyne receivers translate incoming radio signals to one or more intermediate frequencies where successive stages of amplification, baseband filtering and gain control are applied. Direct conversion architecture uses a single mixer with a single oscillator tuned to the desired signal frequency, converting a signal directly to baseband-the zero intermediate frequency “ZIF” signal. Direct conversion receivers have been gaining in popularity in the electronic communications environment.
DC offset correction is one of the most critical problems in the design of direct conversion receivers.
FIG. 1
is a block diagram of a conventional DC offset correction loop
100
such as would be used in the baseband path of a direct conversion receiver, cell phone, or other communication device. Correction loop
100
is shown coupled to a mixer
102
and generally includes a summer
104
, a baseband filter
106
, an attenuator
108
, an integrator
110
, and an operational transconductance amplifier (OTA)
112
. In operation, mixer
102
receives a radio frequency (RF) signal
114
and a second radio signal, such as a local oscillator (LO) signal. The output of the mixer is a zero IF (ZIF) signal which gets corrected by adding or subtracting it with a feedback signal
116
to produce a baseband signal
118
. The baseband signal
118
is filtered through the baseband filter
106
to produce an I and Q output signal
120
. The I and Q output signal
120
is attenuated through attenuator
108
, and the attenuated signal
122
is then integrated through integrator
110
to produce the feedback signal
124
(Ioffset, Qoffset). The output of the correction loop
100
is taken at the output of the baseband filter
120
.
Mismatches between devices in a direct conversion radio system create DC offset in the down mixer and baseband filter. This offset can introduce tremendous distortion in the FM demodulation and even make the baseband filter nonfunctional. In the analog-to-digital conversion processes that occur in digital circuits, the DC offset is converted to a digital input along with I/Q data. During the A/D conversion process the offsets can also saturate the A/D converters.
Slow varying offsets create even more difficulties for DC offset correction. Traditionally, the integrator
110
has included a programmable time constant that is used to remove DC offsets after baseband filtering. However, since the integrator
110
has an infinite gain at DC, the delay introduced is so long that DC estimations cannot quickly follow the actual slow varying DC. As a consequence, the demodulated signal is severely distorted.
Other approaches to DC offset correction include quantization schemes and least mean square (LMS) techniques. However, these methods tend to require heavy computational steps in order to achieve any significant improvement in performance reduction in the DC offset.
Accordingly, there is a need for an improved method and apparatus for correcting DC offsets, particularly those offsets which occur in zero IF and direct conversion receivers.
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Fang Jing
Traylor Kevin B.
Chin Stephen
Doutre Barbara R.
Jiang Lenny
Motorola Inc.
Vaas Randall S.
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