Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Amplitude control
Reexamination Certificate
2000-02-29
2001-05-01
Wells, Kenneth B. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Amplitude control
C327S362000, C330S009000
Reexamination Certificate
active
06225848
ABSTRACT:
TECHNICAL FIELD
This invention relates to techniques and apparatus for minimizing DC offsets in electronic circuits.
BACKGROUND
FIG. 1
is a block diagram of a traditional analog DC offset correction loop
100
such as would be used around the baseband path of a direct conversion receiver or zero IF receiver of a radio, cell phone, or other communication device. Correction loop
100
is used for single ended applications and generally includes a baseband filter
102
, an integrator
104
, an operational transconductance amplifier (OTA)
106
, and a mixer
108
whose output impedance and DC bias current are represented by a current source
110
and a resistor
112
. The desired DC voltage at the output of the baseband filter
102
is analog ground, Vag, however, the interaction of the current source
110
, resistor
112
, and the input referred DC offset of the baseband filter
102
generates an undesired DC offset at the filter input. The DC offset at the input of the baseband filter
102
is amplified through the baseband filter and produces a large offset at the baseband filter output. The integrator
104
and OTA
106
provide a feedback path to alter the current through the resistor
112
to adjust the voltage presented to the input of the baseband filter
102
thereby reducing the input referred DC offset.
Modern communications systems often require fast settling times. Even small DC offsets can saturate the signal path (e.g. baseband filters) causing all linear loop equations to be invalid making it very difficult to settle the loop within the allotted time. Once the offset has been corrected, the correction loop must be moved to a much lower corner frequency (i.e., narrowed greatly) or opened completely. Making a transition from a very wide offset correction loop bandwidth to a very narrow bandwidth poses a problem due to the transient response produced when making such a large transition. Opening the loop in an analog DC offset loop causes the correction voltage to drift from the desired value due to leakage on the integrator's
104
capacitor.
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 and transmitters.
REFERENCES:
patent: 4653117 (1987-03-01), Heck
patent: 5079526 (1992-01-01), Heck
patent: 5483691 (1996-01-01), Heck et al.
patent: 5539779 (1996-07-01), Nagahori
patent: 5584059 (1996-12-01), Turney et al.
patent: 5617473 (1997-04-01), Wietecha et al.
patent: 5789974 (1998-08-01), Ferguson, Jr. et al.
patent: 5893029 (1999-04-01), Bastani
patent: 6006079 (1999-12-01), Jaffee et al.
Ferrer Enrique
Graham David J.
Meraj Atif A.
Salvi Raul
Tilley Keith A.
Doutre Barbara R.
Lamb James A.
Motorola Inc.
Wells Kenneth B.
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