Coded data generation or conversion – Converter compensation
Reexamination Certificate
2000-09-14
2001-08-21
Williams, Howard L. (Department: 2819)
Coded data generation or conversion
Converter compensation
Reexamination Certificate
active
06278391
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
This invention relates to digital-to-analog converters, and more particularly to D.C. offset error correction in digital-to-analog converters.
II. Description of the Related Art
Digital-to-analog converters, commonly referred to as “DACs” or “D-to-A” converters, are used to translate information from the digital domain to the analog domain. DACs typically transform digital signals into a range of analog values. DACs represent a limited number of different digital input codes by a corresponding number of discrete analog output values. Examples of input code formats accommodated by existing DACs include simple binary, two's complement binary, and binary-coded decimal. A number of techniques for implementing digital-to-analog converters are well known in the art.
Digital-to-analog converters are used in a wide variety of applications including digital wireless communications. For example, DACs are used in digital wireless cellular telephones to convert digital voice signals to “baseband” analog signals (i.e., signals having frequencies near D.C.).
FIGS. 1
a
and
1
b
show a block diagram of an exemplary digital wireless cellular telephone
900
that utilizes DACs to convert digitally encoded voice signals into filtered baseband analog signals. The cellular telephone
900
is manufactured in accordance with the TIA specification entitled “Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System,” TIA/EIA/IS-95-A, published in May 1995 by the Telecommunications Industry Association, and referred to hereafter as the “IS-95 specification.”
As shown in
FIGS. 1
a
and
1
b,
the exemplary digital cellular telephone
900
primarily comprises a user interface section
916
, a mobile station modem (MSM) application specific integrated circuit (ASIC)
914
, a baseband analog ASIC
912
, receive and transmit amplifiers
902
and
904
respectively, an upconverter
918
, a power amplifier and driver
920
, an antenna
906
, a duplexor
908
and a low-noise amplifier (LNA) and mixer circuit
910
. The cellular telephone
900
and its component parts are described in more detail in a related commonly assigned U.S. Pat. No. 5,880,631, issued on Mar. 9, 1999, entitled “High Dynamic Range Variable Gain Amplifier,” which is hereby incorporated by reference. An understanding of the function and operation of many of the components of the cellular telephone
900
are not essential to understanding the present invention and therefore are not described herein. However, a brief description of the MSM
914
and baseband analog ASIC
912
is useful in understanding one exemplary application and operating environment for the present invention.
The MSM
914
performs a variety of functions for the cellular telephone
900
including voice coding, decoding, interleaving, data modulation, spreading and filtering. For example, when information is transmitted from the telephone
900
to a CDMA base station (“reverse link” transmissions), voice information is first coded by the vocoder
950
and transferred to the modulator interleaver circuit
952
where the data is encoded, interleaved, modulated, spread and filtered. The digitized and modulated data is supplied to a pair of DACs
954
,
956
in the baseband analog ASIC
912
(
FIG. 1
b
) for further processing. The MSM
914
provides a baseband modulated digital representation of the CDMA waveform to the DACs
954
and
956
in the baseband analog ASIC
912
. The frequency range of the baseband digital signals is between D.C. (or 0 Hz) and approximately 630 kHz. The baseband analog ASIC
912
(largely due to the operation of the DACs
954
,
956
) converts the modulated digital data received from the MSM
914
into baseband analog signals. The baseband analog ASIC
912
filters the baseband analog signals generated by the DACs
954
,
956
and “upconverts” the filtered signals to an analog intermediate frequency (IF) signal. The IF signal is supplied to the transmit automatic gain control (AGC) amplifier
904
and further processed for eventual transmission to a wireless base station.
A better understanding of the operation of the DACs
954
,
956
can be obtained by describing the transmit section of the baseband analog ASIC
912
in more detail. One embodiment of the transmit section
100
of the baseband analog ASIC
912
of
FIG. 1
b
is shown in FIG.
2
. As shown in
FIG. 2
, the transmit section primarily comprises a pair of transmit DACs
102
(one each for the in-phase modulated baseband digital signals (I) and the quadrature-phase modulated baseband digital signals (Q)), a pair of CDMA filters
104
,
106
, and a transmit up-converter circuit
108
. The well known quadrature modulation scheme preferably is used to up-convert to the IF frequency in the CDMA path of the transmit section
100
shown in FIG.
2
. Therefore, two DACs are needed to perform the digital-to-analog conversion of the baseband digital signals received from the MSM ASIC
914
. The IDAC
110
converts the received baseband digital in-phase signals to baseband analog in-phase signals. Similarly, the QDAC
112
converts the received baseband digital quadrature-phase signals to baseband analog quadrature-phase signals. In the embodiment shown in
FIG. 2
, the transmit DACs
102
have differential outputs to reduce the detrimental effects caused by external noise that may be generated elsewhere on the baseband analog ASIC
912
.
The I and Q channel CDMA filters
104
,
106
remove unwanted noise that is generated by the DACs
110
and
112
, respectively. The CDMA filters
104
,
106
comprise anti-alias filters which perform a smoothing function on the baseband analog signals generated by the transmit DACs
102
and thereby remove any high frequency components introduced by the DACs
102
. Similar to the transmit DACs
102
, the CDMA filters
104
,
106
have differential outputs as shown in FIG.
2
. The outputs of the CDMA filters
104
,
106
are input to the transmit up-converter
108
which converts the baseband analog signals to an IF frequency for further processing and eventual transmission to a CDMA base station.
Disadvantageously, the transmit section
100
shown in
FIG. 2
introduces errors which are manifest as added D.C. offsets (referred to hereafter as “offset induced errors”) in the transmit signals of interest before the signals are output to the remainder of the cellular telephone circuitry. In particular, and referring again to
FIG. 2
, the offset induced errors can be imposed upon the transmit signals by the transmit DACs
102
and by active components in the CDMA filters
104
and
106
. Because the CDMA filters
104
and
106
can be relatively complex the induced offset errors can be significant. Disadvantageously, the offset errors introduced into the signal path, and specifically into the input of the mixers
114
,
116
, can cause a carrier signal to appear in the IF signal generated at the output of the transmit up-converter circuit
108
. To meet certain carrier suppression specifications it is necessary to reduce or eliminate the offset induced errors introduced by the transmit section
100
. Unfortunately, the offset induced errors have proven difficult to eliminate in the past. Because the magnitude of the offsets vary widely depending upon the operational characteristics (i.e., voltage, temperature, etc.) of the baseband analog ASIC
912
the errors have proven difficult to eliminate. Therefore, a need exists for a method and apparatus which can reduce or eliminate the D.C. offset errors that appear at the input of the transmit mixers
114
,
116
.
A prior art approach at reducing the D.C. offsets is shown in FIG.
3
. The prior art uses a fuse-based D.C. offset error correction circuit
120
to reduce the errors produced at the output of the CDMA filters
104
,
106
. The error correction circuit
120
primarily comprises a series of fuses and a relatively small DAC which is capable of adding an error adjustment to the signals at the input
Brown Charles D.
Qualcomm Incorporated
Streeter Tom
Wadsworth Philip R.
Williams Howard L.
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