Telecommunications – Receiver or analog modulated signal frequency converter – Noise or interference elimination
Reexamination Certificate
1999-06-25
2001-10-30
Legree, Tracy (Department: 2681)
Telecommunications
Receiver or analog modulated signal frequency converter
Noise or interference elimination
C455S303000, C455S311000, C455S312000, C455S313000, C375S319000
Reexamination Certificate
active
06311051
ABSTRACT:
BACKGROUND
1. Technical Field
The present invention relates generally to a mobile communication system and, more particularly, to a mobile communication system having a baseband analog processor with DC offset compensation circuitry.
2. Description of Related Art
Code division multiple access (CDMA) is based on technology originally developed by the Allies during World War II to resist enemy radio jamming. It has been significantly refined during the intervening decades, and is used today in digital cellular services and personal communication services (PCS), as well as a variety military applications.
The earliest radio transmissions were unintentionally spread across a broad frequency spectrum, much like CDMA. However, the proliferation of uses for radio, coupled with the inherent inability to discern the difference between one radio signal and another, led to change. This change was the division of the radio spectrum into specific bands and frequencies, or channels, to prevent one transmission from interfering with another. This change was made possible by improvements in radio filter technology.
With CDMA technology, the signal is spread over a broad frequency of spectrum. In particular, the CDMA process involves dividing a digitized voice transmission into small packets of encoded data which are then transmitted along with other transmissions across a broad band of spectrum. Each transmission is spread in bandwidth by its own encoding sequence so that no transmission has the same code. Although all transmissions are sent out simultaneously, the unique code allows the receiver to separate one transmission from all the others.
Referring to
FIG. 1
, a block diagram illustrates a conventional mobile communication system which may be implemented, e.g., in a CDMA mobile phone. The conventional system comprises an input circuit
100
, a gain control circuit
200
, a baseband analog processor
300
, a digital mobile station modem (MSM)
400
, and a coder-decoder (CODEC)
500
, all of which are serially connected between an antenna
10
and a speaker
20
in the order as illustrated.
The conventional mobile communication system of
FIG. 1
operates as follows. A very weak signal RFA received from a base station via the antenna
10
is down-converted into an intermediate frequency (denoted by IFA
1
) band by the input circuit
100
. The IFA
1
signal is provided to the gain control circuit
200
, wherein it is amplified to produce an IFA
2
signal. This IFA
2
signal is then received by the baseband analog processor
300
, wherein it is down-converted to generate a first baseband analog signal which is then converted to a first baseband digital signal BBD
1
. The BBD
1
signal is input to the MSM
400
, wherein it undergoes CDMA demodulation. The CDMAdemodulated signal is output as a second baseband digital signal BBD
2
, which is then processed by the CODEC
500
to extract the voice data. The voice data is then output via the speaker
20
.
Referring now to
FIG. 2
, a detailed block diagram illustrates a conventional baseband analog processor
300
of the system illustrated in FIG.
1
. As shown, the conventional baseband analog processor
300
comprises a down converter
310
, a low-pass filter
320
, an analog-to-digital converter (ADC)
330
, and an offset compensation circuit
340
. The down converter
310
down-converts the IFA
2
signal to the baseband analog signal BBA
1
. The baseband analog signal BBA
1
is then filtered by the low-pass filter
320
to eliminate noise. The low-pass filter
320
is typically a high order low-pass filter. Typically, the filtered baseband analog signal BBA
2
contains a DC offset voltage AOV, which is one of the corrupting influences present in the analog signal. The control of the DC offset voltage AOV at the input of the ADC
330
is considerably affected by the receiving signal path and the MSM digital processing.
Accordingly, an offset compensation signal OCAS
1
is provided to control the DC offset voltage AOV. In particular, the MSM
400
senses the DC offset of the digital baseband data (BBDI) and produces a pulse density modulated (PDM) signal or a pulse width modulated (PWM) signal as the offset compensation signal OCAS
1
to compensate the DC offset voltage AOV. The offset compensation signal OCAS
1
is settled between 50% and 100% of an ADC full scale. However, since the baseband analog processor
300
is a monolithic integrated circuit (MIC), the DC offset voltage AOV is typically greater than the full scale of the ADC
330
. Thus, the offset voltage AOV may not be fully controlled by the offset compensation signal OCAS
1
.
A conventional solution to the foregoing problem is the use of the offset compensation circuit
340
to compensate the offset voltage AOV. The offset compensation circuit
340
comprises a comparator
341
, a counter
342
, a latch
343
, and a digital-to-analog converter (DAC)
344
. When supplied with power, the baseband analog processor
300
generates a “power on” signal POW, which causes the comparator
341
to generate a detection signal COM by comparing the DC offset voltage (AOV) of the filtered baseband analog signal BBA
2
and a reference voltage VREF. The counter
342
counts the detection signal COM in synchronism with a clock signal CLK (which is an internal clock signal of the baseband analog processor
300
). The counted compensation signal OCDS
2
is latched by the latch
343
. The latch
343
then sends the OCDS
2
signal to the DAC in response to the detection signal COM. The DAC
344
converts the counted compensation signal OCDS
2
into a second offset compensation signal OCAS
2
, which is then received by the filter
320
. In this manner, during the “power on” period of, e.g., the CDMA mobile phone, the offset compensation circuit
340
will generate the second offset compensation signal OCAS
2
to preset the DC offset voltage AOV, so that the offset voltage is settled below the full scale of the ADC
330
.
After the DC offset voltage AOV is preset as described above, the MSM
400
will generate the offset compensation signal OCAS
1
to compensate the DC offset voltage AOV of the filtered baseband analog signal BBA
2
when the first baseband digital signal BBDI is received by the MSM
400
from the ADC
330
.
One disadvantage associated with the conventional baseband analog processor
300
is that the “power on” preset offset voltage of the baseband analog processor
300
is fixed at a constant value. Therefore, when a sudden variation of ambient environment, such as temperature and/or humidity, is encountered during operation of the system, the DC offset voltage AOV may exceed the compensable range, causing the compensation of the offset voltage AOV to be restricted. This may result in an undesired erroneous voice signal. Accordingly, there is a need in the art for a baseband analog processor which provides improved DC offset compensation to overcome the above described disadvantages.
SUMMARY OF THE INVENTION
The present invention is directed to a mobile communication system having a baseband analog processor with improved DC offset compensation circuitry.
In one aspect of the present invention, a mobile communication system comprises:
a first down-converter for converting a radio frequency (RF) signal into an intermediate frequency (IF) signal;
a baseband analog processor for converting the IF signal into an analog baseband signal and for converting the analog baseband signal into a digital baseband signal; and
a modem for demodulating the digital baseband signal;
wherein the baseband analog processor is configured for generating a first DC offset compensation signal to preset a DC offset voltage of the analog baseband signal when the mobile communication system is powered on, and for generating a second DC offset compensation signal to re-preset a DC offset voltage of the analog baseband signal when the mobile communication system enters a predetermined mode of operation, and wherein the modem is configured for generating a third DC offset compensation signal to compensate for a DC o
Davis Temica M.
Legree Tracy
Samsung Electronics Co,. Ltd.
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