LG KE590 Service Manual page 17

3. Circuit Description
3.2.1.1 Baseband PGA/Low pass Filter Specifications
The baseband programmable amplifier comprises one stage with variable gain followed by a fixed
gain amplifier. The overall gain control range is 36dB with 6dB Steps. The filtering is provided by a
single R/C low pass filter with an on-chip capacitor followed by on-chip Chebychev low pass filters.
The filters have been specified to achieve maximal group delay flatness in the pass-band combined
with the required levels of suppression of interfering signals. The distribution of the gain and filtering
has been designed to ensure that the receiver does not compress under blocking conditions. The
final fixed gain amplifier is included to match the on-chip levels to the input dynamic range of the
ADC.
3.2.1.2 DC offset auto-calibration system
B6PLD implements a system for cancelling the DC offsets in the baseband programmable gain
amplifiers(PGA). This prevents a small DC offset at the input giving a large DC offset at the output,
even\ at high gain settings. When the B6PLD receiver is performing an auto-calibration, the
sequencer cancels the offsets locally around the PGA, then the Digital filter. The system includes
switches to short out the signal path whilst the cancellation is occurring. The switches are opened in
sequence as the calibration progresses. For PGA the A/D converter system employs a successive
approximation technique and achieves 6 bit resolution. The PGA stage has an associated 6 bit
current DAC which cancels the DC offset at the output. The sequencer ensures that on-chip filters
have sufficient time to settle before applying correction in the next digital offset cancellation stage.
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