LG MG230 Service Manual page 11

3. TECHNICAL BRIEF
A. Low Noise Amplifiers
The AD6548 includes four fully integrated Low Noise Amplifiers (LNAs), to support quad band
applications without further external active components. The LNAs have differential inputs which
minimize the effect of unwanted interferers. The inputs are easily matched to industry standard FEMs
or discrete Rx SAW filters. The outputs of the LNAs are directly coupled to the down-converting
mixers. The voltage gain of the LNAs are typically 24 dB. Each LNA can be switch to a low gain mode
when receiving large input signals as part of the AGC system.
B. Down-Converting Mixers
Two quadrature mixers are used to mix down the signals from the LNAs, one for the high bands (1800
and 1900 MHz) and one for the low bands (850 and 900 MHz). The outputs of the mixers are
connected to the baseband section through an integrated single pole filter with nominal cut-off
frequency of 800kHz. This acts as a "roofing filter" for the largest blocking signals (i.e. those _ 3MHz)
and prevents the baseband amplifiers from being overloaded.
C. Baseband Amplifiers / Low Pass Filters
The baseband amplifiers provide the majority of the analog receiver gain. The filtering is provided by
an integrated 5th order Chebyshev filter giving the necessary adjacent channel and blocking filtering, it
is also acting as an anti-alias filtering for Baseband IC's converters. A final low pass pole is possible at
each of the baseband outputs via internal series resistor along with an external shunt capacitor. The
external capacitor is not required with ADI baseband ICs. The on chip filter has an auto calibration
feature ensuring that the filters are tuned for optimum performance. The baseband amplifiers have
programmable gain for system AGC. A total of 57 dB of gain control is provided in 3dB steps
programmable over the serial interface. This together with the LNA gain control gives a total of 77dB of
gain control range. The receive baseband outputs are routed to the common Rx/Tx I/Q ports for
connection with the baseband converters.
D. Baseband Output D.C. Offset Correction
In order to minimize D.C. offsets inherent in the receiver and maximize dynamic range a D.C offset
correction circuit is integrated. This correction is triggered over the serial bus and then an offset
tracking loop is enabled to minimize residual offsets under all conditions. The tracking loop is fully
hardware integrated, requiring no software intervention.
E. Receiver Local Oscillator (LO) Generator
The Rx LO generator is used to avoid DC offset problems associated with LO leakage into the receiver
RF path. By operating the VCO at a frequency other than the desired receive frequencies, any leakage
of the VCO (e.g. via package) will fall out of band. The LO generator is used to convert the offset
synthesized VCO output to the on-frequency quadrature LO required by the chipset. The LO generator
is implemented as a regenerative frequency divider, performing a 2/3 multiplication of the VCO output
for the high band (DCS1800/PCS1900) and a 1/3 multiplication for low band (E-GSM/GSM850).
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