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LGE Internal Use Only
Copyright © 2009 LG Electronics. Inc. All right reserved.
Only for training and service purposes
3. TECHNICAL BRIEF
LGE Internal Use Only
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 Front End Modules (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.
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.
Baseband Amplifiers / Low Pass Filters
The baseband amplifiers provide the majority of the analog receiver gain. The filtering is provided by an
integrated 5
th
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 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.
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 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 900/GSM850).
