3. TECHNICAL BRIEF
Figure 3-2: The RF Receiver Block
9
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 adja-
cent 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.
To Next Page
Loading...
