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3. TECHNICAL BRIEF
A. RF front end
RF front end consists of Antenna Switch(U405), dual band LNAs integrated in transceiver(U411).
The Received RF signals (GSM 925MHz ~ 960MHz, DCS 1805MHz ~ 1880MHz) are fed into the
antenna or mobile switch. An antenna matching circuit is between the antenna and the mobile
switch.
The Antenna Switch (U405) is used to control the Rx and TX paths. And, the input signals VC1 and
VC2 of a U405 are connected to 2-Input AND Gates(U401) to switch either TX or RX path on.
When the RX path is turned on, the received RF signal then feeds either Rx_900_RF or
RX_1800_RF path selected by GSM-RX and DCS-RX respectively. This Rx_900_RF path contains
one SAW filter, followed after the Antenna Switch (U405), to filter any unwanted signal apart from
the DCS RX band. And, the RX_1800_RF path is the same case.
The logic and current for Antenna Switch is given below Table 3-1.
GSM TX
DCS TX
GSM/DCS RX
These two paths are then connected to the LNAGSMN (#11) and LNADCSIN (#13) of CX74017
(U411), respectively. A low-noise bipolar RF amplifier, contained within the U411, amplifies the RF
signal. The RF signals from the front-end pass to the receiver mixers within the U411 device.
B. Demodulator and baseband processing
In direct conversion receiver there is only one mixer down-converting received RF signal to BB
signal directly. The gain down converting mixer is 40dB at high gain mode and 22dB at low gain
mode.
The Rx gain setting is done in the AGC algorithm. The nominal gain of the receiver is set as a
function of the expected signal strength at the antenna input so that a desired level is reached at
the Rx I/Q. 7 blocks in the receiver chain have variable gains, LNA, Mixer, LPF1, VGA1, gmC Filter,
Auxiliary gain control and VGA2. The gain settings can be adjustable via 3-wire bus control lines.
The baseband signals pass via integrated low-pass filters to the baseband A/D converters.
The remainder of the channel filtering is performed by the baseband chipset. The demodulator
contains switches to maintain the sense of the baseband I/Q outputs with respect to the incoming
RF signal on both GSM900 and DCS 1800.
C. DC Offset Compensation
Three correction loops ensure that DC offsets, generated in the CX74017, do not overload the
baseband chain at any point.
After compensation, the correction voltages are held on capacitors for the duration of the receive
slot(s). A rising edge on the RXEN signal, selected via the serial interface, placed the DC
compensation circuitry in the track mode.
Table 3-1. The logic and current
VC1
0 V
2.7 V
0 V
- 14 -
VC2
2.7 V
10.0 mA max
0 V
10.0 mA max
0 V
< 0.1 mA
Current
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