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FL801
Antenna Switch
DCS
PCS
EGSM
D
D
I
I
P
P
VC1,VC2,VC3
3.3.1. IF Modulator
The baseband converter(BBC) within the GSM chipset generates I and Q baseband signals for the
Transmit
vector modulator. The modulator provides more than 40dBc of carrier and unwanted side-
band. Rejection and produces a GMSK modulated signal. The baseband software is able to cancel
out differential DC offsets in the I/Q baseband signals caused by imperfections in the D/A converters.
The TX-Modulator implements a quadrature modulator. A quadrature mixer upconverts the differential
I/Q signals with the IFLO to generate a SSB IF signal, which is filtered and used as the reference input
to the OPLL. The IFLO frequency is generated between 766 and 896 MHz and internally divided by 2
to generate the quadrature LO signals for the quadrature modulator, resulting in an IF between
383MHz and 448 MHz.
3.3.2. OPLL
The OPLL consists of a feedback mixer, a phase detector, a loop filter, and a fully integrated TXVCO.
The TXVCO is centered between the DCS 1800 and PCS 1900 bands, and its output is divided by 2 for
the GSM 850 and E-GSM 900 bands. The RFLO frequency is generated between 1272 and 1483 MHz.
To allow a single VCO to be used for the RFLO, high-side injection is used for the GSM 850 and
E-GSM 900 bands, and low-side injection is used for the DCS 1800 and PCS 1900 bands. Low-pass
filters before the OPLL phase detector reduce the harmonic content of the quadrature modulator and
feedback mixer outputs. The cutoff frequency of the filters is programmable with the FIF[3:0] bits in
register 04h. The OPLL requires no external duplexer to attenuate transmitter noise and spurious signal
s in the receive band. Additionally, the output of the transmit VCO (TXVCO) is a constant-envelope
signal which reduces the problem of spectral spreading caused by non-linearity in the PA.
Figure 3. RF Transmit Block
EGSM
PA
PA
RFOD
RFOG
÷1
÷1
÷2
÷2
PAM RF3146 (U804)
DCS/ PCS
PA_BAND
IF PLL
IF PLL
RF PLL
RF PLL
Φ
Φ
DET
DET
Triple Buffer
Triple Buffer
-16-
Si4205 (U803)
÷2
÷2
VC1,VC2,VC3
Baseband
Baseband
(TI)
(TI)
U101
U101
IOTA
IOTA
Q
+
+
U102
U102
Calypso
Calypso
I
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