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3. TECHNICAL BRIEF
B. 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. The I and Q signals are
automatically swapped when switching bands. Additionally, the SWAP bit in register 03h can be used to
manually exchange the I and Q signals.
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 (Figure 3-3), and should be set to the recommended settings detailed in the register
description.
3.2 PAM (SKY77328)
Figure. 3-4
RF input and output ports of the SKY77328 are internally matched to a 50 Ω load to reduced the number of
external components for a quad-band design. This chip contains band-select switching circuitry to select
GSM (logic 0) or DCS/PCS (logic 1) as determined from the Band Select (BS) signal. The BS pin selects the
PA output (DCS/PCS out or GSM850/900 out) and the Analog Power Control (VAPC) controls the level of
output power.
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