PG-1400 Service Manual
4.2.4 Transmit Section
4.2.4.1 An Overview of Transmit Section
Fig.4-8. Transmitter block diagram
The transmit section consists of an I/Q baseband upconverter, an offset phase-locked loop (OPLL),
and two 50
Ω
output buffers that can drive an external power amplifier (PA). One output is for the
E-GSM 900 (880–915 MHz) bands and one output is for the DCS 1800 (1710–1785 MHz) bands.
The OPLL requires no external filtering to attenuate transmitter noise and spurious signals in the
receive band, saving both cost and power. The output of the transmit VCO (TXVCO) is a
constant-envelope signal that reduces the problem of spectral spreading caused by non-linearity in
the PA. Additionally, the TXVCO benefits from isolation provided by the transmit output buffers. This
significantly minimizes any load pull effects and eliminates the need for off-chip isolation networks.
A quadrature mixer upconverts the differential in-phase (BIP, BIN) and quadrature (BQP, BQN)
baseband signals to an intermediate frequency (IF) that is filtered and which is used as the reference
input to the OPLL. The OPLL consists of a feedback mixer, a phase detector, a loop filter, and a fully
integrated TXVCO.
Low-pass filters before the OPLL phase detector reduce the harmonic content of the quadrature
modulator and feedback mixer outputs.
The transmit I/Q interface must have a non-zero input no later than 94 quarter bits after PDN is
asserted for proper operation. If the baseband is unable to provide a sufficient TX I/Q non-zero input
preamble, then the CWDUR bits can be used to provide a preamble extension.
The receive and transmit baseband I/Q pins are multiplexed together in a 4-wire interface (BIP, BIN,
BQP, and BQN). In transmit mode, the BIP, BIN, BQP, and BQN pins provide the analog I/Q input
from the baseband subsystem. The full-scale level at the baseband input pins is programmable with
the BBG[1:0] bits. The I and Q signals are automatically swapped within the Aero II transceiver when
switching bands. The transmit output path is automatically selected by the ARFCN bits and the
BANDIND bits.
4.2.4.2 Transmitter Part
A. 3 dB attenuator
15
PANTECH
