OVERVIEW: Transmitter Path Section
ROADRUNNER Transmit Operation. The transmitter implemented a Direct Conversion
Transmitter (DCT), where base band data is up-converted into RF with only one stage. The
ICs that perform the transmit operations are ROADRUNNER and SLEDGEHAMMER.
Power cutback control is shared between both ICs, the fine cutback step (1-> 4dB) is
performed by ROADRUNNER and the coarse cutback step (5 dB steps up to 35 dB) is
performed by SLEDGEHAMMER. To generate the RF signal, the SLEDGEHAMMER IC
uses a TX_LO that is centered at the carrier frequency. This TX_LO is generated INSIDE
SLEDGEHAMMER.
The digital samples are generated by the DSP and then sent to ROADRUNNER via
Transmit SSI (TX_S SI). ROADRUNNER then implements the Digital-to-Analog
Conversion function, which provides some power control function. The resulting I!Q
analog signals are then sent to the SLEDGEHAMMER IC, which up-converts the base
band signal to RF (similar to what the JAVELIN did on previous architectures).
The I!Q path circuits will be similar to the lineup used in the Tomahawk IC. The "main"
reconstruction filter for the I!Q paths will be located in the SLEDGEHAMMER IC, and
will have programmable BW options of 25, 50, and 100 KHz.
These functions of the IQ modulator consist of the interface to the digital baseband, the
digital baseband sub-block, the analog baseband sub-block, and the TX DAC bridging the
digital and analog sub-blocks. The ROADRUNNER IC will produce the baseband I and Q
signals needed for IQ modulation on the SLEDGEHAMMER IC.
1.7.2 Sledgehammer IC (TX Path)
The SLEDGEHAMMER is the heart of the transmitter; it incorporates all of the circuitry
necessary to implement a Cartesian feedback closed-loop system.
The differential baseband signals from the ROADRUNNER are input into the
SLEDGEHAMMER. They go through a variable attenuator and are then summed with the
down-converted I & Q feedback. The baseband signal is then amplified and sent to the up
mixers.
1.7.3 JANUS
JANUS is a RF front end module (FEM) intended to support iDEN and WiDEN protocols.
The JANUS FEM contains the RF front end functions for the iDEN receiver, iDEN
transmitter and GPS receiver. The JANUS FEM is intended to be used as part of the iDEN
Phoenix Platform, and it is also backwards compatible with the existing Falcon platform.
The transmitter section of Janus comprises a single lineup that support iDEN 800MHz,
iDEN 900MHz and Mototalk. The TX balun presents a differential impedance to the
modulator on the RFIC, and it also feeds RAW _B+ to the modulator via the TX_IN and
TX_INB pins. The nominal differential impedance to the TX balun is 200ohms.
The RFPA also features a 2.5dB load line select line as well as reduced quiescent current
mode for low output power operation.
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