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OVERVIEW: Transmitter Path Section
1.7.3.1
TX BALUN
The TX BALUN transforms the RFPA 50ohms single ended input impedance into 200ohms
differential (SLEDGEHAMMER's output impedance is 200 ohms differential). The BALUN also
feeds RAW_B+ to the TX modulator portion of the RFIC that drives Janus through the TX_IN and
TX_INB pins.
1.7.3.2
POWER AMPLIFIER
The signal is then routed to the RF PA. All control signals will be asserted before TX_ENB is
asserted, and such control signals will be de-asserted after TX_ENB is de-asserted.
1.7.3.3
QUIESCENT CURRENT SELECTION
The JANUS module is able to select between two discrete steps of RFPA quiescent current to
reduce current drain at high levels of TX cut back. Notice that when quiescent is reduced the
specification on gain flatness is reduced quite a bit. The threshold to activate the quiescent current
selection is 7dB cutback. See table 3.6 to determine the setting of LOAD_SEL and QUIES_SEL
vs. the radio cutback level. The quiescent current selection is not intended to be used in the 902-
928MHz band.
1.7.3.4
DIRECTIONAL COUPLER
The directional coupler samples the incident power going into the T/R switch and it feeds it back
to the RFIC for linearization and power control purposes.
1.7.3.5
HARMONIC FILTER
The harmonic filter is necessary to meet transmitter FCC emissions at the iDEN_ANT pin.
1.7.3.6
T/R SWITCH
The T/R switch selects the path that connects to the iDEN_ANT pin. The possible paths are iDEN
transmitter, iDEN receiver 800MHz, iDEN receiver 900MHz and Mototalk.
The switch should be able to maintain very high linearity under high VSWR conditions in order to
meet the stringent iDEN adjacent channel specifications as well as the harmonic emissions. The
lines SEL_1 and SEL_2 control the switch selection. The Figure 1-14 shows the diagram of the
switch.
38
Figure 1-11.TX Balun
Figure 1-12. T/R Switch
6880401P59-O
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