THEORY OF OPERATION: Transmitter Path Section
10
Isolator
The isolator protects the PA from any impedance changes that may occur at the
antenna. The isolator provides a constant impedance to the PA so that its gain does
not vary depending on load, which could result in splatter. The isolator is a circu-
lator with the third port internally terminated to absorb any reflected power from
port 2.
Antenna Switch
The antenna switch consists of two PIN diodes, one series and one shunt. The
diodes are biased On in transmit mode and Off in receive mode. The bias voltage
is VDP (3.6Vdc pulsed). The DC return is through the isolator, since it is a DC
short.
Cartesian Feedback
iDEN transmitters use MQAM modulation, which requires a highly linear PA
with wide dynamic range. Linear PAs are highly inefficient so a class AB PA is
used for better efficiency and longer battery life. The class AB PA is fairly linear,
but not totally; and this causes splatter in the RF spectrum around the transmitted
frequency band. To reduce splattering into the adjacent channels and to meet sys-
tem specifications, the transmitter uses cartesian feedback to linearize the PA and
reduce splatter. Negative feedback is a commonly used method to linearize cir-
cuits.
Cartesian feedback is the process of down converting the feedback signal to base-
band and summing it with the input signal in the I & Q paths separately. One can
control the 180
o
phase shift at baseband more precisely than at RF frequencies.
The ODCT is the heart of the cartesian feedback system, and as such, is the heart
of the transmitter. There is a forward path and a feedback path in the transmitter.
This a closed loop system and the loop cannot be opened without drastic conse-
quences. See Figure 5 on page 11.
The forward path includes the following:
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ODCT ASIC
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Balun
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Image filter