Rf Power Amplifier; Directional Coupler And Rf Feedback Path; Isolator; Antenna Switch - Motorola MTM700 Detailed Service Manual

THEORY OF OPERATION (800 MHz)
ferentially on pins 51 and 52 (RFOUTB, RFOUT), where the differential RF signal is converted to a
single-ended unbalanced output by means of BALUN L5807. The output signal from the BALUN is
applied to the RF Power Amplifier.

RF Power Amplifier

The RF Power Amplifier consists of attenuator R5413, R5414, R5415, pre-driver IC U5401, driver
transistor Q5421, and output transistor Q5431. The overall RF gain of the power amplifier string is
fine tuned by the BIAS_PA_1 voltage from the controller, which is applied VCNTRL pin 1 on pre-
driver U5401. The BIAS_PA_1 signal is buffered by amplifier Q5501. The operating bias point of
driver Q5421 and PA Q5431 is set by BIAS_PA_2 and BIAS_PA_3 respectively, buffered by Q5521
and Q5542.

Directional Coupler and RF Feedback Path

A directional coupler exists at the RF PA output, and is used to pass signal to Isolator FL5451 and
sample the signal thus providing the necessary feedback for the linearization and feedback correc-
tion. The sampled signal is routed via attenuator R5453, R5454, R5455 to the LNODCT (pin 37,
RFIN). Internally to the LNODCT, the RF feedback signal is mixed down to base-band in a quadra-
ture down mixer, and summed with the base-band input signal to provide cancellation of modulation
distortion generated in the power amplifier.

Isolator

The signal in the forward path is fed to the input port of isolator FL5451. The isolator is placed at the
PA output to decrease the influence of antenna impedance variations on PA performance. Reflected
power returned from the antenna is absorbed in a 50 Ohm resistor inside the isolator, preventing the
reflected power from reaching the PA output device, where it could degrade the operation of the car-
tesian feedback linearization loop.

Antenna Switch

The RF signal from the Isolator is applied to PIN DIODE D5451. This PIN diode is turned on during
the transmission time slot, to route the transmitter output signal to the harmonic filter (C5462-6467,
L5461, and L5462), and from there on to the antenna connector. During the transmit time slot, pin
diode D5551 is also turned on, preventing power from the transmitter from reaching the receiver
LNA. During receive time slots, PIN diodes D5451 and D5551 are OFF, allowing receive signals to
pass from the antenna connector through the harmonic filter, through L5452, and on to the receiver
input preselector.

Harmonic Filter

From the antenna switch the signal is routed to the harmonic filter, consisting of C5462-6467,
L5461, and L5462. The filter is required to attenuate transmitter harmonics during transmit mode,
and leakage of local oscillator harmonics during receive mode.
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