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

THEORY OF OPERATION (400 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 R5830, R5831, R5832, 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 BIAS1 voltage from the controller, which is applied to VCNTRL pin 1 on pre- driver
U5401. The BIAS1 signal is buffered by amplifier Q5501. The operating bias point of driver Q5421
and PA Q5431 is set by BIAS2 and BIAS3 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 the signal to the Antenna
Switch and sample the signal thus providing the necessary feedback for the linearization and feed-
back correction. The sampled signal is routed via attenuator R5453, R5454, R5455 and the BALUN
L5812 to the LNODCT (pin 37, RFIN, pin 36, RFINB). Internally to the LNODCT, the RF feedback
signal is mixed down to base-band in a quadrature down mixer, and summed with the base-band
input signal to provide cancellation of modulation distortion generated in the power amplifier.

Antenna Switch

The RF signal from the Directional Coupler 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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