Rf Power Amplifier; Directional Coupler And Rf Feedback Path; Antenna Switch; Harmonic Filter - Motorola MTM800 User Manual

THEORY OF OPERATION (TRANSCEIVER)
variable attenuator and then they are summed with the down-converted I & Q feedback signal. The
base-band signal is then amplified and sent to the up-mixer. The up-mixer consists of two mixers,
one for the I channel and the other for the Q channel.
The split I & Q LO signal is mixed with the base-band I & Q signals to produce an I and Q modulated
signal at RF frequency. The signal is then output differentially on balls E8 and D8 (RFOUT2B,
RFOUT2), where the differential RF signal is converted to a single-ended unbalanced output by
means of BALUN T5802. 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 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 via the isolator FL 5451 and to sample the signal thus providing the necessary feedback for
the linearization and feedback correction. The sampled signal is routed via attenuator R5453,
R5454, R5455 and the BALUN L5812 to the JAVELIN (ball D1, RFIN, ball E1, RFINB). Internally to
the JAVELIN, 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
(C5455-C5457,C5461-C6467, 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 C5455-C5457,
C5461-6467, 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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