Motorola MTP700 Detailed Service Manual page 138

4.2 - 16
The U800 amplifier gain is set in the factory using the VBLIN signal (from WPIC). The
VBLIN voltage is amplified via a DC amplifier U907. Output voltage from U907 sets the
quiescent current, and hence the gain of the LDMOS driver IC.
The gain of the final stage PA is set in the factory through serial SPI commands (from
RedCap2) to the E-pot. The output of the E-pot is dependent on the resistance divider
ratio internal to it. During transmission, the 2.775V supply to the E-pot is enabled using
PA Bias line signal (from RedCap2). When this line signal goes to 2.775V (HIGH), the
voltage to bias the final stage device is enable.
Coupler and Feedback Path
A coupler U806 exists at the RF PA output, it is used to pass the signal to Isolator FL801
and sample the signal thus providing the necessary feedback for the linearization and
feedback correction. The sampled signal is routed via attenuator R913, R914, R915 to
the LNODCT (pin 37, RFIN). The feedback signal is then mixed down to baseband in a
quadrature down mixer, amplified and summed with the baseband input signal. The loop
is closed.
Isolator
The signal in the forward path is fed to pin 1 of the isolator (FL801). The isolator is placed
at the PA output to decrease the influence of the antenna impedance variation on the PA
performance. The reflected power returned from the antenna, is absorbed in a 50 Ohms
resistor inside the isolator. The isolator also protects the Cartesian loop from sudden
VSWR variations that could lead to loop instability.
Antenna Switch
The RF signal from the Isolator is applied to PIN diodes D901 and D903. The PIN diodes
are turned on during the transmission time slot, and the DC current flows through the Iso-
lator to the ground. The RF signal is routed thru the harmonic filter (C943, L912 and
C944) to the antenna.
In the Receiving mode the PIN diodes are turned off, the input RF signal is directed to the
receiver path via λ/4 LC equivalent circuit (C943, L915 and C938).
The condition of the PIN diodes is controlled by the voltage switch Q806 by applying the
ANT_EN signal (from RedCap2) to the switch. The resistor R911 determines the DC cur-
rent through PIN diodes. Pin diodes D802 and D803 provide the additional capacitance
so as to improve isolation to receiver path during transmission mode. Capacitors C939
and C931 compensate the parasitic inductance of D802 and D803 respectively. The
VCNTO signal applied to the voltage switch Q806 is set LOW during the Receiving and
Transmission modes.
The RF output signal is disabled during the CLCH (Training) mode by setting the VCNTO
signal HIGH thus providing the low output power during the training slot.
Harmonic Filter
From the antenna switch the signal is routed to a one section of LC filter, consisting of the
coil L912 and capacitors C943 and C944. The filter is required to attenuate the harmonics
of the amplifying stages at the transmission path, and the Local Oscillator leakage at the
receiver path.
Resistor R809 is added to provide a discharge path for any static electricity buildup at the
antenna.
MTP700 Portable Radio / Detailed Service Manual TRANSCEIVER 800 MHz