Motorola MTP700 Detailed Service Manual page 137

TRANSCEIVER 800 MHz
The WPIC sends a 2.4MHz low-level differential reference clock signal (pins B11, TCLK;
C10, TCLKB) to the LNODCT. It also sends a differential signal (pins A11, TSLOT; A10,
TSLOTB) that marks the beginning and the end of each transmission time slot (whenever
pin 8, WPIC_TXE signal is received from the RedCap2). After receiving the TSLOT sig-
nal, the LNOCDT toggles the ASW line (pin J5) which signals the WPIC to set VCNTO
signal LOW (pin J6) which enables the Antenna Switch during transmit time.
TX LO Line-Up
The TX LO line-up includes the ESCORT IC which generates a signal of frequency equal
to the transmit frequency. The signal level is about +7dBm. This is then split thru a 3dB
power splitter consisting of R457, R458 and R459.
One output of the power splitter U801 (MVCO signal) goes into the LO input of the
LNODCT downconverter. This signal downconverts the RF feedback input signal to base-
band.
The other output of the power splitter (MVCO_PS signal) goes into the phase splitter
which generates I and Q signals for the upconverter in the LNODCT. These signals
upconvert the baseband signal into RF frequency.
LNODCT
The LNODCT (Low Noise Offset Direct Conversion Transmitter) U803 is the heart of the
transmitter.
The differential baseband signals from the WPIC are inputted into the LNODCT on pins
58 – 61 (BINQB, BINQ, BINIB and BINI). They pass through a variable attenuator and
then they are summed with the down converted I & Q feedback. The baseband 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 chan-
nel. The split I & Q LO signal is mixed with the baseband I & Q signals to produce an I
and Q modulated signals at RF frequency. The signal is then output differentially on pins
51 and 52 (RFOUTB, RFOUT).
Balun - Filter
The differential RF signal is converted to a single-ended (unbalanced) signal by passing
the Balun-Filter T800 (balanced-unbalanced filter). The Balun-Filter has a 200 Ohm input
and 50 Ohm single-ended output. The Balun-Filter output voltage amplitude is two times
higher than the differential voltages amplitudes. Thus the LNODCT output signal is
increased by 6dB when compared to each of the differential signals. The insertion loss of
the Balun-Filter is 3dB.
Resistors R921 - R923 form an attenuator to compensate for changes in the foward path
RF PA gain stages.
RF Power Amplifier
The signal is routed through DC coupling capacitor C901 to the RF power amplifier (PA).
The PA consists of 2 devices, namely U800 driver and Q800 final PA. U800 is a 2-stage
IC that is capable of amplifying the signal to about 24dBm level. The capacitors and
inductors between U800 output and Q800 input form a high pass - low pass matching
network. Q800 output is then matched to the 50 ohms input of the isolator thru a low pass
pi matching stage using microstrips.
MTP700 Portable Radio / Detailed Service Manual 4.2 - 15