Motorola IMPRES Tetra MTP700, Detailed Service Manual

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4.1 - 14 MTP700 Portable Radio / Detailed Service Manual TRANSCEIVER 400 MHz
The output signals are routed to the LNODCT IC. 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 RedCap2). After receiving the TSLOT signal, the LNOCDT toggles the
ASW line (pin J5) which signals the WPIC to set VCNTO signal LOW (pin J6) which ena-
bles the Antenna Switch during transmit time.
TX LO Line-Up
The TX frequency signal TX_SOURCE is generated in the TX PLL of the Frequency
Generating Section. This signal passes an attenuator R6001-R6003, the LO driver
Q6001 and a band pass filter (C6006-C6012, L6003, L6004) to attenuate unwanted spu-
rious frequencies. The signal level is about +7dBm. This is then split by a simple, resis-
tive 6dB power splitter consisting of R6008, R6009 and R6010.
One output of the power splitter drives phase splitter U6001 that converts the single input
signal into two quadrature (90 degrees phase shift) ‘I’ and ’Q’ signals and then routes
them to the LO input (LOI, LOIB, LOQ, LOQB) of the LNODCT.
The second output of the power splitter U6001 is applied to the LNODCT LO feedback
input (MVCO).
LNODCT
The LNODCT IC (Low Noise Offset Direct Conversion Transmit IC) U6002 is the heart of
the transmitter.
The differential baseband signals from WPIC are applied to 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 ampli-
fied 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 I & Q
modulated signals at RF frequency. The signals then are output differentially on pins 51
and 52 (RFOUTB, RFOUT).
Balun
The differential RF signal is converted to a single-ended (unbalanced) signal by passing
the Balun (balanced-unbalanced) wide band transformer T6001. The Balun has a 200
Ohm balanced input and 50 Ohm single-ended output, set by C6032 and C6053 respec-
tively. This Balun configuration matches the LNODCT differential output for optimum per-
formance in the 380-430MHz frequency range. The insertion loss of the Balun is about
1dB.
Resistors R6024–R6026 form an attenuator to compensate for changes in gain in the for-
ward path RF PA stages.
RF Power Amplifier
The signal is routed through coupling capacitor C6034 to the RF power amplifier (PA).
The PA consists of 2 devices, namely U4001 driver and Q4001 final PA. U4001 is a 2-
stage IC that is capable of amplifying the signal to about 18dBm . The capacitors and
inductors between U4001 output and Q4001 input form a high pass - low pass matching
network. Q4001 output is then matched to the 50 ohms input of the isolator by a low pass
pi matching stage using microstrip-lines and discrete components.