Motorola MTP700 Detailed Service Manual page 140

4.2 - 18
Main VCO and Main Synthesizer
The main synthesizer components are WPIC, Loop Filter, VCO and buffer. It produces
the LO signal.
The main synthesizer functions are as follows:
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The WPIC is programmed by the DSP through the SPI bus.
The VCO (Q700 and associated circuitry) generates the LO signal which is fedback to the
synthesizer (inside the WPIC) through the PRSCIN. The TCXO provides the 16.8MHz
reference frequency.
The synthesizer then generates the VCTRL tuning voltage through WPIC's CPGITR pin
to control the VCO frequency. The Loop FIlter (LPF) consists of C641, R624, R623, which
filters noise in the tuning voltage.
The Q702 and Q459 buffers prevent loading and interference to the next stage - ie RX
Mixer and ESCORT respectively.
LMX - Dual Synthesizer
The U730 (LMX233XL) is an integrated dual frequency synthesizer that includes prescal-
ers. It is used with the second LO VCO at the RF section.
The LMX uses the 16.8 MHz from the TCXO for the synthesizers and divides the TCXO
frequency into 300 kHz reference frequency for the 2nd LO.
Second LO VCO
The Second LO VCO is a discrete VCO (Q730) that is controlled by the LMX Dual Syn-
thesizer IC (U730).
The VCO produces a frequency of 219.3 MHz, that routes into WPIC's pin D1 (LOIN).
This signal is further divided by two before being applied to the second mixer in the
WPIC.
The output signal of the VCO is also routed via the phase detector of Dual Synthesizer,
pin 18 (FINIF2). The internal phase detector is of the charged pump type.
The output of the phase detector is present at Dual Synthesizer pin 20 (DOIF). This
signal is directed to loop filter R733, C738 and C739 and to tuning diode D730 of the tank
circuit.
MTP700 Portable Radio / Detailed Service Manual
To serve as the local oscillator for the mixer of the receiver. It is used to down-mix the
RF frequency to 1st IF (109.65 MHz).
To provide reference for the transmit frequency PLL
To serve as the WPIC feedback signal for locking the main synthesizer.
TRANSCEIVER 800 MHz