Motorola IMPRES Tetra MTP700, Detailed Service Manual

Page: 51 / 250

TRANSCEIVER 400 MHz MTP700 Portable Radio / Detailed Service Manual 4.1 - 17
Main VCO and Main Synthesizer
The main synthesizer components are WPIC, Loop Filter, VCO and buffer. It produces
the LO signal.
The synthesizer functions are as follows:
• To serve as the local oscillator for the mixer of the receiver. It is used to down-mix the
RF frequency to the IF frequency 109.65 MHz
• To provide the reference signal for the ESCORT PLL IC of the Tx synthesizer.
• To provide as the WPIC feedback signal for locking the main synthesizer.
The WPIC is programmed by the DSP through the SPI bus.
The VCO (Q3002 and associated circuitry) generates the LO signal which is fed back to
the synthesizer (inside the WPIC) through the PRSCIN. The TCXO provides the 16.8
MHz 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 C1810, R1805, R1802 and
C1816 which filters noise in the tuning voltage. A pre-setting of the VCO frequency is pro-
vided by additional tuning via the DC signals from the “Coarse Tune DAC” in the WPIC.
The Q3003 and Q3004 buffers prevent loading and interference to the next stage - i.e.
the RX Mixer and ESCORT.
LMX - Dual Synthesizer
The U1501 (LMX233XL) is an integrated dual frequency synthesizer IC that includes
prescalers. It is used with the second LO VCO at the RFCU section.
The LMX uses the 16.8 MHz from the TCXO for the second LO synthesizers and divides
the TCXO frequency into 300 kHz reference frequency for the second LO.
Second LO VCO
The Second LO VCO is a discrete VCO (Q1521) that is controlled by the Dual Synthe-
sizer IC (U1501). The VCO produces a frequency of 219.3 MHz that routes into LOIN pin
of WPIC.
The output signal of the VCO is also routed via C1544 to the phase detector of Dual Syn-
thesizer (pin 18, FINIFPOS). The internal phase detector is of the charge pump type.
The output signal of the phase detector is present at Dual Synthesizer (pin 20, DOIF) and
then routed via the loop filter R1511, C1511 and C1512 to tuning diode D1521 of the tank
circuit.