Motorola PM1200, Базовое руководство по обслуживанию

Страница: 37 / 138

6880309U09-A October 26, 2007
Basic Theory of Operation : PLL Synthesizer 3-7
3.7 PLL Synthesizer
The 1st LO maintains stability from the PLL synthesizer by using a 17.25 MHz reference signal from
crystal X1001. PLL synthesizer IC Q1029 (SA7025DK) consists of a prescaler, a reference counter,
a swallow counter, a programmable counter, a serial data input port to set these counters based on
the external data, a phase comparator, and a charge pump. The PLL-IC divides the 17.25 MHz
reference signal by 1725 using the reference counter (10.0 kHz comparison frequency). The phase
detector comparison frequency to be two times the channel spacing(5 kHz). The VCO output is
divided by the prescaler, swallow counter and programmable counter. These two signals are
compared by the phase comparator and input to the charge pump. A voltage proportional to their
phase difference is delivered to the low-pass filter circuit, then fed back to the VCO as a voltage with
phase error, controlling and stabilizing the oscillating frequency. This synthesizer also operates as a
modulator during transmit.
The RX-VCO comprises Q1015 (2SK508) and D1017, D1016 (HVU306A x 2). It oscillates between
47.7 and 54.7 MHz according to the programmed receiving frequency. The TX-VCO is comprised of
Q1016 (2SC4226), and D1015, D1016, D1013, D1012 (HVU300A x 4). It oscillates between 29.7
and 37.0
MHz (37.0 and 50.0 MHz) according to the programmed transmit frequency. The VCO
output passes through a buffer amplifier Q1021 (2SC5107), and a portion is fed to the buffer
amplifier Q1023 (2SC4215) of the PLL IC, and at the same time amplified by Q1027 (2SC5415) to
obtain stable output. The VCO DC supply is regulated by Q1007 (2SC4154E). Synthesizer output is
fed to the 1st mixer by diode switch D1022 (1SS321) during receiving and transmitting. The
reference oscillator feeds the PLL synthesizer, which is composed of crystal X1001 (17.2500 MHz),
the temperature compensation circuit (D1028 (MC2850) and thermostats TH1002 and TH1001), and
transmit (DCS) modulation circuit D1029, D1025 (HVU300A x 2) and D1026 (HVU306A).