Motorola MTM700, Detailed Service Manual

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4.2 - 14 MTM700 Mobile Radio / Detailed Service Manual THEORY OF OPERATION (800 MHz)
Frequency Generating Section Detailed Circuit Description
For the Frequency Generating Section Block Diagram See Figure 4.
This section describes the generating circuits that supply all the required frequencies for the
required transmitter and receiver functions. These circuits are described as follows:
• 16.8 MHz Reference Oscillator - TCXO.
• Main VCO and Main Synthesizer.
• Transmit frequency translation loop.
Reference Oscillator - TCXO
16.8 MHz TCXO (Y5362) is a temperature compensated crystal oscillator producing an accurate
and stable 16.8 MHz master clock to the radio RF circuits. Its operating frequency is fine tuned by
means of a warp voltage signal generated by the LVFRACN IC (U5331, pin 23). On radio power-
up, an initial warp voltage setting is programmed into the LVFRACN IC by the microprocessor via
the SPI bus, using a stored value programmed into the radio codeplug during factory tuning. While
the receiver is registered to the radio system, an automatic frequency control algorithm is executed
by the microprocessor, to fine tune the LVFRACN warp voltage to bring the TCXO frequency into
alignment with the radio system frequency standard.
The output signal at pin 3 of the TCXO is passed to LVFRACN IC U5331, where it is used as a ref-
erence for the main frequency synthesizer. The TCXO output is also buffered by Q5351, and
passed to other radio circuits via REF16_8_OSC. The LVFRACN IC buffers the 16.8 MHz refer-
ence, and provides it at FREFOUT (pin 41) for use by other radio circuits.
Main VCO and Main Synthesizer
The synthesizer components are LVFRACN, Loop Filter, VCO and buffer. It produces the receiver
first LO signal, as well as serving as the reference for the transmit frequency translation loop.
The operating frequency of Voltage Controlled Oscillator Q5712 is determined by resonator W5701,
in conjunction with C5735, C5737, C5730, L5733, and varactors D5732 and D5734. The operating
frequency of the oscillator is varied by means of varactor tuning voltage VCTRL, supplied by the
LVFRACN U5331 IOUT signal (pin 43), and filtered by loop filter components C5303, R5304,
C5305, R5303, R5302, and C5301. The RF output of the VCO is buffered by U5701. U5701 pin 12
provides a sample of the VCO output to the LVFRACN prescaler input (pin 32), where the signal is
divided down to a lower comparison frequency by means of a programmable frequency divider. The
divided down VCO signal is compared to a signal derived from the 16.8 MHz reference clock to gen-
erate an error signal (IOUT) which is filtered by the loop filter and tunes the VCO to the programmed
operating frequency.
A LOCK_FN signal is available at LVFRACN pin 4 to provide feedback to the microprocessor that
the synthesizer has successfully locked to the programmed frequency.
The “RX_OUT” signal from buffer U5701 pin 8 is amplified by Q5701, and supplied to the first mixer
LO port.
The “TX_OUT” signal from buffer U5701 pin 10 provides the frequency reference for the Transmit
Frequency Translation Loop.