Motorola GM1200E Service Manual page 60

Frequency Synthesis
13.0 Frequency Synthesis
The complete synthesizer subsystem consists of the Reference Oscillator (Y3701 or Y3702), the
Fractional-N synthesizer (U3701), the Voltage Controlled Oscillator (Q3741, Q3751), the RX and TX
buffer stages (Q3760, Q3770, Q3780) and the feedback amplifier (Q3790).
13.1 Reference Oscillator
The Reference Oscillator (Y3702) contains a temperature compensated crystal oscillator with a
frequency of 16.8 MHz. An Analogue to Digital (A/D) converter internal to U3701 and controlled by
the microprocessor via serial interface (SRL) sets the voltage at the warp output of U3701 pin 16 to
set the frequency of the oscillator. The output of the oscillator (pin 2 of Y3702) is applied to pin 14
(XTAL1) of U3701 via a RC series combination.
In applications were less frequency stability is required the oscillator inside U3701 is used along with
an external crystal Y3701, the varactor diode D3702, C3708, C3710 and R3704. The crystal may
not be replaced in case of failure. Instead of the crystal, the reference oscillator Y3702 must be
soldered in along with C3706, C3707, R3703. Components Y3701, C3708, C3710, R3704, D3702
must be removed and the value of C3709 must be changed. Afterwords the radio must be retuned.
13.2 Fractional-N Synthesizer (U3701)
The FRAC-N synthesizer IC (U3701) consists of a pre-scaler, a programmable loop divider, control
divider logic, a phase detector, a charge pump, an A/D converter for low frequency digital
modulation, a balance attenuator to balance the high frequency analogue modulation and low
frequency digital modulation, a 13V positive voltage multiplier, a serial interface for control, and
finally a super filter for the regulated 9.3 volts.
A voltage of 9.3V applied to the super filter input (U3701 pin 19) supplies an output voltage of 8.6
VDC at pin 18. It supplies the VCO (Q3741 / Q3751), VCO modulation bias circuit (R3714) and the
synthesizer charge pump resistor network (R3723, R3724). The synthesizer supply voltage is
provided by the 5V regulator U3801.
In order to generate a high voltage to supply the phase detector (charge pump) output stage at pin
VCP (U3701-32), a voltage of 13 VDC is being generated by the positive voltage multiplier circuitry
(D3701-1-3, C3716, C3717). This voltage multiplier is basically a diode capacitor network driven by
two (1.05 MHz) 180 degrees out of phase signals (U3701-9 and -10).
Output LOCK (U3701-2) provides information about the lock status of the synthesizer loop. A high
level at this output indicates a stable loop. IC U3701 divides the 16.8 MHz reference frequency down
to 2.1 MHz and provides it at pin 11. This signal is used as clock signal by the controller.
The serial interface (SRL) is connected to the microprocessor via the data line SPI DATA (U3701-5),
clock line SPI CLK (U3701-6), and chip enable line FRACN CE (U3701-7).
13.3 Voltage Controlled Oscillator (VCO)
The Voltage Controlled Oscillator (VCO) uses 2 colpitts oscillators, FET Q3741 for transmit and FET
Q3751 for receive. The appropriate oscillator is switched on or off by FRAC-N IC output AUX3
(U3701-1) using transistors Q3742 and Q3752. In RX mode AUX3 is nearly at ground level and
Q3742 enables a current flow from the source of FET Q3751 while Q3752 is switched off.
3.1-30 Introduction/Theory of Operation