Frequency Synthesis; Reference Oscillator; Fractional-N Synthesizer (U3701); Voltage Controlled Oscillator (Vco) - Motorola GM950E Service Manual

Frequency Synthesis

10.0 Frequency Synthesis
The complete synthesizer subsystem consists of the Reference Oscillator (Y3702 or Y3701), 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).
10.1

Reference Oscillator

The Reference Oscillator (Y3702) contains a temperature compensated crystal oscillator with a
frequency of 16.8 MHz. An analog 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.
10.2

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 analog 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 22) 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).
10.3

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. In TX
mode AUX3 is about 5V DC and Q3742 is switched off. Q3752 is switched on and enables a current
flow from the source of FET Q3741 while Q3751 is switched off. When switched on the FETs draw a
drain current of 8 mA from the FRAC-N IC super filter output. The frequency of the receive oscillator
is mainly determined by L3752, C3752, C3754 - C3756 and varactor diodes D3751 / D3752.
6.3-22 Theory of Operation