Motorola GM950E Service Manual page 66

Frequency Synthesis
Rise and fall time of the output power during transmitter keying and dekeying is controlled by the
comparator formed by opamp U0701-3.
During normal transmitter operation the voltage at U701-3 pin 13 is higher than the voltage at pin 12
causing the output at pin 14 being low and switching off transistor Q0711. Diode D0732 reduces the
bias voltage BIAS VLTG for low control voltage levels.
The temperature of the PA area is monitored by opamp U0701-1 using thermistor R3611 (located in
the PA section). If the temperature increases, the resistance of R3611 decreases, decreasing the
voltage PA TEMP. The inverting ampliÞer formed by U0701-1 ampliÞes the PA TEMP voltage and if
the voltage at opamp pin 1 approaches 4.6 V plus the voltage (ON) across D0721, U701-1 simulates
an increased power which in turn decreases the power control voltage until the voltage at U0701-4
pin 9 is 4.6V again. During normal transmitter operation the output voltage of opamp U701-1 pin 1 is
below 4.6V. Diode D3601 located in the PA section acts as protection against transients and wrong
polarity of the supply voltage.
13.0 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 ampliÞer (Q3790).
13.1 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.
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 analog modulation and low
frequency digital modulation, a 13V positive voltage multiplier, a serial interface for control, and
Þnally a super Þlter for the regulated 9.3 volts.
A voltage of 9.3V applied to the super Þlter 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).
4-28 Theory of Operation