Frequency Synthesis; Reference Oscillator; Fractional-N Synthesizer (U2701) - Motorola GM-950 Manual

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 R2611 (located in
the PA section). If the temperature increases, the resistance of the thermistor decreases, decreasing
the voltage PA TEMP. The inverting amplifier formed by U0701-1 amplifies 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. Resistor R0724, R0722, R0723 set the factor of the decrease in output
power per temperature increase while R0721 through R0723 set the threshold were the temperature
starts reducing the output power. During normal transmitter operation the output voltage of opamp
U701-1 pin 1 is below 4.6V. Diode D2601 located in the PA section acts as protection against
transients and wrong polarity of the supply voltage.
10.0

Frequency Synthesis

The complete synthesizer subsystem consists of the Reference Oscillator (Y2701 or Y2702), the
Fractional-N synthesizer (U2701), the Voltage Controlled Oscillator (Q2741, Q2751), the RX and TX
buffer stages (Q2760, Q2770, Q2780) and the feedback amplifier (Q2790).
10.1

Reference Oscillator

The Reference Oscillator (Y2702) contains a temperature compensated crystal oscillator with a
frequency of 16.8 MHz. An Analogue to Digital (A/D) converter internal to U2701 and controlled by
the microprocessor via serial interface (SRL) sets the voltage at the warp output of U2701 pin 16 to
set the frequency of the oscillator. The output of the oscillator (pin 2 of Y2702) is applied to pin 14
(XTAL1) of U2701 via a RC series combination.
In applications were less frequency stability is required the oscillator inside U2701 is used along with
an external crystal Y2701, the varactor diode D2702, C2708, C2710 and R2704. The crystal may
not be replaced in case of failure. Instead of the crystal, the reference oscillator Y2702 must be
soldered in along with C2706, C2707, R2703. Components Y2701, C2708, C2710, R2704, D2702
must be removed and the value of C2709 must be changed. Afterwords the radio must be retuned.
10.2

Fractional-N Synthesizer (U2701)

The FRAC-N synthesizer IC (U2701) 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 (U2701 pin 19) supplies an output voltage of
8.6VDC at pin 18. It supplies the VCO (Q2741 / Q2751), VCO modulation bias circuit (R2714) and
the synthesizer charge pump resistor network (R2723, R2724). The synthesizer supply voltage is
provided by the 5V regulator U2801.
Theory of Operation
Frequency Synthesis
5C.3-23