Motorola XTL 1500, Detailed Service Manual, Page: 94 / 452

6815854H01-A June 15, 2005
Theory of Operation: Frequency Generation Unit (FGU) 3-45
Protection Mechanisms
Final-stage temperature is sensed in the RFPA resulting in VTEMP, which is proportional to
temperature. VTEMP is compared against a reference voltage TEMP_1 (U0959, pin 9) via U0957-1.
When VTEMP exceeds TEMP_1, the U0957-1, pin 1, voltage increases and forward biases one of
the D0951 diodes, which cuts back power. Power continues to cut back with rising temperature until
the voltage level at the junction of R0978 and R0983 is high enough to forward bias D0952, thus
clamping the cut back so that the radio meets its duty cycle specification while providing protection
against high-temperature conditions. The clamping level is set via TEMP_2 (U0959, pin 10) and
U0957-2. U0957-3 is used to sense if a high A+ battery voltage condition exists and, if it does, the
Q0955 gate is biased on, which increases the clamp voltage allowing for additional power cutback
for a high A+, high temperature condition.
Final-stage current is also monitored via VCURRENT, which is proportional to current. VCURRENT
is compared against a reference CURR_LIM_SET (U0959, pin 1) which is tuned after power
characterization. If VCURRENT exceeds CURR_LIM_SET, then U0957-4, pin 14, voltage rises and
forward biases one of the D0951 diodes, which limits power.
Finally, control voltage is limited by U0956-4 and D0950. RFPA_CNTRL can rise to the control
voltage limit set by R0942-4.
3.13 Frequency Generation Unit (FGU)
This section of the theory of operation provides a detailed circuit description of the frequency
generation unit (FGU).
3.13.1 VHF (136-174 MHz) Band
The FGU ( Figure 3-34 on page 3-46 ) provides the XTL 1500 radio with a 16.8 MHz reference
frequency, receiver 1st local oscillator, and a modulated transmitter RF carrier that is further
amplified by the power amplifier section of the radio.
The FGU consists of the following:
• Reference oscillator (Y3750)
• Low-voltage Fractional-N (LV Frac-N) synthesizer (U3751)
• Two transmitter VCOs
• Three transmitter buffer/amplifier stages
• Two Receiver VCOs
• Three receiver buffer/amplifier stages