Section 6: 2-6 UHF Band 2 (450–512/527 MHz) 1–25 W Frequency Synthesis
6881091C63-F
U5502 is a temperature-sensing device, which monitors the circuit board temperature in the vicinity
of the transmitter driver and final devices, and provides a DC voltage to the PCIC (TEMP, pin 30)
proportional to temperature. If the DC voltage produced exceeds the set threshold in the PCIC, the
transmitter output power will be reduced so as to reduce the transmitter temperature.
4.0 UHF Band 2 (450–512/527 MHz) 1–25 W Frequency Synthesis
The synthesizer subsystem consists of the reference oscillator (Y5261 or Y5262), the Low Voltage
Fractional-N synthesizer (LVFRAC-N, U5201), and the Voltage Controlled Oscillator VCO.
4.1 Reference Oscillator
The reference oscillator (Y5262) contains a temperature compensated crystal oscillator with a
frequency of 16.8 MHz. An analog-to-digital (A/D) converter internal to U5201 (LVFRAC-N) and
controlled by the µP via serial interface (SRL) sets the voltage at the warp output of U5201 pin 25 to
set the frequency of the oscillator. The output of the oscillator (pin 3 of Y5262) is applied to pin 23
(XTAL1) of U5201 via a RC series combination.
In applications where less frequency stability is required the oscillator inside U5201 is used along
with an external crystal Y5261, varactor diode D5261, C5261, C5262 and R5262. In this case,
Y5262, R5263, C5235 and C5251 are not used. When Y5262 is used, Y5261, D5261, C5261,
C5262 and R5262 are not used, and C5263 is increased to 0.1 uF.
4.2 Fractional-N Synthesizer
The LVFRAC-N synthesizer IC (U5201) 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 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 5 volts.
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