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6 VCXO
6.1
What is VCXO
The phone has got a reference crystal of 13 MHz which signal is use for both the radio and the
logic.
The logic uses the clock signal MCLK as master clock and for the synchronisation of the digital
circuits of the logic.
The radio uses the 13 MHz signal as a reference signal for frequency regulation of both the
transmitter and the receiver.
The frequency fault of both the transmitter and the receiver must be inside the valid limits.
The phone has to have the possibility to control the frequency of the reference crystal to be able
to maintain the limits during different circumstances.
This is possible since the reference crystal is a Voltage Controlled Crystal Oscillator, VCXO.
The schematic is shown below Fig.6.1.
The crystal B510, the capacitors C510, C512, C513 and the varicap diode V510 are forming an
oscillating circuit.
The active part of the oscillating circuit is in N500
By changing the DC voltage of the varicap diode its capacitance changes, this changes the
frequency of the oscillating circuit.
The control voltage VCXOCONT for the varicap diode comes from a DAC in N800.
The range of the DAC is between 0 and 3FF Hex, that is equivalent to a control voltage
between zero V
The frequency of the oscillating circuit is amplified in N500 and goes to the radio and the logic
through two outputs called 13MHz and MCLK.
4/00021-3/FEA 209 544/18 B
Fig.6.1
and +3 V
.
DC
DC
Trouble Shooting Guide, Advanced
27(121)
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