Trouble Shooting Guide, Advanced
4/00021-3/FEA 209 544/25 C
Ericsson Mobile Communications AB
32(78)
9VCXO
9.1 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 in the figure belowFig. 9.1.
Fig. 9.1
The crystal B320, the capacitors C321, C322, C323 and C324 and the varicap diodes V321
and V322 are forming an oscillating circuit.
The active part of the oscillating circuit is in N300.
By changing the DC voltage of the varicap diodes their capacitance changes, this changes the
frequency of the oscillating circuit.
The control voltage VCXOCONT for the varicap diodes comes from a DAC in N500.
The range of the DAC is between 0 and 3FF Hex, that is equivalent to a control voltage
between 0.0 and 2.5 V
DC
.
The frequency of the oscillating circuit is amplified in N300 and goes to the radio and the
logic through two outputs called 13MHz and MCLK.
The VCXO (13 MHz crystal) is trimmed in EFRA-calibration.
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