Nokia Mobile Phones NSE-5 Series Service Manual page 76

NSE–5
System Module
AFC function
In order to maintain the clock of the transceiver, i.e. the 13 MHz VCTCXO,
locked to the frequency of the base station an AFC (Automatic Frequency
Control) is used. The AFC reduces variations in the frequency of the
VCTCXO due to temperature drift. The AFC voltage is generated by
baseband with an 11 bit DAC in the COBBA. There is a RC-filter in the
AFC control line to reduce the noise from the converter.
The AFC voltage is obtained by means of Pure Sine Wave (PSW) slots,
which are a part of the signaling from the base station. The PSW slots are
repeated every 10 frames, meaning that there is a slot in every 46 ms.
Since changes in the VCTCXO -output frequency due to temperature
variations are relatively slow compared to the 46 ms, the transceiver has a
stable clock frequency.
When the transceiver is in sleep mode and "wakes" up to receive mode,
there is only about 5 ms for the AFC-voltage to settle. When the first burst
arrives the system clock has to be settled to +/- 0.1 ppm frequency
accuracy. The VCTCXO-module requires about 4 ms to settle into the final
frequency. The amplitude rises to maximum in about 3 ms, but because
the frequency–settling time is higher, the oscillator must be powered up
early enough to avoid frequency errors.
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PAMS
Technical Documentation
Issue 1 07/99