AFC is used to lock the transceiverfrequencyto the frequency ofthe base station. AFC-voltage is generated in COBBA with 11 bit DA-converter. There is a RC-filter in AFC control line to reduce the noise from the converter. Settling time requirement for the RC-network comes from signaling, how often PSW (pure sine wave) slots occur. They are repeated after 10 frames, meaning that there is PSW in every 46 ms. AFC tracks base station frequency continuously, so transceiver has got a stable frequency, because changes in VCTCXO-output don't occur so fast (temperature).
Settling time requirement comes also from the start up-time allowed. When 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 comes in system clock has to be settled into +/-0.1 ppm frequency accuracy. The VCTCXO-module requires also 5 ms to settle into final frequency. Amplitude rises into full swing in 1...2 ms, but frequency settling time is higher so this oscillator must be powered up early enough.
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