Implementing Automatic Frequency Control (AFC)
By S. Hellan and O. Stengel
Chipcon AS
Introduction
Chipcon's CC1020 and CC1021 transceivers have a built-in feature called Automatic Frequency Control (AFC), which can be used to compensate for frequency errors due to crystal inaccuracy, temperature drift, or aging.
The receiver can be calibrated against the transmitter by changing the operating frequency according to a measured offset from the nominal intermediate frequency (IF). The AFC feature reduces the crystal accuracy requirement thus enabling the use of cheaper crystals. The AFC can be used for an FSK/GFSK signal, but not for OOK/ASK.
This application note outlines the procedure for implementing AFC. Plots of receiver sensitivity versus frequency offset with and without AFC are provided.
Automatic frequency control
The average frequency offset of the received signal (from the nominal IF) can be read in the AFC register. The signed (2's-complement) 8-bit value AFC[7:0] can be used to compensate for frequency offset between an external transmitter and the receiving device.
The average frequency offset of the received signal (from the nominal IF) is given by:
where AFC is the digital value read from the AFC register converted to decimal.
Programming the frequency word in the configuration registers detemines the operating frequency. There are two frequency words, termed FREQ_A[22:0] and FREQ_B[22:0], which can be programmed to two different frequencies. One of the frequency words can be used for Rx and the other for Tx in order to be able to switch very fast between Rx mode and Tx mode. They can also be used for Rx (or Tx) at two different channels. In this application note, we assume FREQ_A[22:0] is used for Rx mode and FREQ_B[22:0] is used for Tx mode. The receiver can be calibrated against the transmitter by changing the operating frequency according to the measured offset. The new frequency must be calculated and written to the FREQ_A[22:0] register by the microcontroller. After this compensation the center frequency of the received signal will better match the digital receiver channel filter bandwidth.
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