Si446X AFC OPERATION Application Note
This document describes the operation of the Automatic Frequency Control circuitry on the Si446x RFIC family. In a radio link it is inevitable that there is some residual frequency offset between the transmit and receive side. This frequency offset can disturb the detection of an (G)FSK modulated digital transmission. The Automatic Frequency Control (AFC hereafter) in the receive side works towards eliminating this frequency offset to maintain good reception and sensitivity.
Fundamentally the AFC is built up from two main blocks: the frequency error detector device and the error compensation device. The two devices can be arranged in feedback configuration (in a control loop fashion) or in feed-forward configuration (the error compensation device will not modify the original frequency error). There are three kinds of frequency error detectors on the Si446x RFIC and both feedback and feed-forward compensation can be set.
The selected detector and compensation circuits are driven by the data packet structure which the chip is configured to receive. The receiver cannot acquire and track frequency errors immediately; it always needs a certain amount of time (and even better, some knowledge of the packet's beginning) to achieve frequency alignment. Therefore a preamble is always necessary in a packet before the real data fields begin. (The preamble is also used to settle the other two control algorithms in the receive side: Automatic Gain Control (AGC hereafter) and the Bit Clock Recovery (BCR hereafter). As a rule of thumb the longer the preamble and the more data transition it contains the more robust the link will become.
The AFC only works in (G)FSK mode; OOK signal detection is not sensitive to frequency offsets as long as the signal stays within the receive channel. However, when operating in OOK mode the receiver bandwidth is typically set wide enough to incorporate even relatively big frequency misalignments.
From above, one can conclude that there are a lot of configuration possibilities on the RFIC. However, all these settings will be automatically generated by the Wireless Development Suite (WDS hereafter) software based on higher level input parameters. References will be made in each section as to how to achieve a certain set of settings using WDS and which API properties contain the relevant settings.
In the following sections the different modes of operations, their prerequisites, and their performances are discussed in detail.
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Application note & Design Guide |
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Please see the document for details |
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English Chinese Chinese and English Japanese |
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11/14 |
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Rev. 0.2 |
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AN734 |
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1.6 MB |
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