AN307 Datasheet, PDF (1/14 Pages) Silicon Laboratories – RECEIVE POWER SCAN PRIMER

AN307
Si4712/13/20/21 RECEIVE POWER SCAN PRIMER
1. Introduction
This document provides an overview of receive power scan (RPS), provides two algorithms for selecting the
optimal transmit frequency and details their implementation, and provides an algorithm test procedure and test
data.
The host controller uses the RPS feature of the Si4712/13/20/21 to select one or more optimal FM transmit
frequencies. This feature can eliminate the need for user intervention in the selection process, greatly reduce time
to select a transmit frequency, and select optimal frequencies because all channels in the band can be evaluated.
The Si4712/13/20/21 incorporates a power scan measurement function that provides total power received at the
antenna. This received power is referred to as the Receive Noise Level (RNL).
2. Algorithms for Selecting Candidate Channels
There are two algorithms for selecting candidate transmit channels using RPS. The first algorithm finds the
minimum noise level in a given band, and the second algorithm uses the minimum noise level averaged over
several channels in a given band. By averaging the received noise level over several channels, the second
algorithm will select candidate stations away from adjacent channels, which may improve performance in FM
receivers with poor adjacent channel selectivity. While averaging requires slightly more complexity on the host, it is
the recommended method.
2.1. Minimum Noise Level
The noise floor of the received spectrum is typically shaped with frequency due to environmental noise. To reduce
the effects of noise floor shaping, the algorithm divides the FM spectrum into several separate bands and finds the
lowest power in each. This prevents environmental noise from dominating the station selection criteria and spreads
the candidate stations throughout the FM band. Figure 1 shows an RPS spectrum along with the selected
candidate stations using this method.
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Figure 1. Selecting Stations Using Minimum Noise Level in Five FM Frequency Bands
Rev. 0.2 3/08
Copyright © 2008 by Silicon Laboratories
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