Rev. 0.2 3/08

Copyright © 2008 by Silicon Laboratories

AN307

AN307

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