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Tackling FM antenna design roadblocks

Posted: 12 Mar 2009 ?? ?Print Version ?Bookmark and Share

Keywords:antenna design FM? sensitivity reception maximizing? portable mobile?

By Natalian Zhai
Silicon Laboratories Inc.

FM radio has been used for years in high-fidelity music and speech broadcasting, offering excellent sound quality, signal robustness, and noise immunity. Recently, FM radio has witnessed an explosion of interest from the market for its applications in mobile and personal media players; however, the traditional FM design approach requires a long antenna, such as a wired headphone, which limits its usefulness for many users who do not carry the wired headset. Also, as wireless usage models continue to be a growing trend in portable devices, more customers can benefit from wire-free FM radio reception using other FM antennas while listening with either a wireless headset or a speaker output.

This article introduces an FM radio receiver solution that enables the antenna to be integrated or embedded inside the portable device enclosure, making the headphone cable optional. It starts with the goal of maximizing sensitivity; follows with methods for achieving the maximum sensitivity, including maximizing efficiency at the resonant frequency, maximizing antenna size, and maximizing efficiency across the FM band with a tunable matching network. Finally, this article describes an implementation of the tunable matching network.

Maximizing sensitivity

Sensitivity can be defined as the weakest signal that an FM receiver system can receive while achieving a certain signal-to-noise ratio (SNR). It is an important parameter of FM receiving system performance and is related to both signal and noise. The received signal strength indicator (RSSI) indicates only the RF signal strength at a particular tuned frequency. It does not provide any information about noise or signal quality. The audio signal-to-noise ratio (SNR) is perhaps a better measure for comparing receiver performance with different antennas. Therefore, maximizing SNR is essential for listeners to experience good audio quality.

Antennas are the connection between the RF electrical circuits and electromagnetic waves. For FM reception, an antenna is a transducer that converts energy from electromagnetic waves to a voltage that can be used by an electrical circuit, such as a Low Noise Amplifier (LNA). The sensitivity of an FM receiving system is directly related to the electrical voltage received by the internal LNA. To maximize sensitivity, the electrical voltage must be maximized.

There is a variety of antennas, including headphone, stub, loop, and chip antennas, on the market, but all antennas can be analyzed using equivalent circuits. Fig. 1 shows a generalized equivalent antenna circuit model:

Fig.1: A generalized equivalent antenna model

In Fig. 1, X can be either a capacitor or an inductor. The choice of X is determined by the antenna topology, where the value of the reactance (inductive or capacitive) is related to the antenna geometry. The loss resistance, Rloss, is related to the power dissipated in the antenna as thermal energy. The radiation resistance, Rrad is related to the voltage generated from the electromagnetic wave. For simplicity, we will analyze the loop antenna model in the remainder of this article. Similar calculations can be made for other antenna types, such as the short monopole and headphone antennas.

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