This article refers to the address: http:// In Figure 1, X can be a capacitor or an inductor. The choice of X depends on the antenna topology, and its reactance value (inductance or capacitance) is related to the geometry of the antenna. The loss resistance (Rloss) is related to the power dissipated in the form of heat in the antenna. The radiation impedance (Rrad) is related to the voltage generated by the electromagnetic wave. For ease of explanation, this paper only analyzes the loop antenna model, and the same calculations can be applied to other types of antennas, such as short monopole antennas and earphone antennas. As can be seen from the above figure, the best efficiency can be achieved at 98 MHz, but the closer the frequency is to the band edge, the efficiency decreases. This is not a big problem for earphone antennas because the antenna size is large enough to collect enough electromagnetic energy throughout the frequency band and convert it to a higher voltage into the RF receiver. However, compared to longer earphone antennas, the short antenna size is small and the collected energy is small, so the efficiency is also rapidly reduced when the frequency is far from the resonance point. This may cause reception problems when using a fixed resonance scheme at the edge of the band, mainly because the short antenna has a higher Q value than the earphone, making its efficiency extremely low at the edge of the band. The above design uses a mixed-signal digital low-IF architecture with a digital signal processor (DSP) to propose an advanced signal processing algorithm that includes a self-tuning embedded short antenna. The antenna algorithm automatically adjusts the capacitance of the variable capacitor based on each frequency tuning point of the device for optimal performance. Power X (Qingdao) Energy Technology Co., Ltd. , https://www.qdpowerxsolar.com
FM radio has begun to appear in more market applications such as mobile and personal media players. However, conventional FM design methods must use very long antennas, such as wired headsets, and thus limit the number of users who do not have wired headsets. In addition, with the increasing popularity of wireless usage modes in portable devices, more and more users are hoping to use wireless FM radios with other FM antennas while listening to sound using wireless headphones or speakers.
This article describes an FM radio receiver solution that integrates or embeds an antenna inside a portable device, making the headphone cord an optional accessory.
Maximizing Sensitivity Sensitivity can be defined as the minimum signal that the FM receiver system can receive and achieve a specific signal-to-noise ratio (SNR). This is an important parameter in the performance of the FM receiving system, which is related to both signal and noise. The Received Signal Strength Indicator (RSSI) can only indicate radio frequency (RF) signal strength at a particular tuning frequency and does not provide any information about noise or signal quality. The audio signal-to-noise ratio (SNR) may be a better parameter when comparing receiver performance using different antennas. Therefore, it is very important to maximize the SNR. The antenna is a bridge connecting the RF circuit and the electromagnetic wave. In the case of FM reception, an antenna is a transducer that converts energy from electromagnetic waves into voltages that are available to electronic circuits such as low noise amplifiers (LNAs). The sensitivity of the FM receiving system is directly related to the voltage received by the internal LNA. In order to maximize sensitivity, this voltage must be maximized.
There are a variety of antennas on the market, including headsets, metal stubs, loops and chip antennas, but all antennas can be analyzed with equivalent circuits. Figure 1 shows a general antenna equivalent circuit model. 
Maximizing Resonant Frequency Efficiency To maximize the antenna's switching energy, we used a resonant network to counteract the antenna's reactive impedance, which may cause the antenna to switch to the internal LNA's voltage value. For inductive loop antennas, the capacitor (Cres) can be used to cause the antenna to resonate at the desired frequency: 
The resonant frequency (fres) is the frequency at which the antenna can convert electromagnetic waves into the highest efficiency of voltage. Antenna efficiency is the ratio of the power of Rrad to the total power of the antenna, expressed as Rrad/Zabt, where Zant is the antenna impedance with an antenna resonant network. Zant can be expressed as: 
When the antenna is in resonance, the efficiency η can be expressed as: 
The efficiency at other frequencies is: 
The antenna efficiency η other than the resonant frequency is lower than the maximum efficiency ηres because the antenna input impedance Zant at this time is inductive if it is not capacitive. 
The Q value represents the quality factor, which is proportional to the energy and loss or radiant energy stored in the antenna network per unit time. For an antenna equivalent circuit with an antenna resonant network, the Q value satisfies: 
Compared with the short antenna, the earphone antenna has a high radiation resistance Rrad due to its large size, and therefore also has a lower Q value. Since embedded applications must use short antennas with higher Q values, the problem of drastic efficiency is particularly noticeable.
The Q value of the antenna is also related to the antenna bandwidth, and the relationship can be expressed as: 
Where fc is the resonant frequency and BW is the 3 dB bandwidth of the antenna. Compared to longer earphone antennas, short antennas with high Q values ​​have narrower bandwidths and therefore have higher losses at the edge of the band.
In order to overcome the bandwidth limitation problem of a high-Q fixed resonant antenna, a self-regulating resonant circuit can be used to change the fixed resonance to a tunable resonance, so that the circuit is often at a resonant frequency that maximizes the receiving sensitivity. A high signal-to-noise ratio can also be achieved with a self-tuning resonant antenna because the gain from the resonant antenna reduces the system noise figure of the receiver, while the inherently high Q value of the embedded antenna helps to filter out possible local oscillators. Interference with harmonics mixed together.
Construction of a Tunable Matching Network Figure 3 shows a conceptual block diagram of an enhanced FM receiver architecture. The FM receiver architecture can be used to support embedded short antennas, and its tunable resonance is achieved using on-chip tunable variable capacitance and tuning algorithms. 
Conclusion In order to maximize sensitivity, this article discusses how to improve the FM reception using embedded antennas, and further explore its implementation. Due to the limited space available for portable devices with embedded antennas, a self-tuning resonant network can be considered to maximize the sensitivity of the receiver over the entire FM band, allowing the short antenna to achieve maximum efficiency at each frequency.