Built-in antenna FM radio small low noise amplifier in mobile phone

The FM radio module has become a standard for many mobile phones and earphone devices. They basically use the earphone cable as the FM antenna. Because the receiving margin of the earphone cable as an antenna is not large, it is not convenient and the Bluetooth earphone cannot be used when the earphone cable is used, so the earphone cable as a radio antenna is not an ideal solution. Another solution is to integrate a passive antenna into the inside of the mobile phone. However, since the wavelength of the FM band is on the order of one meter, and the small size required by the mobile phone device greatly limits the electrical size of the built-in antenna, the gain of the antenna is reduced, and the receiving sensitivity of the FM receiver is greatly reduced.

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However, the sensitivity of the FM receiver can be greatly improved by using so-called active antenna technology. Active antenna technology refers to the direct connection of the FM's internal antenna with a suitable low noise amplifier. The so-called active antenna module consists of a single, specially fabricated passive structure that is directly connected to the corresponding amplifier circuit. This technology quickly gained recognition in new mobile device applications. High gain, small package and high antistatic capability The low noise amplifier is an important factor in the design of active antennas. Infineon Technologies' new generation BGB719N7ESD can meet all the requirements of this active antenna.

Figure 1: The BGB719N7ESD requires fewer peripherals than previous generations (BGB707L7), saving PCB space.

Built-in antenna design challenges

While providing several of the above advantages, there are some challenges in designing an active antenna. Two problems arise when integrating an antenna into the inside of a mobile phone. First, an antenna with a small size and a relatively low position has a high height. The input reactance, low input impedance and low radiation efficiency make it difficult to get a good match with other circuits. The second problem involves the isolation of the transmitted signals of other antennas inside the mobile phone that are close to the FM antenna. This effect RF current in the chassis of a mobile phone that is related to most radiation. Other antennas connected to the chassis of the mobile phone or antennas similar to the chassis as the main radiator will suffer serious signal leakage.

The small size of the built-in antenna will severely reduce the antenna gain, resulting in lower receiving sensitivity than using the earphone as an external antenna. To compensate for the receiving sensitivity, active antenna technology must be used. In addition to the main component of the low noise amplifier. External ESD protection devices and single-pole double-throw switches for diversity antennas are also necessary in active antenna systems. Single-pole double-throw switches can be used to switch between external earphone antennas and internal antennas.

Using a high-performance low-noise amplifier can improve the sensitivity of the receiver. However, there are some difficulties in the low-noise design of the above-mentioned built-in antenna system. In mobile applications, in order to extend battery life, low current is necessary. However, low current will affect the linearity of low noise. Other requirements include shutdown function, strong antistatic capability of RF pins, small size and few peripheral devices.

Solution - Active Antenna

Reducing the size of the internal antenna will result in higher losses and thus lower the sensitivity of the receiver. The application of a low noise amplifier and its associated antenna radiator can solve this problem. Due to the very small size, the impedance of the antenna is very high, so Low noise figure, low noise and must match the high impedance of the antenna. Active antenna solution in mobile phones, low current consumption, shutdown function and high linearity (because there will be more in the mobile phone) Band interference). At the same time, low noise release needs to support the worldwide FM band (76-108MHz), high antistatic capability to support system level static protection requirements.

Table 1: Electric small size antenna design requires low noise

Small package, high gain and low current

Infineon Technologies' latest generation of low noise amplifiers can solve the main problems of built-in active FM antennas. It can improve receiver sensitivity and consume less power meter to save board area. BGB719N7ESD is a low MMIC dedicated to FM antenna systems. Noise. It provides high gain, low noise figure, high antistatic capability, low power loss and low distortion. It is a low-cost silicon germanium carbon technology developed by Infineon Technologies. Has a high cost performance.

Figure 2: Infineon Technologies' latest generation of FM low noise amplifiers for mobile devices, the BGB719N7ESD, offers high gain and low current consumption in a tiny package.

The main features of the BGB719N7ESD include:

- Support for FM bands worldwide (76 to 108MHz)

- Less peripherals (only 3 external capacitors required)

- Small package leadless TSNP-7-6

- Higher gain at 2.8mA

- Built-in dynamic bias circuit to maintain a stable operating point during temperature changes and power supply changes

- ESD protection (1.5KV HBM) for all pins

- Higher input compression point (IP1dB=-6dBm typical)

- High input impedance

- Excellent noise figure (1.2dB) with the latest SiGe:C process

- Wide power supply range: 1.5V to 4.0V

- Shutdown function

- Lead-free (RoHS compliant), halogen-free (WEEE compliant) product

The BGB719N7ESD is available in a very small leadless TSNP-7-6 (1.26mm x 1.4mm x 0.31mm) package that performs all of the above functions. The integrated dynamic bias circuit ensures stable DC temperature and environmental changes. Work point. It can be applied to all mobile devices such as mobile phones, PDAs, portable FM radios, MP3s, etc.

Compared to previous generations of devices (BGB707N7ESD), the new generation of BGB719N7ESD has the following improvements: Small PCB area (6mm2 Vs 20mm2), high gain (23dB V S21 dB), reduced supply current (2.8mA Vs) 3.0mA), high stability and few peripherals (3 Vs 7).

Figure 3: BGB719N7ESD has several enhancements in FM applications compared to previous generations (BGB707L7), such as high gain, etc.

The BGB719N7ESD integrates up to 2KV (HBM) of ESD protection. If it is equipped with Infineon Technologies' ESD protection diode ESD0P8RFL at its front end, it can improve the system's ESD protection to 8KV contact discharge (according to IEC61000-4-2). The ESD0P8RFL is packaged in TSLP-4-7 (1.2mm x 0.8mm x 0.39mm) with a capacitance of only 0.8pF.

Infineon Technologies offers a low noise protection solution for high static protection with built-in FM antennas. Due to the small TSLP package, this design is suitable for small mobile applications. When applying the 0402 capacitor, the entire low noise amplifier circuit only occupies approximately 8mm x 8mm PCB area. Since the BGB719N7 integrates a dynamic bias circuit internally, the application circuit requires few peripheral components. This not only reduces the PCB area but also reduces material costs.

The BGB719N7ESD can be easily matched to short half-loop antennas and monopole antennas. Only one passive device at the input is required. In the case of a half-loop antenna, a capacitor of approximately 30 pF is connected in parallel to the low noise input. If it is a monopole antenna, the antenna can be matched with the low noise amplifier by connecting an inductor of about 300nH in parallel with the low noise input.