Mobile communication relies on electromagnetic waves as the medium for transmitting signals, enabling wireless connectivity. As a result, the radio frequency (RF) circuit plays a crucial role in mobile communication devices. The performance of the RF system directly affects the device's ability to receive and transmit signals, as well as its overall communication quality with the base station. Therefore, the design of RF circuits for mobile terminals and the development of RF components have become key concerns for manufacturers.
In modern mobile terminals, the RF circuit typically consists of three main parts: the RF front-end, the transceiver, and the frequency source. While these components differ significantly in design, materials, and manufacturing processes, they are all moving toward miniaturization and cost reduction. However, due to material constraints and the challenges of signal transmission, these three components are difficult to integrate into a single chip. As a result, they continue to develop independently while influencing each other, collectively shaping the evolution of RF technology in mobile devices.
The RF front-end is increasingly moving towards miniaturization and integration. Semiconductor companies have long been working on integrating multiple functions into large-scale integrated circuits, which has helped reduce terminal costs and bring more value to consumers. In the RF section of mobile devices, many components have either been integrated into chips or eliminated through the use of direct digital up/down converters.
Despite these advancements, two critical components—RF front-end filters and RF power amplifiers—have yet to be fully integrated. This is due to the incompatibility of their fabrication techniques with standard CMOS processes. Traditionally, filters have been made using ceramic or surface acoustic wave (SAW) technology, while power amplifiers rely on gallium arsenide (GaAs) heterojunction bipolar transistors (HBTs) or FETs. These technologies are fundamentally different from silicon-based processes used in RF chips, so these components remain as discrete devices, often combined with large-scale integrated circuits that handle most of the RF functions in today’s phones.
The RF front-end circuit usually includes three key elements: the RF power amplifier, the antenna switch or duplexer, and the front-end (image frequency) filter. Due to challenges in isolation and process compatibility, integrating these components into a single chip remains a significant challenge. To address this, antenna switch or duplexer manufacturers embed receive front-end filters on low-temperature co-fired ceramic (LTCC) substrates, creating compact RF front-end modules (FEMs) such as MURATA’s LMSP54HA and Hitachi Metals’ LSHS-M085FE. These modules offer advantages in size and cost, making them widely used in GSM mobile devices.
Another popular RF front-end module is created by integrating antenna switches or duplexers with power amplifiers using GaAs technology. Examples include RFMD’s RF7115 and SKYWORKS’ SKY77506. These modules also benefit from miniaturization and cost efficiency, and are commonly used in current GSM designs. Both antenna switch manufacturers and power amplifier producers are now focusing on the integration of these three components, making the three-in-one RF front-end monolithic an inevitable trend in the development of mobile communication systems.
The development of RF transceivers has evolved through three major stages. The first stage involved a superheterodyne architecture with fixed intermediate frequency (IF). The second stage introduced a low-IF superheterodyne system, reducing the need for external IF filters. The third stage uses direct conversion, where the received and transmitted signals are directly converted into I/Q signals without an IF stage. This approach eliminates the need for IF filters, simplifying the design and lowering costs.
In the first stage, RF transceiver chips integrated the classic superheterodyne architecture, using a fixed IF filter to select and process signals. A typical example was INFINEON’s PMB6253, which used a 360MHz IF filter for signal conversion.
The second stage improved upon this by increasing the first local oscillator frequency, resulting in a lower IF signal. This allowed the integration of channel filters and enabled the elimination of external IF filters, reducing costs. A common example at this stage was MT612X from MediaTek.
The third stage introduced direct conversion, where the local oscillator signal matches the operating frequency, allowing the IF to be zero. This eliminates the need for IF filters, further streamlining the RF transceiver design. This advancement represents a significant step forward in the evolution of RF technology in mobile devices.
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