The demand for higher data rates is driving the evolution of mobile communication systems from 2G to 3G. As these systems offer faster speeds, they place more stringent requirements on the design of RF components in mobile devices. This shift has led to the adoption of more sophisticated modulation techniques that improve spectrum efficiency and overall performance.
To maximize bandwidth efficiency within the allocated frequency bands, third-generation systems employ linear modulation schemes such as Quadrature Phase Shift Keying (QPSK), 8-Phase Shift Keying (8PSK), and Quadrature Amplitude Modulation (QAM). These methods ensure better spectral utilization but also require linear RF power amplifiers to maintain signal integrity. Linear amplifiers are essential to preserve good Adjacent Channel Power Rejection Ratio (ACPR) and Error Vector Magnitude (EVM) characteristics.
In CDMA-based systems, a common approach is to use Class A power amplifiers, which provide high linearity but suffer from low efficiency. Typically, Class A amplifiers operate at around 30% efficiency at the 1dB compression point, and their efficiency drops further when operating below this point. In practice, RF power amplifiers in IS-95 and CDMA2000 systems often work with a back-off of 6dB to 40dB from the peak power level, resulting in very low efficiency most of the time.
Given that RF power amplifiers consume a significant portion of the battery power—up to 20% to 40% in typical mobile applications—it's crucial to optimize their energy efficiency. Improving this efficiency directly translates to longer talk time and better battery life, making it a key focus for modern mobile device design.
This paper introduces a simple power tracking technique designed to enhance the efficiency of RF power amplifiers. The method utilizes a dB-linear RF power detector, such as the LMV225, along with a DC-DC converter. By dynamically adjusting the supply voltage (VCC) of the RF power amplifier based on output power levels, this approach can significantly boost energy efficiency. Off-the-shelf CDMA2000 RF power amplifiers can benefit from this technology, leading to improved performance in mobile devices.
RF power amplifiers play a central role in wireless communication systems. When paired with a DC-DC converter, they form an efficient power management circuit. For example, the SKY77152, a popular CDMA2000 RF power amplifier, can achieve over 40% power added efficiency (PAE) near the 1dB compression point.
CDMA RF power amplifiers typically have multiple supply voltage pins, such as VCC and VBIAS, as well as a reference voltage pin (VREF), which must be set to 2.85V. These amplifiers usually support two operational modes: high power and low power. The VCONT pin controls the mode, allowing the amplifier to switch between them based on the RF output level. However, switching modes can introduce phase differences between signal paths, complicating baseband processing.
Figure 2 illustrates how reducing the DC supply voltages VCC and VBIAS affects the RF output power. It shows that lower supply voltages result in reduced RF power, demonstrating the relationship between power supply and output performance.
Power Amplifier Efficiency (PAE) is a critical metric defined as the ratio of RF output power to DC input power. While manufacturers often highlight peak PAE at maximum output power, real-world usage rarely reaches this level. In mobile devices, the average PAE is more relevant, as it reflects actual power conversion efficiency during normal operation.
As shown in Figure 3, most of the time, the RF power amplifier operates below +15dBm, making it important to optimize efficiency at lower power levels. Reducing the supply voltage can lower DC power consumption, as demonstrated by Equations 1 and 2.
However, lowering the supply voltage requires careful consideration of several factors, including ACPR, EVM, and the transition time between different supply voltages. Maintaining signal quality while improving efficiency is a delicate balance.
Adjacent Channel Power Rejection Ratio (ACPR) measures the amount of power leaking into adjacent channels. Table 1 outlines the ACPR requirements for CDMA2000 systems. Although earlier standards like IS-95 may not have formal ACPR specifications, designers should still aim to meet these performance levels to avoid signal distortion and interference.
In summary, optimizing RF power amplifier efficiency is essential for extending battery life and improving the overall performance of mobile devices. Techniques like power tracking offer a practical solution to this challenge, ensuring both efficiency and signal quality in modern communication systems.
Y40 Pressure Gauge
ZHOUSHAN JIAERLING METER CO.,LTD , https://www.zsjrlmeter.com