The megatrend toward achieving seamless voice connectivity has significantly transformed the requirements for wireless infrastructure networks. As the global wireless market continues to evolve, these networks operate at different frequencies depending on regional development and technological maturity. The advancement of RF technologies and the emergence of new wireless standards have further increased the demand for fully integrated, adaptive chipsets that can support a wide range of applications. To meet the ever-changing needs of the wireless market, it is essential to develop comprehensive solutions that enhance design efficiency, reduce costs, and ensure superior voice quality and transmission performance. This article explores Texas Instruments’ family of radio frequency transceiver (TRF) analog signal chain solutions, which offer flexible design options and improved sound quality. Additionally, we will examine how OEMs, service providers, and the broader RF market can benefit from these innovations.
Seamless voice and data connectivity technologies are reshaping the design requirements for wireless infrastructure. System capacity must continue to grow, requiring higher bandwidth signals and multi-carrier capabilities. The power of the transmitter and the sensitivity of the receiver directly impact the coverage area of the network. To ensure ubiquitous signal availability, smaller pico base stations (TITIs) are deployed throughout cities to provide reliable services. In response, OEMs must deliver high-performance devices while maintaining efficient and cost-effective designs.
Moreover, OEM systems must support a wide range of existing wireless standards, such as CDMA2000, WCDMA, GSM, and EDGE, as well as emerging standards like TD-SCDMA in China and WiMAX for global broadband data services. A flexible architecture is crucial to meeting various modulation requirements, maximizing design resources, and improving system reliability. The diversity of global operating bands adds complexity, with voice communications typically ranging from 800 MHz to 2.1 GHz and data services from 3.5 GHz to 5.6 GHz.
To address this complexity, TI has introduced a highly flexible direct upconversion solution that meets multiple frequency band and wireless standard requirements. This solution offers superior RF performance, aligning with strict base station specifications, and its high level of integration makes it ideal for compact, low-cost designs.
In terms of transmitter architecture, two main options exist: direct upconversion and superheterodyne. The traditional superheterodyne architecture involves two mixing stages, where the signal is first converted to an intermediate frequency (IF) and then filtered using a narrowband SAW filter. In contrast, the direct upconversion scheme eliminates the IF stage, converting the baseband signal directly to the desired RF channel. Figure 1 illustrates the structures of both architectures.
Figure 1: Superheterodyne architecture and direct upconversion architecture
The direct upconversion method uses quadrature modulators and removes the need for additional mixing stages, synthesizers, and SAW filters. This not only simplifies the design but also reduces the bill of materials (BOM) cost. Furthermore, this architecture supports a variety of modulation technologies, including CDMA, GSM, and OFDM, making it highly flexible.
Without the need for narrowband filters, the architecture supports a wide range of signal bandwidths based on the selected modulation scheme. For example, it supports various bandwidths associated with CDMA2000 and WCDMA, as well as WiMAX signal bandwidths ranging from 3.5 MHz to 10 MHz. Multi-carrier applications are also supported due to the absence of bandwidth limitations. Additionally, the direct upconversion architecture supports digital predistortion (DPD) linearization signals, which require a bandwidth up to five times the desired signal bandwidth to account for nonlinear effects of the power amplifier.
The direct upconversion modulator consists of differential in-phase (I) and quadrature-phase (Q) signals, which are combined at the output. A quadrature modulator is essential for this process. Due to the inherent characteristics of the quadrature modulator, the local oscillator (LO) signal and unwanted image signals are naturally suppressed without the need for a filter.
The amount of sideband suppression depends on the amplitude and phase balance of the input quadrature components. LO leakage is influenced by the DC offset balance between the two input paths of the IQ signals. It is preferable for the device to achieve better than 35 dBc rejection of LO leakage and unwanted sidebands, as these parameters may degrade with temperature. If further suppression is needed, fine-tuning can be performed in the digital-to-analog converter (DAC). Data converters like the TI DAC5687 provide an I/Q interface with built-in regulation to meet amplitude and phase balance requirements and support DC offset correction.
The key parameters of the TI TRF3703 modulator are listed in Table 1 below. The linearity and output noise parameters of the modulator are critical for system performance. These parameters define the operational output range of the device and set the upper limit for the maximum output power of the entire radio system. For modulated signals with a high peak-to-average ratio (PAR), such as CDMA and OFDM, the modulator must avoid negatively impacting adjacent channel power ratio (ACPR) performance during signal peaks, ensuring compliance with industry standards.
Table 1: TRF3703 RF parameters
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