The majority of current 2G and 2.5G cellular networks operate across four frequency bands in the RF spectrum. These bands are globally standardized, meaning that when a user travels with their mobile phone to different countries, the RF components of the device may need to support different frequencies. This necessitates a handset design that includes multiple configurations of parallel switches and key components like power amplifiers (PAs) and surface acoustic wave (SAW) filters. However, due to the maturity of 2G technology, market demand, and advancements in packet data and integrated IC transceiver designs, these multi-band solutions have become cost-effective and compact.
Figure 1 illustrates the architecture of a typical quad-band GSM transceiver. Through integration, only three main components are needed on the signal path: 1. A multi-band PA and integrated switch module; 2. A quad-band Rx SAW filter bank; 3. A silicon-based transceiver.
The situation for 3G is somewhat similar to earlier systems. Initially, UMTS 3G systems were allocated one frequency band, making the RF front-end relatively simple and allowing for the use of external filters without significant cost or PCB space. However, due to regional differences, regulatory restrictions, and the licensing of new spectrum, there are now 10 bands allocated globally for 3G, some of which overlap. As users expect their 2G devices to work internationally, 3G users also anticipate the same flexibility. This is especially important for high-end users who rely heavily on data and voice services, as they tend to be business professionals and early adopters of new technologies. Therefore, a more efficient multi-band transceiver design is necessary.
Figure 2 shows the current global band allocation map. A modern multi-band handset should ideally cover at least three bands—low, medium, and high. The most straightforward way to achieve this is by using a single-band transceiver design with multiple front-end modules operating in parallel. While the RF transceiver circuitry remains largely similar, external matching and filters must be specifically designed for each frequency band. This approach can be successfully implemented with careful frequency planning, wideband PLL systems, and advanced sub-micron CMOS integration. However, from a cost, integration, and PCB area perspective, this solution isn't always ideal.
A practical example of such a design is shown in Figure 3, where the GSM section has been omitted for simplicity.
Figure 1: An example of a quad-band GSM transceiver using only three devices.
Figure 2: Frequency allocation diagram for the 3GPP band.
Figure 3: Example of a 3G multi-band transceiver front end built from single-band designs.
The Othello-3 family of 3G transceivers introduces a bold processing architecture that reduces the challenges associated with front-end integration. One of the main obstacles is the need for external filters, which define the input and output frequency ranges for a given band. In full-duplex systems like WCDMA, both the transmitter and receiver operate simultaneously, requiring a duplex filter at the antenna to prevent high-power transmission signals from degrading the sensitive receiver. However, due to circuit limitations, it's often necessary to include separate transmit and receive filters, as shown in Figure 3.
Transmitter Architecture
A common superheterodyne transmitter architecture is illustrated in Figure 4. The filtered I/Q baseband signal is mixed with an orthogonal local oscillator (LO) to produce a constant intermediate frequency (IF) signal. This signal is then filtered to remove unwanted spurious components before being mixed again with a variable frequency oscillator to generate the final RF output. Gain control is applied throughout the system. External SAW filters help reduce the complexity of the integrated Tx channel by filtering out unwanted frequencies and ensuring compliance with noise and harmonic requirements. The filtered signal is then sent to the PA and duplexer, completing the transmit chain.
The Othello-3 family uses a direct conversion or "zero heterodyne" transmitter architecture, directly upconverting baseband signals to the RF carrier. This eliminates the need for secondary mixing and reduces the number of external filters required. However, without a Tx filter, additional attention must be paid to noise levels in the receiver band.
To address this, the Othello-3 includes a special modulator core that minimizes LO channel noise, traditionally found far from the carrier. Its 86dB gain control is integrated into the modulator, reducing the need for extra circuitry and minimizing noise contribution. Low-noise design techniques are embedded throughout the transmitter, eliminating the need for additional filtering. Calibration is fully self-calibrating, requiring no user input. The modulator operates with fully differential signal processing, and a built-in balun provides a 50Ω single-ended output directly to the PA module.
The Othello-3 transmit channel delivers excellent EVM and ACLR performance. Removing the external SAW filter on the transmit side significantly reduces costs and PCB space, enabling multi-band PA integration. As seen in the GSM market, all four bands can fit into a single PA package.
Direct Conversion Receiver
The direct conversion receiver architecture used in all Othello transceivers is highly integrated and proven. In a GSM receiver, the entire receive signal chain can be integrated. For 3G systems, where transmitter leakage to the receiver is a concern, an interstage filter was previously required to reduce signal interference. This helped maintain receiver sensitivity but posed challenges for integration.
Othello-3 features three LNA modules—two for high bands and one for low bands. Each LNA has a single-ended input that easily matches to its respective duplex filter. They also include optimized bandpass responses to minimize out-of-band signals and transmitter leakage. The high linearity mixer design eliminates the need for an external interstage filter, as shown in Figure 5. Receiver gain control is distributed optimally between the RF signal chain and baseband stages. Integrated gain distribution logic simplifies programming and calibration, allowing the baseband to set optimal gain with a single word.
Figure 5: AD6551 front-end architecture with the external filter removed.
This example highlights the integration of 3G RF devices, with vendors also exploring antenna switch modules that include all front-end modes and integrated band switches with GSM Rx SAW filter banks.
The ADI Othello-3 transceiver consists of the AD6551, suitable for WCDMA 3G handsets, and the AD6552, designed for 3G TD-SCDMA. Both support 3GPP Release 5 and HSDPA operations.
Summary
Current single-mode 2G handsets can implement a full quad-band RF solution using just three major chip packages. This design is compact and meets all necessary requirements. Early 3G handsets were limited to single-band operation, relying on external filters, but this approach wasn't ideal for multi-band designs in terms of size and cost. The Othello-3 eliminates the need for external filters, enabling greater integration of front-end devices and PAs. With ongoing advancements in integration and switch design, more front-end components can be incorporated over time. However, with a transceiver like Othello-3, a fully optimized multi-mode architecture becomes achievable.
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