The most appropriate multi-mode mobile phone transmission architecture

In the fiercely competitive GSM phone terminal market, terminal manufacturers are beginning to expect solutions that can speed up development and shorten development cycles. As more and more new feature-rich mobile phones begin to integrate megapixel digital cameras, Bluetooth technology and multimedia functions, designers are beginning to look for smaller and more integrated transmit and receive RF modules that can provide excellent performance levels while occupying less board space.

In order to enable more designers to quickly complete the design and development of these increasingly complex new mobile phone terminals, component solution suppliers are faced with the challenge of how to meet some inherently contradictory design requirements. To meet these market demands, designers at RF Micro Devices (RFMD) have developed the RF7115 transmitter module (TxM).

This transmit module brings the GSM RF front end one step closer to full integration. As a major component supplier for this application market, we have witnessed the design complexity of discrete RF front end solutions, which often prevented GSM mobile phone manufacturers from smoothly entering the mass production stage. This experience led us to develop a module that embeds the power amplifier (PA) die and matching circuits, which we call a power amplifier module (PAM). From this foundation, we have further embedded integrated power control in the PAM, which can replace the complex and sensitive control loop designed to make the PA output well converted to the GMSK burst template.

In GMSK mode, in order to meet the timing specifications or restrictions specified by ETSI, the PA has burst timing requirements. A burst template is simply the recommended or implemented timing of the TxM module control signal, which is used to ensure that all ETSI restrictions are met during the rise and fall phases of a transmit burst pulse. This revolutionary technological advancement can reduce the development time of the telephone platform by 1/4.

Benefits of TxM

Besides the obvious advantages of reduced size and component count, in what ways does TxM really benefit designers compared to discrete implementations?

Reduce insertion loss on Tx and Rx paths

Generally speaking, the transmit module implements the antenna switch function by integrating any number of switch configurations and internal architectures. The common TxM module architecture contains two Tx path switches and 3 or 4 Rx path switches, connected to the antenna through a common node. The typical configuration uses SPXT or cascade design.

While early switch modules were mostly implemented using PIN diodes, newer designs have utilized custom pHEMT switch designs to achieve similar or better performance. Among the various switch design options, one tangible benefit of pHEMT-based designs is reduced insertion loss (IL) through the switch.

Insertion loss in the Tx signal path can be minimized by careful design of the pHEMT switch pins and harmonic matching filters. By minimizing insertion loss, the PA output power can be delivered to the antenna more efficiently. This is possible in TxM because the designer has a good understanding of the PA output matching and control of the matching/filet and pHEMT inputs. This gives the designer a degree of flexibility compared to the current PAM and ASM (antenna switch module) combination solution. This is possible because the TxM designer is no longer restricted to a 50 ohm output environment. He can choose the appropriate impedance to match the output of the amplifier (typically 2-4 ohms) and match it directly to a pHEMT switch. This means that only one matching network is used, thereby reducing losses (see Figure 1).

Insertion loss is reduced because you no longer have to perform the task of transforming the impedance at the PA output and ASM input to 50 ohms. This reduces transmit chain loss by up to 0.5 dB, saving 150 mA for GSM mode and 90 mA for PCS/DCS mode.

Similarly, the receive path (Rx) internal matching topology also benefits from the elimination of the 50 ohm transition, and the same benefits can be obtained on the Rx side of the TxM. The reduction in Rx insertion loss improves Rx sensitivity.

Improved harmonic performance

As time to market becomes more important, design reuse has become a common issue for developers, leading to the emergence of more and more mobile phone development platforms. We have seen this trend towards platform-based development becoming more and more obvious, and many future mobile phone designs will adopt RF transceiver and PA+ASM combination solutions. However, many difficulties are encountered when applying this "well-known" platform to designs with different form factors. In particular, clamshell, candy bar and slider mobile phone form factors all cause mismatches between antennas and RF modems and variations in PCB performance. Therefore, a platform that works well in one form factor may perform very differently in another form factor, which usually results in a decrease in overall performance. In these cases, antenna differences are usually the culprit.

RF platform designers can benefit from having a tightly controlled PA output, harmonic filter and switch interface. TxM provides designers with a controlled environment where they can optimize the harmonics emitted from the PA. This also makes the RF design a true platform that can adapt to various form factors, and the performance included in TxM can be guaranteed by the electrical specifications set by the manufacturer. This is completely impossible for a typical PA+ASM platform design.

TxM was developed to address a design difficulty faced by most handset designers. Furthermore, even after the handset designer has achieved a robust design, any modification to the precision matching can severely change the impedance and affect the harmonic levels measured at the antenna. By maintaining the integrity of this interface, the TxM designer has a greater chance of suppressing harmonics under extreme conditions, even under mismatched conditions. For example, a VSWR (voltage standing wave ratio) of 4:1 with harmonic levels below -35dBm under all extreme conditions is typically achievable for TxM designs (see Figure 2). This results in a very robust solution that can be used in many different form factor considerations.

Flexible transceiver signal routing

As mentioned earlier, the latest TxMs use pHEMT switches in their designs. In the antenna switch section, the advantage of using a pHEMT switch comes from the ability to set the Rx port in any way required for routing. The pHEMT input frequency response is broadband, in fact, it allows any port to be designated as high band or low band. This allows the GSM TxM to be used independently of any specific transceiver. We predict that this will be particularly useful in an ODM environment, as ODMs will typically maintain 2 to 3 different transceiver configurations based on baseband selection, but for the PA, will generally only choose one from the major volume suppliers.

Potential for transmitting WCDMA signals

A feature that is particularly convenient for emerging multimode applications is that the TxM module can designate an unused port as a pass-through port for independent WCDMA Tx/Rx signals. In today's era of transition to multimode applications, the market often creates a need for a quick bundling solution that utilizes independent WCDMA and GSM/GRPS RF chains. The ability to utilize an unused port for switch control in a GSM/GPRS TxM eliminates the need for a separate ASM and matching components to support the WCDMA RF chain. This solution saves both cost and size, both of which are very important for this emerging market.

RFMD has tested and characterized the linearity and intermodulation capabilities of this pHEMT switch port at WCDMA frequencies and power levels. The results show that it is feasible to use one of the Rx ports as a WCDMA channel. The characterization includes intermodulation measurements, insertion loss, ACP degradation, harmonic generation, and heat generated by the pHEMT switch.

Other features of the TxM modules include ease of use, built-in 8KV ESD protection circuitry, and reduced external component count. TxM modules offer both improved performance and ease of use, which are often not available simultaneously in mobile phone designs. One of the "ease of use" features found in most commercial TxM modules is the integrated ESD protection at the antenna port. It includes an on-board susceptor circuit to suppress any high voltage glitches due to damage to modem components. This is a very useful feature because the protection capabilities of most SAW filters in the Rx path against ESD transients are very questionable. In addition, these ESD filters fully comply with the 8KV contact discharge requirements of IEC 61000-4-2.

In addition to allowing designers to achieve higher levels of performance, TxM modules also require fewer external components, which is also very important. As part of the future development trend, new generations of integrated modules continue to face the challenge of reducing the number of external components required to implement RF solutions.

The same challenges are faced by TxM modules. In fact, TxM modules specifically take these integration requirements as a fundamental premise of their existence. TxM modules not only reduce the number of external components, but also eliminate the need for sensitive internal component matching circuits. Compared with other implementations of the same function, TxM modules can help designers reduce the overall layout size. Typical figures show that compared with competitive PA+ASM solutions, a 30% to 50% layout area reduction can be achieved.

Reduce phone platform development time

The TxM module is a very robust solution, which can be directly attributed to the use of internal matching circuits. The real benefit is the reduction of development costs. In addition, the TxM solution will greatly reduce development time and time to market.