RF filter modules for GSM/WCDMA mobile phones must be highly integrated for worldwide use in all frequency bands. EPCOS supplies suitable modules based on LTCC technology.

Nine different pairs of frequency bands are currently specified for WCDMA worldwide (Table). Not all of them are accessible across the globe and some are currently still under construction. Bands III and VII, for example, will not be in service until 2009 at the earliest. In order to reduce the complexity and cost of combined GSM/UMTS mobile phones, only up to three WCDMA frequency bands are currently used.

The possible combinations depend on where the mobile phone is used. The following combinations have been agreed upon:

• World (I, II, V)
• Japan (I, IX, VI)
• Europe (I, III, VIII)
• USA (II, IV, V)

WCDMA FREQUENCY BANDS

Quad-band GSM and band-I WCDMA front-end

Fig. 1 shows a block diagram of a dual-mode mobile phone for GSM/WCDMA. It uses a quad-band GSM transceiver in combination with a single-band WCDMA transceiver. The receive part of the GSM block contains a front-end module with a switch and four SAW filters, with one assigned to each GSM frequency band. These filters are typically equipped with an asymmetrical input - i.e., referred to ground - and a symmetrical output, so that a balun (impedance converter) is no longer needed in front of the GSM transceiver. While use of a balun involves an additional insertion loss, this does not occur when the asymmetrical signal is converted to a symmetrical one within the SAW filter.

	FIGURE 1: COMBINED GSM/WCDMA ARCHITECTURE FOR FIVE BANDS
Schematic diagram of the front-end for a quad-band GSM and a single-band WCDMA mobile phone.

A dual power-amplification module is used for the two GSM transmit bands (1 GHz and 2 GHz). GSM power amplifiers with high efficiency require low-pass filters to suppress the harmonics. These are typically LC filters that are integrated into the front-end module as three-dimensional structures in the multilayer LTCC substrate. The complex matching process needed at the input of the GSM transceiver can also be incorporated, thus saving twelve separate components.

On the WCDMA side, this kind of module operates with a duplexer, a WCDMA power amplifier and intermediate stage filters for the Rx and Tx paths. The duplexer, power amp and intermediate stage filter (Tx path) can also be integrated in a PAiD (power amplifier with integrated duplexer). In the receive path, intermediate stage filters are placed after the LNA, whereas the intermediate stage filter in the transmit path is integrated in front of the power amplifier as part of the PAiD. The front-end module contains a SP7T switch to connect the GSM and WCDMA transceivers with the antenna. A CMOS multiplexer with seven switching stages is used here.

As a result, the front-end for the dual-mode mobile phone can consist of three modules:

• A front-end module with a SP7T switch, a quad-band filter bank and low-pass filters in the transmit path
• A dual power-amplifier module for GSM
• A WCDMA PAiD module with a power amplifier, an intermediate-stage transmit filter and duplexers (discrete solutions are still widespread here)

Another concept is to implement a combination of a power-amplifier switch module (PSM), a PAiD and a filter bank (in module form). The decision to modularize the front-end depends strongly on the space requirements, cost aspects, time-to-market considerations and logistics.

Front-ends for quad-band GSM and tri-band WCDMA

Advanced multimode, multiband mobile phones support four GSM frequency bands and up to three WCDMA frequency bands. This concept will now be presented in detail for mobile phones operating in frequency bands I, II and V. The designs described may also be applied to other band combinations.

a) Separate GSM and WCDMA transceivers

The GSM front-end described above can be extended for further WCDMA bands within the scope of this approach. This means that the SP7T switch is replaced by a SP9T switch so that the two additional frequency bands can also be switched. The WCDMA front-end must be extended by another two duplexers, four additional intermediate-stage filters, two power amplifiers as well as another two sections within the WCDMA transceiver.

Use of this concept reduces the expenditure needed for the development of a dual-mode, multi-band mobile phone. This means time-to-market and development costs are also lower.

Despite their increased complexity, such systems must be realized with low costs. As seen in Fig. 2, the number of receivers increases to seven and the number of transmitters to five, leading to a large number of terminal pins as well as large package dimensions. The costs of the transceiver and filter then rise quite significantly.

	FIGURE 2: COMBINED MODULE FOR SEVEN BANDS
Block diagram of the front-end for a quad-band GSM as well as a triple-band WCDMA mobile phone.

b) Dual-mode receivers

In order to minimize costs, new approaches will have to be taken. A possible solution is shown in Fig. 3. This concept requires the use of reconfigurable multimode receivers to allow the same receive path to process both GSM and WCDMA signals. In this way, the number of receivers can be reduced by two to a total of five, which means the front-end can be designed with fewer filters and simpler switches.

	FIGURE 3: SIMPLIFIED SOLUTION
Schematic block diagram of the front-end for a quad-band GSM and a triple-band WCDMA mobile phone. In contrast to Figure 2, the receive path can handle both GSM and WCDMA signals, which simplifies the design.

A glance at the frequency bands shown in Table 1 makes clear that GSM 850 and WCDMA band V operate in the same receive frequency band. The same applies to GSM 1900 and WCDMA band II. Therefore, the receive filter of the corresponding WCDMA band can also be used for GSM. The same concept may be applied to the GSM/WCDMA mobile phones operating in bands I, III and VIII released for Europe as well as in bands I, III (IX) and V (VI) used in Japan.

Two conventional GSM receive filters and two dual-mode intermediate-stage receive filters are required to implement this design. The dual-mode filters must satisfy the specifications of both standards. Fig. 4 shows the frequency response (S₂₁) of such a filter in comparison with the typical GSM 1900 blocking-frequency specification. Fig. 5 shows the power transfer function (PTF) as well as the error vector magnitude (EVM) of the same filter in relation to the typical specification for the WCDMA band II.

	FIGURE 4: ATTENUATION CURVE OF A DUAL-MODE FILTER
Frequency response of the dual-mode filter for the intermediate receive stage (red), which falls well within the GSM 1900 specification (green) .

The PTF is a measurement for the attenuation of a WCDMA-modulated signal. The EVM specifies the signal distortion on the basis of non-uniform attenuations as well as of non-uniform group delays via a 5-MHz wide WCDMA channel. Both these parameters are plotted as a function of the center frequency of the corresponding WCDMA channel.

	FIGURE 5: POWER CURVE OF THE DUAL-MODE FILTER FOR WCDMA II
Power transfer function (red) and error vector magnitude (blue) of a dual-mode filter for the intermediate receive stage in comparison with the WCDMA-II specification (green).

c) Dual-mode receivers without intermediate stage filters

A possible next step is to remove the intermediate-stage filters in all the WCDMA bands. The receive filter of the WCDMA duplexer must then satisfy not only the WCDMA, but also the corresponding GSM specifications. In addition, the balun functionality of the intermediate-stage filters must also be shifted to the duplexer.

A duplexer with a symmetrical receive path will now be described for WCDMA band V in order to illustrate the requirements on its receive filter. Fig. 6 shows the frequency response (S₂₁) of such a dual-mode receive duplex filter in comparison to the typical GSM 850 specification. In particular, the GSM specifications for blocking frequencies above the pass band must be satisfied. Fig. 7 shows the power transfer function (PTF) as well as the EVM of the same filter in comparison to the typical specification for WCDMA band V. Despite high suppression of the transmit frequencies, this filter must be specially matched to the third-order intermodulation product’s frequencies in order to suppress them.

	FIGURE 6: ATTENUATION OF A DUAL-MODE FILTER FOR WCDMA V
Frequency response of a dual-mode duplex filter (red) for the receive branch compared with the GSM 850 specification (green).

	FIGURE 7: POWER CURVE OF THE DUAL-MODE FILTER FOR WCDMA V
Power transfer function (red) and error vector magnitude (blue) of a dual-mode duplex filter for the intermediate receive stage compared with the WCDMA-V specification (green).

Example of a multimode front-end module

The EPCOS module D2027 based on LTCC technology will be used here as an example of a state-of-the-art multimode front-end module. A block diagram of this module is shown in Fig. 8. It contains a SP7T CMOS switch, four SAW filters for the GSM receive paths, low-pass filters for the low and high GSM transmit paths, output paths for WCDMA operation, circuits for protection against electrostatic discharges and all necessary matching elements for the filters and switches. The antenna pin of the module is connected to the input of the switch via the ESD protection circuit. The seven output paths of the switch are connected to the four GSM receive filters, the two GSM low-pass transmit filters and the WCDMA output pin.

	FIGURE 8: FRONT-END MODULE M104
Block diagram of multimode module M104 for four GSM paths and a WCDMA path.

Fig. 9 shows the module currently in volume production. The encapsulation was removed to show the components mounted on the upper side. All necessary passive component functions are embedded in a multi-layer LTCC substrate in order to implement the two low-pass filters, the matching components for the receive path, and the matching elements between the sections of the circuit. This enables considerable space savings compared with non-modular solutions.

	FIGURE 9: UNPACKAGED FRONT-END MODULE
Multimode module M104 with encapsulation removed to reveal the SAW filters in their CSSP2 packages, the SP7T switch mounted in the wire-bond procedure as well as discrete SMD components.

This makes it possible to implement customized Rx output impedances as part of the module, thus saving customers space on the circuit board as well as development effort. Two SAW filters in CSSP2 technology, each containing two GSM receive filters as a 2-in-1 solution, are mounted on the upper side of the substrate. The SP7T switch is placed underneath as a wire-bonded individual die. Discrete EPCOS SMD components were then added for the ESD protection circuit and can be seen in the lower right corner. The module has an overall footprint of 5.4 x 4.7 mm².

The key parameters for good processing performance of multimode modules are the insertion loss, predefined attenuation levels, minimized harmonics in the GSM transmit modes and excellent intermodulation properties.

With current consumption of only 50 µA and an ESD strength of 8 kV at contact discharge to IEC1000-4-2, the D2027 offers unrivaled performance data. All this is implemented into a minimum amount of space at a low cost.

The next steps are aimed at further miniaturization. This will be achieved by using the ultra-compact SAW packages from EPCOS in CSSP3 technology. This, in combination with flip-chip mounting technologies for the switch, makes it possible to reduce the footprint of the module by almost one half while also further decreasing the insertion height.

Development trends

Successful development of RF front-ends for multimode, multiband mobile phones depends on the management and availability of key materials and technologies, advanced filter techniques and comprehensive system know-how. EPCOS relies on these competencies with the development and manufacture of LTCC technologies, as well as expertise in the development, synthesis, simulation and production of SAW and BAW filters.

Various partitioning solutions for the RF front-end designed for multimode multi-band mobile phones lead to technologically equivalent system solutions. While the reuse of existing GSM subsystems may result in shorter time-to-market, it increases either the number of components or the complexity and thus the cost of the system.

If the requirements of the GSM and WCDMA specifications are combined, a single receive filter can be used twice – namely for both systems. This eliminates the need for a receive filter and reduces the number of filters, as well as the complexity of the quad-band GSM/tri-band WCDMA front-end modules. By adding the GSM blocking specifications and the balun functions to the receive filter in the duplexer, elimination of the intermediate-stage filters is facilitated.

Authors: Ulrich Bauernschmitt, Christian Block, Peter Hagn, Günter Kovacs, Enrico Leitschak, Andreas Przadka und Clemens C.W. Ruppel