For a PDF version of this article, click here.

By selecting the highest integration-level components and with careful layout, a state-of-the-art GSM/GPRS transceiver with a footprint of 1.5 cm² can be designed. This design consists of a signal path with only three critical components:

• Fully integrated transceiver silicon integrated circuit.

• Integrated transmit and receive (Tx/Rx) switch and power amplifier.

• SAW filter module with integrated matching.

However with such a high level of integration, care must be taken to ensure the components are fully compatible. The flexibility of the circuit designer to optimize the performance is reduced. System-level validation by the component suppliers is now essential to ensure performance and that time-to-market requirements are met.

Minimum RF performance is dictated by the GSM specification, developed by the 3^rd Generation Partnership Project (3GPP) as the 3GPP TS 45.005. Early on in the design of the transceiver system, the GSM specifications must be correctly translated to design parameters, and then the budget must be allocated to each component that contributes to that area of the system. If a component is a separate device, the suppliers must agree on a specification so the overall performance target is met. With a transceiver design where all the voltage-controlled oscillators (VCOs), tanks and filters are fully integrated, the onus of meeting the system-level specification lies with the silicon provider. It is essential that the system architecture has been fully validated with appropriate system analysis tools. The included tables relate the GSM specification parameters to the blocks of the transceiver that contribute to that area of the specification.

Due to the nature of a communications network, with the RF spectrum being a shared resource, the transmitter section is tightly controlled by the GSM specification. This is to ensure that the network can be managed efficiently and that one handset does not degrade the quality of another user's spectrum. To this end, spectral masks must be adhered to and it is imperative that the transmitter does not produce unwanted spurious outputs or noise in the receive band that can jam other users' channels. The translational loop architecture offers a highly integrated approach to the transmit section, and with a correct design, removes the need for external filtering on the transmit section. With dual- or quad-band GSM designs, the benefit is even greater.

It can be seen from Table 1 that the transmitter performance depends on both good transceiver and PA design and implementation.

The receiver section in a modern transceiver design has all the active circuits fully integrated. The only external components are the switch and band-select surface acoustic wave (SAW) filter at the front end of the receiver. The filters are required to ensure good performance in the presence of strong out-of-band blocking signals. These are particularly troubling because this RF spectrum is uncontrolled by the network operators and prone to high-power signals from other communications systems and interference caused by other electronic equipment, such as microwave ovens or automobile ignition systems.

With the choice of a direct-conversion architecture for the receiver, several key benefits are obtained that allow the GSM specification to be met with a fully integrated design.

• Less complexity in the receiver signal path.

• Single local oscillator (LO) is required.

• The image frequency is essentially removed.

• IF sections are at baseband, using less current and enabling on-chip filtering.

The elegance of this architecture translates into benefits for the customer in terms of a cost-effectiv, integrated solution. The single LO minimizes the risks of oscillator crosstalk and coupling, which can degrade sensitivity on selective channels, produce unwanted spurious responses, or spurious emissions that will cause a failure in the type approval or network interoperability test stages of a handset. Such problems are notoriously difficult to predict and can be highly sensitive to external printed circuit board (PCB) layout or routing. The lower complexity also provides for lower risk and, ultimately, lower power consumption, prolonging standby time of the handset.

By combining the AD6548 transceiver with TriQuint's TQM6M4001 transmit (Tx) module and a SAW filter module, phone designers quickly achieve the design goals of a high-performance 1.5 cm² GSM/GPRS radio design.

The AD6548 (Othello-G) is the latest of the Analog Devices direct-conversion GSM transceivers, packaged in 5 × 5 mm 32-pin lead frame chip-scale packaging (LFCSP). The device is built on the Othello technology, which was introduced as the first open-market direct-conversion GSM Radio in 1999. A combination of patented circuit designs, architectures and system knowledge were used to solve the historical problems of self detection, dc offset and VCO phase noise. Consequently, GSM radios experienced a significant reduction in components and cost with the elimination of IF SAW filters, VCOs and associated components. The AD6548 Othello-G transceiver sets new standards for integration and total solution size in GSM/GPRS radio design. Othello-G is a true quad-band design, with independent programmable-gain low-noise amplifiers (LNAs) for 850, 900, 1800 and 1900 MHz frequency bands. LO generation for both transmit and receive bands is performed on-chip with a fast-locking fractional-N phase locked loop (PLL) synthesizer with integrated loop filters, Tx and receive (Rx) VCOs, and tank circuits. The AD6548 also includes an on-chip crystal oscillator and calibration system, eliminating the traditional external voltage-controlled temperature-compensated crystal oscillator (VCTCXO) and reducing cost. The translation-loop transmitter architecture eliminates the need for external filtering between the transceiver and power amplifier (PA).

In terms of power management, the AD6548 offers an attractive feature in that the low dropout voltage (LDOs) regulators for the transceiver are fully integrated into the device. This means the device can be connected directly to the battery supply, thus eliminating the need for additional external components and complexity. The power control for the LDOs and the device is handled by the common serial interface bus, eliminating the need to address an additional external device and ensuring synchronization of radio and LDO timing commands, for tight control of power consumption during transitional phases.

State-of-the art processing, packaging and design techniques make a highly integrated module feasible, decreasing size and improving performance. The Triquint TQM6M4001 incorporates all RF transmit functions between the transceiver and the antenna in the market's smallest form factor — 6 mm × 6 mm × 1.1 mm — offering a significant size reduction compared to competing solutions. The TQM6M4001 transmit module architecture and interface is optimized for transceivers like the AD6548 and ensures optimal line routing on the phone PCB level.

The module leverages the discrete GSM850/900 and Digital Cellular System (DCS1800)/Personal Communication System (PCS1900) power amplifier (PA) blocks and integrated power control found in current-generation power amplifier modules. It then adds the low insertion loss quad-band pseudomorphic high-electron mobility transfer (pHEMT) switch, Tx harmonics filtering, integrated switch decoder, four receive ports, and full electrostatic discharge (ESD) protection necessary to implement full RF transmit. The TQM6M4001 requires no external PA matching.

To design its circuits, TriQuint used in-house six-inch gallium arsenide (GaAs) processes, including indium gallium phosphide (InGaP) GaAs HBT and GaAs pHEMT for the PA, low-pass filter (LPF) and switch, respectively. All control functions for the PA and switch are incorporated into a proprietary complementary metal oxide semiconductor (CMOS) design. The transmit module is laminate-based, and all die are connected through wire bonds. TriQuint was able to achieve size reduction by using integration to eliminate all matching and biasing surface-mount device (SMD) components inside the module. A major design goal for the TQM6M4001 transmit (Tx) module was to further improve dc and RF performance of the complete transmit chain for GSM/GPRS operation in all four frequency bands and to provide enhanced performance results compared to a stand-alone PA and antenna switch module line-up.

The front-end design is vital to the receiver performance, as described in the previous section. Traditionally, this has required numerous components. For each band, a SAW filter and LNA matching components are required, amounting to four components. Thus, for a quad-band design, this amounts to 16 components taking up considerable board area. Due to the sensitivity of the SAW filter and LNA input circuits, the matching components need to be trimmed by an experienced RF engineer to obtain satisfactory performance, which is time consuming.

Another approach is viable today, providing a superior solution. Module technology from leading suppliers such as Fujitsu, Murata, SAWTEK and EPCOS allows for the SAW filters and matching components to be fully integrated into one package with a footprint as small as 5 mm × 3 mm. On top of space and design time reduction, this approach can provide improved performance as parasitics associated with PCB traces are reduced due to the improved proximity of the components. Vendors can provide solutions for dual-, tri- and quad-band systems, with drop-in replacement packaging allowing cost-effective solutions for all types of handset design.

To optimize the LNA matching, the transceiver supplier will provide accurate s-parameters for the LNA input, including bond wire and package parasitics, to the module manufacturer. A custom part is then made to mate with that specific integrated circuit (IC). Fine-tuning, test and validation will be done jointly by the two suppliers, and the solution will be evaluated on a reference design. This approach has clear advantages for the handset manufactures, as the task is completed even before the handset design is done, thus reducing time to market, as well as form factor.

Once the RF system has been designed and implemented, the next essential stage is full validation. Initially, this can be done in terms of the basic RF parameters, such as noise figure, second-order intercept point (IP2) or IP3, but this exercise is purely to ensure that the required design parameters have been achieved. The major testing should focus on the GSM specification, which is at a system validation level. This can only be achieved by testing on a complete reference design with the baseband chip set and GSM software protocol stack. A call must be successfully set up with a tester to complete the required loopback bit error rate (BER) tests.

For basic testing, the Rohde & Schwarz CMU 200 radio communication tester or a similar solution can be used for quickly and accurately measuring parameters in accordance with the GSM specification. The limitation of a single test setup is that spurious emissions, receiver blocking, amplitude modulation (AM) suppression, intermodulation and adjacent-channel tests cannot be completed. It is costly and time consuming if a handset fails these tests at the type approval stage. A more sophisticated setup is required, as shown in Figure 3.

To ensure fast, repeatable and accurate characterization across temperature and voltage, an automated laboratory test setup is essential. This will consist of a general-purpose interface bus (GPIB)-controlled switch box and filter bank, PC and a software control program to control the instrumentation and then to post-process the data into a format that can be easily reviewed. This is necessary because for a quad-band GSM phone design, the number of channels totals 975. For each channel, numerous tests are required, and for an individual test, numerous data points are required for completion. This amounts to thousands of data points, so post-processing and formatting are also essential so the test results can quickly be digested and conclusions can be reached.

In the test cases where the unwanted signal is much higher than the wanted signal, such as for blocking or adjacent channel, test equipment phase noise can affect the measurement result. In this case, additional filtering should be included in the switch box. A fading simulator can also easily be added to the test setup to fully comply with the GSM requirements.

The following results are taken with the AD6548 on Analog Devices' hardware platforms using the automated test setup described in the previous section. This enables full and rapid evaluation and optimization of reference designs to accelerate customers' design cycles. A summary of the key plots is shown for the highest frequency, PCS1900 band.

The plots show good margin on the GSM specifications. This translates into a high handset yield for the manufacturer and improved performance for the customer. The sensitivity, for example, has a 7.1 dB margin over the GSM-specified -102 dBm. However, this is the measurement for static sensitivity. Once fading conditions are included, although baseband-dependent, typically 3 dB extra signal-to-noise ratio (SNR) is required. The RF design still provides more than 4 dB of performance margin. Blocking tests also demonstrate good BER margin over the GSM specification of 2% even with a blocker at 2 dB over the required level.

On the transmitter side, the modulation spectrum shows excellent results. The 400 kHz offset, for examples, shows greater than 5 dB margin, with the further-out spectrum yielding even greater margin. RMS and peak phase error also demonstrate good margin over the specified 5° and 20°, respectively.

The design of a 1.5 cm² GSM quad-band transceiver is possible, meeting the rigorous demands of the cellular handset marketplace in terms of RF performance, price and time to market. This size profile is important to the manufacturers so they can include more advanced features and modes in a handset without increasing the size. By selecting state-of-the-art components, the RF signal chain is contained in only three packaged parts. However, with this level of integration, the handset manufacturer must rely on the component suppliers doing a thorough job of not only sub-block but also system validation.

Mike Durrant holds a BEng in electrical engineering from Brunel University, England, and a post-graduate diploma in RF IC design from Delf University, The Netherlands. He has spent 13 years in the semiconductor industry working in a variety of design and application roles. Durrant is responsible for the product application engineering of the Othello^™ range of radio transceiver ICs with Analog Devices. He can be reached via e-mail at mike.durrant@analog.com.

Andreas Nitschke is a product marketing manager for Triquint Semiconductor based in Munich, Germany. He manages the company's GSM/EDGE/WCDMA radio frequency product portfolio. Nitschke has held similar positions in the field of handset RF ICs at Infineon Technologies in Germany and the United States.

RSS    Save to Del.icio.us  Digg This