Traditional radio designs incorporate key circuit components comprised of different material structures. These include ceramic-based RF filters, gallium-arsenide-based power amplifiers (PAs) and transmit/receive (T/R) switches, to name a few. In order to achieve optimum performance, designers have been using a multichip solution. This solution, while producing high performance, requires a high cost of materials and assembly and handling difficulties. In addition, it frequently results in a module with high DC power consumption and a large footprint.

One solution is to move circuit components on-chip, which not only reduces the material costs but also can achieve a high level of integration. Considerations that should be included when determining which components to design on-chip are desired required performance, the potential on-chip performance and which components could, or should, remain external to the radio.

The advantages of single-chip design include:

• Better matching of on-chip devices, which can reduce requirements for filtering as signal interference and image-band rejection improves.

• The ability to include high performance complex digital filtering, though some analog filtering may still be required.

• The inclusion of adaptive cancellation techniques, which further reduces the need for external filtering.

Performance limitations that need to be considered include increased phase-noise levels for crystal oscillators, decreased balance for baluns and performance limitations due to integration of the T/R switch on-chip. Inclusion of the PA on-chip brings down the material cost but designers must deal with heat dissipation resulting from power-added efficiencies and low voltage breakdown. Parasitic coupling to other analog circuits could be substantial to the high signal level.

Despite the problems, the industry is moving to single-chip solutions as the advantages outweigh the problems and the technology is improving, which will solve the problems over time.

Engineers should consider the capabilities of the target technology and target device before deciding on a single-chip radio design. For example, for low-noise amplifiers, SiGe is preferred due to its lower noise, higher gain and wider-stable temperature range. But when used in power amplifiers, silicone's higher early voltage is a better choice.

Once the designer selects the material, then the performance of the resident devices must be considered. Because of their inherent characteristics of higher transductance, easy impedance matching and higher voltage rails, designers frequently choose bipolar devices for the design of RF circuits.

Choosing an all-CMOS mixed-signal radio can create problems because of reliability issues with currently available RF models. Other issues include the challenges associated with low-voltage rails, low-transconductoance, and difficult impedance matching.

Ongoing foundry relationship and process-specific models cause process selection to become highly important. Other factors for designers to take into consideration are power consumption, wafer costs, isolation, and packaging alternatives.

Designers should consider, when deciding which components to incorporate on-chip and which to leave as components, the available materials, devices and finally process, in regard to performance impact for each particular circuit component. Designers should consider emerging circuit design techniques and new technologies while always keeping the goals of shrinking the footprint, lowering the cost and reducing power consumption for a multichip solution in mind.

Source: Integrating Radios Needs Fine-Tuning
Jim Wight
EE Design
5/21/02
Integrated System Design