The world of wireless communication is very segmented. For example, varied communication standards exist in different regions of the world. Each of these standards is mandated either by governmental regulation, historical technological infrastructure, required data rates, or a lack of alternatives.

In the wide-area-network (WAN) space, the breadth of current 2G standards includes GSM, PDC, TDMA, PCS, and various versions of CDMA. For wireless local-area networks (WLANs) and personal-area networks (PANs), the dominant standards in existence are IEEE 802.11a, IEEE 802.11b, and Bluetooth. Though these standards are meant to utilize unlicensed frequency bands, their existence becomes complicated. Not all of these standards are regulated in different parts of the world.

Yet no matter which standard exists—in an area, for an application, or for whatever reason—the trend toward convergence is ongoing. It is now forcing wireless-device designers to supply products that are capable of supporting more than one standard or frequency band. This pressure is being passed down to integrated-circuit (IC) designers, who design the transceivers that establish and maintain the physical connections between wireless-capable devices. These designers also are being pressured to supply increasingly affordable products for consumers. This issue is causing IC designers to rethink their traditional architecture of choice.

Into this picture enters the direct-conversion architecture for transceiver design. Historically, most wireless-enabled devices have utilized the superheterodyne architecture. This choice was largely made because direct conversion has traditionally had design and production difficulties. These difficulties hampered its adoption into wireless devices. Some firms have solved these design difficulties, however. They are now producing transceivers based on the direct-conversion architecture that are more cost effective than superheterodyne. As such, these transceivers are more suited toward multiband and/or multimode solutions.

WIRELESS RECEIVER
The typical block-level diagram for a wireless radio receiver is shown in Figure 1. Although this structure has often been associated with the superheterodyne architecture, different proprietary advancements in design have blurred the boundaries between this and other architectures. The direct-conversion architecture utilizes a similar format with the exception of the intermediate-frequency (IF) stage. This stage is shown between the two phase-locked loops (PLLs) of Figure 1.

The superheterodyne-based receiver accomplishes all filtering with passive components. It is characterized by two downconversion processes. To be demodulated, these processes produce a baseband signal. The direct-conversion architecture, on the other hand, requires only one downconversion process. It performs its channel filtering by utilizing a low-pass filter.

Some applications require a low-IF solution to be used instead of a direct-conversion solution. The difference between the two is that a low-IF structure has a superheterodyne receiver coupled with a direct-conversion-based transmitter. Because of their overall flexibility, however, true direct-conversion architectures are more preferred by IC designers.

In the signal-reception stage, the radio transceiver amplifies the radio-frequency carrier signal. It then translates and filters that signal to produce the baseband signal. Some of these functions can be accomplished via a digital design. But by and large, most of them are accomplished via an analog design.

After the RF signal is received by the antenna and filtered, it is amplified to meet the dynamic range of the RF system and block out extraneous noise. The total amplification process is performed via a low-noise amplifier (LNA) and a variable gain amplifier (VGA). The LNA is chosen according to its linearity and noise figure, while the IF amplifier is valued for its gain control. When multimode and multiband solutions are considered, the LNA can cover entire bands with minimal hardware overhead. The filtering is required if the desired channel data is going to be selected by suppressing any interference.

The frequency translation is complicated. It is only done with ease because of a function of the channel spacing of the particular wireless standard. For GSM, 200 kHz of channel spacing is required. CDMA requires 1.25 MHz. The integrated phase-locked-loop devices are usually designed to support all required channels. The voltage-controlled oscillator's (VCO's) operating range should be designed to be wide enough to cover the entire operating frequency range. The phase noise should be small enough to eliminate spurious mixing with nearby interference.

The selectivity depends upon the frequency location and channel bandwidth. As the frequency increases and the channel bandwidth is reduced, it becomes difficult for integrated devices to effectively perform the channel-filtering function. In order for a superheterodyne-based system to filter between the RF and IF stages or reception, a passive component is normally utilized. An example is a surface-acoustic-wave (SAW) filter.

Most of the hardware following this channel-selection filter has loose linearity requirements. For instance, say the channel-selection filtering is done quite close to zero frequency or at the baseband in a direct-conversion transceiver. The filter can then be designed utilizing silicon devices and passive elements, such as resistors and capacitors. A low-quality low-pass or bandpass filter may be used in this instance. Yet all of the building blocks—including the channel-selection filter—should have a dynamic range that is high enough to survive the strong interference effects.

In a multiband or multimode RF system, it is difficult to design a receiver structure that has the same structure as the one discussed above. The main design objective is to minimize the overhead in hardware. Ideally, then, each functional block should support as many of the communication standards or frequency bands as possible.

Highly linear and low-loss front-end bandpass filters are typically used at specified bands. They can therefore reject the wideband out-of-band interference. Such interference usually requires signals that are stronger than the desired signal by 70 or 80 dB.