Radio systems use high frequencies to transport data from one location to another. The signal levels and frequencies involved require careful design of the component blocks within the radio/wireless interface. For this reason, integrated circuits that provide the functionality of a radio system tend to comprise dedicated components for a specific purpose interconnected in an optimum way for operation.
The homodyne, otherwise known as Zero IF structure, mixes the radio frequency carrier down in frequency to DC and the data, modulated on the carrier, is mixed to a frequency from DC to half the transmission bandwidth. It is a system that has been used for many years for data communication. In transmission, the data is provided in two data streams, the two streams being phase separated by 90°. The data is then mixed with a local oscillator that also has two signals phase related by 90° and the output of the transmit mixer is summed to create a composite signal.
Low IF, where the carrier is mixed to a low intermediate frequency rather than DC has also been widely used, offering the advantage over zero IF that the center of the band is not at DC so that offsets caused by carrier leak and production tolerances can be easily circumvented. Low IF has the problem that the image channel rejection is wholly dependent on the accuracy of the 90° phase difference between the two channels, whereas with zero IF the phase imbalance manifests as a reduction in EVM error vector magnitude due to the baseband signal mapping into the opposing spectra. Also, with low IF, the whole RF bandwidth is mapped either side of the chosen IF frequency, compared with half this for zero IF. The advantage of the Zero IF and Low IF systems over standard superhetrodyne systems is that the use of low frequencies allows the use of circuits with more easily achievable specifications. The disadvantage is that a quadrature phase system must be accurate and the circuit complexity is double that of a simple higher frequency IF heterodyne system. The system is well suited for integration onto silicon, since accurate high frequency filtering is not necessary and circuit area is less important than in the discrete version.
Integrated circuit technology is often targeted for a specific application. This is especially true for radio systems where the nature of RF circuits requires careful consideration of the various elements and parasitic components in order to provide the required overall RF performance. As a result, application areas outside the target application often need additional components in combination with the integrated circuit to make the product meet the required specification and the total manufacturing cost is elevated.
Generally, user programmable circuits known as Field Programmable Gate Arrays FPGAs are exclusively digital and cannot be used for analogue signal processing. Similarly, Complex Programmable Logic Devices CPLDs and Programmable System-on-Chip devices are also purely digital. Field Programmable Analog Array cells are analogue array based, but these are generally based on a single repeated cell that can be used for analogue baseband processing See FPAA product range from Anadigm -ANAD 2011(www.anadiqm.com/fpaa.asp: Software Download Anadigm Designer 2). None of these products are suitable for RF signals used in the configurations required for transceiver ICs, and all of them are too non-specific for the wireless industry. Component arrays such as those produced by Diodes Incorporated formerly Zetec are simply a selection of analogue components that are too non-specific to be used at RF and are designed to be used at low frequencies.