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5 Dec 2006 18:18:35 Digital-to-Analog Converters Digital-to-Analog Converter (DAC) is an electronic device used for converting a digital code, which is normally binary in nature, to an analog signal such as electric charge. DACs are the bridge between the abstract digital signals and the real analog signals. This kind of conversion is usually carried out by resistors, capacitors and electricity sources. In telecom, DACs help in obviating cross-channel interference, minimise noise and reduce signal distortion in signal processing applications such as communications, wireless infrastructure, video imaging and multimedia. With reduced power dissipation, DACs bring down the operating costs and enhance dependability on the system, simultaneously increasing higher channel density in 3G base stations. Broadband, multi-carrier wireless communications standards require lesser distortion and noise specifications. The DACs in the transmitter circuits have a prominent role in the generation of analog signals. They can considerably influence the performance of the system. Manufacturing parts for a transmitter in multi-carrier base transceiver stations for GSM/EDGE is a challenging task as they have to possess high resolution and quicker update rates at elevated output frequencies, simultaneously minimising noise and spurious emissions across a wide bandwidth. Because of these requirements, GSM/EDGE system designers are of the opinion that communications DACs are good if only they are capable of meeting important specifications in four chief noise and distortion aspects - Spurious-Free Dynamic Range (SFDR), Signal-to-Noise Ratio (SNR), Intermodulation Distortion (IMD), and Multi-Tone Power Ratio (MTPR). The four criteria are very important in enabling the transmitter system conform to the expected spurious and IMD targets. A GSM/EDGE Transmitter mask is employed to determine the DAC requirements for these criteria to specify noise and spurious emissions from the base stations. The GSM/EDGE mask and the associated specifications are established on a single active carrier and the other carriers in the transmitter are incapacitated. For GSM/EDGE systems, the analog signal synthesis circuit blocks - DACs - should have spurious emission levels of less than -80dBc (decibel carrier) for offset frequencies greater than or equal to 6MHz. From the system's angle, the capability of the DACs to provide greater degree of performance under multi-tone conditions smoothens the transmitter path design. Conventionally, Base Transceiver Station systems used one Digital-to-Analog converter per transit conduit. However, a DAC with a substantial multi-tone performance will assist in simplifying the transmitter design and shortens board space and cost by reducing the number of DACs needed in such a system. Links: New 4 Channel Converter for 3G Base Stations Content Team -- BSNL Portal -- Intelligroup Asia Pvt. Ltd. |
