Understanding Smart Sensors - Nomads.usp

Transceivers, Transponders, and Telemetry 177Aligned IC) process provides low-power dissipation at a lower cost and has aneven higher frequency range than BiCMOS. 1 GaAs provides the lowest operatingvoltage at the highest cost but can operate above 2 GHz.Mixed analog and digital process technology is required for the adaptivedifferential pulse code modulation (ADPCM) coder/decoder (codec) shown inFigure 8.1 that translates analog into a digital format in personal communicationdevices. The digitized signal is transmitted over the RF channel. TheADPCM is designed for 32 Kbps, which is the codec technology for many personalcommunication systems worldwide, as well as 64-, 24-, and 16-Kbps datarates. DSP technology is used to achieve optimum data rate and RF bandwidth.Precision analog and high-performance digital technologies are required for thetransition from analog to digital regime and back.GaAs semiconductors are being designed to address the highest speedapplications and are well suited to low-voltage operation. However, they aremore expensive than silicon and should be used only where their performancewould be difficult or impossible to achieve with silicon. The use of GaAs technologyadds another dimension to circuit partitioning that must be consideredfor RF circuits.8.1.2 Spread SpectrumSpread spectrum is one of the secure methods of transmitting information viaradio signals that recently (1991) has expanded from military use to commercialuse. The spread spectrum signal is a much lower magnitude but is generatedby a much broader frequency range, 26 MHz compared to a narrowbandsignal of less than 25 kHz, as shown in Figure 8.2. Narrowband transceivers(combined transmitters and receivers) are highly susceptible to interference byfrequency sources near their carrier frequencies. Because they operate withinthe audio range, the equipment being monitored frequently generates the interference[2].Spread spectrum offers improved interference immunity, low interferencegeneration, high data rates, and nonlicensed operation at practical power limits[3]. To conform to FCC guidelines, the total transmitted power must be 1Wmaximum, and the spectral density (the power at any specific frequency) mustbe no greater than 8 dB in a 3-kHz bandwidth. Two techniques are used to distributethe conventional narrowband signal into a spread spectrum equivalent:direct sequencing and frequency hopping.1. MOSAIC is a trademark of Motorola, Inc.