Understanding Smart Sensors - Nomads.usp

174 Understanding Smart SensorsPortable wireless products require low power consumption to increasetheir useful life before requiring battery replacement. For measurement of lowpower consumption, high-impedance sensors, sleep mode, and smart techniquessuch as the PWM technique (see Chapter 5) are required. Depending onthe type of measurement (static or dynamic) and how frequently it must bemade, periodic readings can be transmitted to distant recording instrumentsin process controls, hazardous material monitoring systems, and a variety ofmobile data acquisition applications. Such readings would have been more timeconsuming, dangerous, prohibitively expensive, or difficult with previouslyavailable technology.8.1.1 The RF SpectrumRecent legislation has expanded the electromagnetic spectrum that is availablefor wireless communications in the United States. The Federal CommunicationsCommission (FCC) regulation Title 47, part 15, covers unlicensedsecurity systems, keyless entry, remote control, and RF LANs. Table 8.1 identifieskey frequency ranges that are already the focus of several industries. Theindustrial, scientific, and medical (ISM) bands are being used for several applications.Integration is the key to higher performance, smaller packages, andlower cost in the RF arena, as well as previously discussed systems, with severaltechnologies used in various frequency ranges.Several technology choices exist in the RF front end of a portable communicationproduct, shown in Figure 8.1 [1]. Silicon competes with GaAs inthe 1- to 2-GHz range; however, in the 2- to 18-GHz range, GaAs is the onlysolution. With a supply voltage of 3V, high-frequency (1 GHz) operation ofGaAs has an efficiency of 50% versus 40% for silicon bipolar or 43% for a lateraldiffused MOS (LDMOS). The tradeoff in efficiency versus cost must beevaluated prior to initiating a custom design. The down converter and lownoiseamplifier (LNA)/mixer, transmit mixer, antenna switch, driver and ramp,and power amplifier can be designed with RF chip sets consisting of differentIC technologies. The chip set approach can be a cost-effective alternative tohigher levels of integration. High-frequency designs for radio frequency areconsiderably different from high-speed digital processes. The frequency rangein RF circuits of 800–2,400 MHz is much higher than the fastest digital circuits.A mixed-signal approach is required that combines not only digital andanalog but also RF technology. Circuit isolation is required to prevent unwantedcoupling of signals, which can range from 3 V p–p to less than 1 mV p–p . RFICs arebeing designed for a number of high-frequency applications using a variety of