Fundamentals-of-Microelectronics-Behzad-Razavi.pdf

Need for such filters example 143 receivers designed

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need for such filters. Example 14.3 Receivers designed for the Global Positioning System (GPS) operate at a frequency of approxi- mately 1.5 GHz. Determine the interferers that may corrupt a GPS signal and the type of filters necessary to suppress them. Solution The principal sources of interference in this case are cellphones operating in the 900-MHz and 1.9-GHz bands. A bandpass filter is therefore required to reject these interferers (Fig. 14.7). Exercise Bluetooth transceivers operate at 2.4 GHz. What type of filter is required to avoid corrupting Bluetooth signals by PCS signals? The former is called the “cellular band” and the latter, the “PCS band,” where PCS stands for Personal Communi- cation System.
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BR Wiley/Razavi/ Fundamentals of Microelectronics [Razavi.cls v. 2006] June 30, 2007 at 13:42 725 (1) Sec. 14.1 General Considerations 725 f 1.5 GHz 900 MHz 1.9 GHz Cellular Signal Signal PCS Signal GPS Required Filter Response Figure 14.7 Figure 14.8 summarizes four types of filters, including a “band-reject” response, which sup- f Low Pass ( ) ω H f ( ) ω H High Pass f 1 f ( ) ω H Band Pass f ( ) ω H Band Reject f 2 Figure 14.8 Summary of filter responses. presses components between and . Another classification of analog filters concerns their circuit implementation and includes “continuous-time” and “discrete-time” realizations. The former type is exemplified by the famil- iar RC circuit depicted in Fig. 14.9(a), where exhibits a lower impedance as the frequency R 1 C 1 out V in V C 1 V C 2 "Resistor" 1 V 2 (a) (b) Figure 14.9 (a) Continuous-time and (b) discrete-time realizations of a low-pass filter. increases, thus attenuating high frequencies. The realization in Fig. 14.9(b) replaces with a “switched-capacitor” network. Here, is periodically switched between two nodes having volt- ages and . We prove that this network acts as a resistor tied between the two nodes - an observation first made by James Maxwell in the 19th century. In each cycle, stores a charge of while connected to and while tied to . For example, if , absorbs charge from and delivers it to , thus approximating a resistor. We also observe that the equivalent value of this resistor decreases as the switching is performed at a higher rate because the amount of charge delivered from to per unit time increases. Of course, practical switched-capacitor filters employ more sophisticated topologies. The third classification of filters distinguishes between “passive” and “active” implementa- tions. The former incorporates only passive devices such as resistors, capacitors, and inductors, whereas the latter also employs amplifying components such as transistors and op amps.
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BR Wiley/Razavi/ Fundamentals of Microelectronics [Razavi.cls v. 2006] June 30, 2007 at 13:42 726 (1) 726 Chap. 14 Analog Filters The concepts studied in Chapter 8 readily provide examples of passive and active filters.
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