Introduction to RF Filters
The need for RF filters to attenuate interference band of frequencies, generated in signal processing circuits is explained with a practical example. The pass band of filters could be positioned in four ways and hence there are four types of filters. The
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Introduction to RF Filters
Abstract The need for RF filters to attenuate interference band of frequencies, generated in signal processing circuits is explained with a practical example. The pass band of filters could be positioned in four ways and hence there are four types of filters. The basic characteristics of low pass, high pass, band pass and band reject filters are explained. Integrated filtering circuits and the industrial applications of filters are also explained with examples.
1.1 Need for Filters Electronic equipments contain circuits for processing signal band of frequencies to satisfy the functional requirements of the equipments. For reasons such as nonlinearity in the signal processing circuits and internal/external noise generation, frequency components outside the signal band are generated. The frequency components outside the signal band are the interference band of frequencies and they affect the satisfactory functioning of electronic equipments. They could cause distortion, jamming of received signals or poor reception in communication equipments. They could lead to erroneous diagnosis in medical equipments. A practical example is provided for the interference band of frequencies. CDMA (Code-Division Multiple Access) services use 800 MHz band and the GSM (Global System for Mobile communication) services use 900 MHz band (GSM low band) and 1,800 MHz band (GSM high band). For simplicity, assume that CDMA system generates interference component in GSM low band at 920 MHz and the interference signal is shown in Fig. 1.1. The interference component causes the distortion or cross talk in GSM low band. The interference component at 920 MHz in GSM band should be attenuated. A filter module is designed to attenuate the entire GSM low band and not just 920 MHz as the CDMA transmission system might generate other interference D. Natarajan, A Practical Design of Lumped, Semi-Lumped and Microwave Cavity Filters, Lecture Notes in Electrical Engineering, DOI: 10.1007/978-3-642-32861-9_1, Ó Springer-Verlag Berlin Heidelberg 2013
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1 Introduction to RF Filters
Fig. 1.1
Example of signal interference in bands
Fig. 1.2
Need for RF filter in electronic system
frequency components in GSM band. The filter module is inserted between CDMA sub-system and antenna as shown in Fig. 1.2 attenuating the interference band of frequencies.
1.2 Filtering Circuits Simple requirements of suppressing interference signals are realised by integrating filter circuits in functional circuits. Two examples could be given. Fig. 1.3 shows a relay switching circuit. When the DC supply to the relay coil is switched off, reverse high voltage noise frequency components are generated as the inductance of the relay coil opposes ‘change of current’. The noise frequency components are suppressed by connecting a diode in reverse direction across the relay coil. The diode does not conduct or load the DC supply but act as a short circuit for the reverse high voltage noise frequency components thus filtering out the noise signals at the
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