Whenever you see a filter topology like a ladder, you are looking multiple element filters derived from replicated L filter sections, T filter sections or
π
filter sections as more elements are usually required to improve certain parameter of filters like stop-band rejection or slope of transition from pass-band to stop-band.Multiple Element Filter comprising L-Topology Elements
4.0 CONCLUSION
In this unit we learnt that filters are electrical networks that alter amplitude and/or phase of electrical signals with respect to frequency and which should not add new frequencies or change the frequencies component of the input signal but change the relative amplitudes of the various frequency components and/or their phase relationships.
We discussed the single element, L, T and the
π
Filter topologies which are just four of the numerous topologies which all filters irrespective of type and response must have. This is the electrical layout of the filter which in some cases can only accommodate certain filter types and frequency responses.187 5.0 SUMMARY
- Filters are electrical networks that alters the amplitude and/or phase characteristics of an electrical signal in the frequency domain
- All filters must have an electrical layout called topology of the filter - Only certain filter types and frequency responses can be accommodated by
specific filter topologies
- Bandpass filters can never be accommodated by an L-filter topology - The order of a filter is determined by the number of reactive elements with
the exception that a parallel or series combination of an LC tuned circuit is treated as a single element
- Capacitors present lower impedance at higher frequencies while inductors present higher impedance at higher frequencies
- Multiple element filters are derived from replicated L filter sections, T filter sections or π filter sections
6.0 TUTOR MARKED ASSIGNMENTS
1 How would you describe a filter to a layman? Would you also be able to explain to this layman what filters do?
2 What do you understand by filter topology? Can you sketch and label the ones you know?
3 Explain how you would determine the order of a filter?
4 If a passive filter is designed with seven reactive elements, and two pairs of these elements are tuned LC circuits, what then is the order of this filter?
5 Describe the filter class known as single element filters ? sketch and label a single element low pass filter which uses a capacitive element?
6 What is a symmetrical filter?
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7 Can you design a symmetrical low pass T filter and label it?
8 List five appliances around you which use filters?
9 Do you recognise this filter topology illustrated below? Which is it and what does it pass?
8 Describe in detail with illustrative sketches how multiple element filters can be derived?
9 Can you think of any filter topology that will not accommodate a filter type? Which is it or which are they?
7.0 REFERENCES/FURTHER READINGS A Textbook of Electrical Technology 2010
By B. L. Theraja and A. K. Theraja. Published By S. C. Chand, Electrical Circuit Analysis
By C. L. Wadhwa Published By New Age International Network Analysis with Applications 4th Edition
By William D. Stanley Published by Prentice Hall Fundamentals of Electric Circuits 4th Edition
By Alexander and Sadiku Published by Mc Graw Hill Analog Filter Design
By M. E. Van Valkenburg Published by Holt, Rinehart and Winston
189 UNIT 2 BASIC FILTER TYPES
CONTENTS 1.0 Introduction 2.0 Objectives 3.0 Main Content
3.1 Filters Classification 3.1.1 Passive Filters 3.1.2 Active Filters 3.1.3 Digital Filters 3.2 Passive Filters 3.3 Active Filter
3.4 Other Filter Methods 4.0 Conclusion
5.0 Summary
6.0 Tutor Marked Assignments 7.0 References/Further Readings
1.0 INTRODUCTION
Filters are used to emphasize signals in certain frequency ranges and reject signals in other frequency ranges frequency dependent gain; often when useful signal at frequency range has been contaminated with unwanted signal of another frequency range.
Filters are classified in any of several ways. They may be classified by Technology, by topology or by design methodology. Each of these classifications defines group parametric characteristics which have merits and drawbacks. We shall discuss the three classifications described above to get a feel of them and then focus our attention on Passive Filters and Active Filters which are two important types of filters classified by technology.
2.0 OBJECTIVES
After you have read this unit, you will be able to 1 Discuss the classification of filters
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2 Classify filters by topology, technology and methodology 3 Distinguish between passive and active filters
4 Explain how digital filters are implemented 5 Understand the Q factor of a filter
6 Sketch the frequency response of filters 7 Explain the transfer function
8 Work in the frequency domain 9 Sketch passive and active filters 10 Sketch twin T filters
11 Sketch the transfer function of Wien bridge filters 12 Explain active filter optimisation
13 Understand the order of a filter
14 Describe the effect of filter order on rolloff slope 15 Sketch ideal filter response curves
3.0 MAIN CONTENT