Download LC Filter Design: Building a Third-Order Low-Pass and High-Pass Chebyshev Filter and more Study notes Electrical Engineering in PDF only on Docsity! Laboratory Exercise 8 1 Laboratory #8: Low Pass RF Filter Design I. OBJECTIVES A. Design a third order (N = 3) low-pass Chebyshev filter with a cutoff frequency of 550 MHz and 3 dB ripple with equal terminations of 50 Ω using: (a) discrete components (pick reasonable values for the capacitors) (b) What is the SWR of the filter in the passband (pick 200 MHz and 550 MHz)? B. Design a third order high-pass Chebyshev filter with a cutoff frequency of 5500 MHz and 3 dB ripple with equal terminations of 50 Ω using: (a) discrete components (pick reasonable values for the capacitors) (b) What is the SWR of the filter in the passband (pick 200 MHz and 550 MHz)? (c) What does the SWR do outside the passband? II. INTRODUCTION Signal filtering is often central to the design of many communication subsystems. The isolation or elimination of information contained in frequency ranges is of critical importance. In simple amplitude modulation (AM) radio receivers, for example, the user selects one radio station using a bandpass filter techniques. Other radio stations occupying frequencies close to the selected radio station are eliminated. In electronic circuits, active filter concepts using OpAmps were introduced. One of the advantages of using active filters included the addition of some gain. However, due to their limited gain-bandwidth product, active filters using OpAmps see little use in communication system design where the operational frequencies are orders of magnitude higher than the audio frequency range. The two types of frequency selective circuit configurations most commonly used in communication systems are the passive LC filter (low, high, and bandpass responses) and the tuned amplifier (bandpass response). LC ladder networks are commonly used as building blocks for passive filters at RF. The values of the inductors and capacitors are varied depending on the type of filter, frequency specifications, and terminations. In this laboratory, passive LC filters at radio frequencies (RF) will be designed and tested. Two common low-pass LC filter configurations are shown in Figures 1 (a) and (b). Each "section" consists of an L-C pair, with each section corresponding to the order of the filter. Two section (or second order) filters are shown in Figure 1. Note that the values of the capacitors and inductors changes with varying input and output resistances. Tabulated "normalized" values for the inductors and capacitors for varying termination ratios are available to the design engineer. The component values in the tables are normalized with respect to the termination ratio and cutoff frequency. Generic representations of the LC low-pass filter are shown in Figure 2. Docsity.com ELEC 412 Laboratory Exercise 8 2 vs RS C1 L1 C2 L2 RL vs RS C1 L1 C2 L2 RL (a) (b) Figure 1. Two Ladder Network Configurations for LC Filters Figure 2. Two Generic Representations of Figure 1. The closeness of the impedance match between the source resistance RS and filter input resistance Rfin is frequency expressed as a return loss defined as: 120logAρ = ρ . (4) Normalized Butterworth (Maximally Flat), Linear Phase, and Chebyshev LC lowpass filters are presented in tabular form on the following pages. The normalized inductors and capacitors are denormalized using: 2 n C CC f R = π (5) and 2 n C L RL f = π , (6) where Cn is the normalized capacitor value, Docsity.com
