ABE425 Engineering Measurement Systems Filters Dr. Tony E. Grift Dept. of Agricultural & Biological Engineering University of Illinois
Agenda Recap complex numbers Relationship Laplace, frequency (Fourier) domain Relationship time, s and frequency domains decibel notation (dB) RC circuit as a Low-Pass and High-Pass filter Bode plots Combination filters
Complex number in complex plane Argument of s Absolute value of s (aka Modulus or Magnitude)
Operations on complex numbers cont. Multiplication/division using Euler’s notation
Operations on complex numbers cont. Complex conjugate Multiplying a complex number by its conjugate gives a real number
Relation Laplace and Fourier Transform Time domain Time domain s-domain (Laplace Domain) -domain (Frequency Domain) Transient response (step, impulse, ramp) Frequency response (filters)
Relation time, s and frequency ( ) domain Time domain i Laplace (s)-domain -domain
Concept of impedance (Capacitor) Laplace transform
Concept of impedance (Inductor (coil)) Laplace transform
Low-Pass filter using RC network
Derivation transfer function with impedance
Decibel notation Addition is much simpler than multiplication Notation in Bel (after Alexander Graham Bell) For Power For Voltages (Power ~ Voltage2) In deciBel (0.1 Bel)
The transfer function of a RC circuit is a complex number
First order system analysis in standard notation (laborious)
First order system analysis in standard notation (laborious)
First order system analysis in Euler’s notation
First order system analysis in Euler’s notation
RC circuit as a Low-Pass filter Transfer function has an Absolute value (Magnitude of complex number) Phase (argument of complex number) Analyze three points: Very low frequencies ‘Corner’ frequency Very high frequencies
Filter response at very low frequency Magnitude Magnitude in dB Phase (argument)
Filter response at corner frequency Magnitude Magnitude in dB Phase (argument)
Filter response at very high frequency Magnitude Magnitude in dB Phase (argument)
Summary 1st order low pass filter characteristics
Bode plot of a Low-Pass filter for t = 1s MatLab: bode([0 1],[1 1])
High-pass filter using RC network
High-Pass filter characteristics
RC circuit as a High-Pass filter Filter response has a Absolute value (Magnitude of complex number) and Phase (argument of complex number)
1st order High Pass filter characteristics
Bode plot of a High-Pass filter for t = 1s MatLab: bode([1 0],[1 1])
Band-Pass filter through cascading
Cascade of High-Pass and Low-Pass filters to obtain a Band-Pass filter Since the sections are separated by a buffer: Add absolute values in dB;s. Add phase angles Buffer
ABE425 Engineering Measurement Systems Filters The End Dept. of Agricultural & Biological Engineering University of Illinois