2. INTRODUCTION
Most of the signals we deal with in real life get
corrupteed in some way or another by some
unwanted signals.
For the purpose of signal processing and analysis, it is
imperative to get rid of these interferences, or at least
reduce their effects.
This is achieved through applying Signal Filtering
techniques.
3. A Filter is an electronic circuit that removes /
attenuates, from a signal, some unwanted component
or feature.
Filter Application
Eliminate background noise
Radio tuning to a specific frequency
Direct particular frequencies to different speakers
Modify digital images
Remove specific frequencies in data analysis
4. FILTER CHARACTERISTICS:
To understand the basics of filtering, it is first
necessary to learn some important terms used to
define filter characteristics.
Cut-Off Frequency (fc): Also referred to as the
corner frequency, this is the frequency or
frequencies that define the limits of the filter
range.
5. Stop Band: The range of frequencies that is filtered out.
Pass Band: The range of frequencies which is let
through and recorded.
Transition Band: Region that Separates the pass band
and stop band.
6. LOW-PASS FILTER:
A low-pass filter is an electronic filter that passes
low-frequency signals and attenuates (reduces
the amplitude of) signals with frequencies higher
than the cutoff frequency. The actual amount of
attenuation for each frequency varies from filter to
filter.
It is sometimes called a high-cut filter, or treble cut
filter when used in audio applications.
7. There are two basic kinds of circuits capable of
accomplishing this objective, and many variations of
each one:
The inductive low-pass filter
The capacitive low-pass filter
8. THE INDUCTIVE LOW-PASS FILTER
The inductor's impedance increases with increasing
frequency. This high impedance in series tends to
block high-frequency signals from getting to the load.
9. THE RESPONSE OF AN INDUCTIVE LOW-PASS FILTER FALLS
OFF WITH INCREASING FREQUENCY.
10. THE CAPACITIVE LOW-PASS FILTER
The capacitor's impedance decreases with increasing
frequency. This low impedance in parallel with the
load resistance tends to short out high-frequency
signals, dropping most of the voltage across series
resistor R1.
11. THE RESPONSE OF A CAPACITIVE LOW-PASS FILTER FALLS
OFF WITH INCREASING FREQUENCY.
12. All low-pass filters are rated at a certain cutoff
frequency. The cutoff frequency for a low-pass filter
is that frequency at which the output (load) voltage
equals 70.7% of the input (source) voltage. This
cutoff percentage of 70.7 is not really arbitrary, all
though it may seem so at first glance. In a simple
capacitive/resistive low-pass filter, it is the frequency
at which capacitive reactance in ohms equals
resistance in ohms. In a simple capacitive low-pass
filter (one resistor, one capacitor), the cutoff
frequency is given as:
13. A simple filter will be two elements - a capacitor or
inductor and a resistor. A capacitor will tend to "trap"
low frequencies.
In the case of a low pass filter made of a capacitor
and resistor, the output voltage will be measured
across the capacitor.
Inductors are the opposite, so the output would be
across the resistor.
14. There are a great many different types of filter
circuits, with different responses to changing
frequency. The frequency response of a filter is
generally represented using a Bode plot.
• First-order filter
• Second-order filter
• Third and Higher order filters
15. First-order filter:
A first-order filter, will reduce the signal strength by
half (about −6 dB) every time the frequency doubles.
The magnitude Bode plot for a first-order filter looks
like a horizontal line below the cutoff frequency, and a
diagonal line above the cutoff frequency.
16. SECOND-ORDER FILTER
A second-order filter does a better job of attenuating
higher frequencies. The Bode plot for this type of
filter resembles that of a first-order filter, except that
it falls off more quickly.
For example, a second-order filter will reduce the
signal strength to one fourth its original level every
time the frequency doubles.
