B - B - 3.16 - Filters PDF

Module 3 Electrical Fundamentals II Topic 3.16: Filters  INTRODUCTION On completion of this topic you should be able to: 3.16.1 Define the operation, construction and uses of the following filters: Low pass...

Module 3 Electrical Fundamentals II Topic 3.16: Filters  INTRODUCTION On completion of this topic you should be able to: 3.16.1 Define the operation, construction and uses of the following filters: Low pass High pass Band pass Band stop 30-03-2024 Slide No. 2  FILTERS For accurate sound reproduction, a good set of speakers will include: A Tweeter A Mid-range speaker A Woofer Sub Woofer – Low frequency base – you don’t hear it you feel it. 30-03-2024 Slide No. 3  FILTER CHARACTERISTICS Output from amplifier is fed to a filter card. Filter splits audio into: High frequencies for tweeters Medium frequencies for mid range Low frequencies for the woofers Therefore only the correct range of frequencies is sent to each speaker. 30-03-2024 Slide No. 4  FILTER CHARACTERISTICS 30-03-2024 Slide No. 5  FILTER CHARACTERISTICS The capacitor in the tweeter line passes high frequency. The capacitor and inductor in the mid range line passes middle frequency. The inductor in the woofer line passes low frequency. 30-03-2024 Slide No. 6  FILTER CHARACTERISTICS Typical use of filters on aircraft Aircraft engine vibration must be Vibration Sensor. selectively monitored – but reject taxi rumble or air turbulence. Filters are used to select frequencies. 30-03-2024 Slide No. 7  FILTER CHARACTERISTICS Low pass filter passes low frequencies but rejects high frequencies. Dividing line between low and high frequencies is referred to as cut - off frequency. Below cut-off frequency, signals pass essentially unaffected. Above cut-off frequency, signals are greatly attenuated or reduced. 30-03-2024 Slide No. 8  LOW PASS FILTERS Two basic configurations for a low pass filter – either, RC or RL. Output is always less than input; therefore, the filter is also an attenuator. Because inductors and capacitors are frequency sensitive, output voltage varies with frequency. Inductive reactance (XL) is directly proportional to frequency. Capacitive reactance (XC) is indirectly proportional to frequency 30-03-2024 Slide No. 9  RC LOW PASS FILTER Above cut-off frequency: As frequency decreases: Capacitive reactance (XC) is XC increases and thus VC very low increases Most of applied voltage will More and more voltage appears be across resistor across capacitor Very little voltage appears Less and less voltage appears across output capacitor across resistor Passes lower frequencies 30-03-2024 Slide No. 10  RL LOW PASS FILTER Above cut-off frequency: Inductive reactance (XL) is very high Most of applied voltage will be across inductor Very little voltage appears across output resistor As frequency decreases: XL decreases and thus VL decreases More and more voltage appears across resistor Less and less voltage appears across inductor Passes lower frequencies 30-03-2024 Slide No. 11  LOW PASS FILTER Typically, RC low pass filter will do exactly same job as RL low pass filter (having same cut-of frequency). RC low pass filters are usually easier to use and less expensive than RL. Furthermore, inductors introduce some resistive loss – capacitors do not. 30-03-2024 Slide No. 12  HIGH PASS FILTERS Performance of RC and RL high pass filters is typically identical. Above cut-off frequency, high pass filters pass signals with little attenuation 30-03-2024 Slide No. 13  RC HIGH PASS FILTER Below cut-off frequency: Capacitive reactance (XC) is very high Most of applied voltage will be across capacitor Very little voltage appears across output resistor As frequency increases: XC decreases and thus VC decreases More and more voltage appears across resistor Less and less voltage appears across capacitor Passes higher frequencies 30-03-2024 Slide No. 14  RL HIGH PASS FILTER Below cut-off frequency: Inductive reactance (XL) is very low Most of applied voltage will be across resistor Very little voltage appears across output inductor As frequency increases: XL increases and thus VL increases More and more voltage appears across inductor Less and less voltage appears across resistor Passes higher frequencies 30-03-2024 Slide No. 15  RESONANCE Inductive and capacitive reactances in an LCR circuit vary with frequency. An increase in frequency causes inductive reactance (XL) to increase and capacitive reactance (XC) to decrease. A decrease in frequency causes XL to decrease and XC to increase. Condition where XL equals XC is called resonance. Resonant circuits are known as tuned circuits. At resonance the Impedance of the circuit becomes Resistive 30-03-2024 Slide No. 16  RESONANT FREQUENCY Resonant frequency (fr) – Frequency where magnitude of XL = magnitude of XC. Below fr – XC is greater than XL and series LCR circuit acts capacitively. Above fr – XL is greater than XC and series LCR circuit acts inductively. Formula for resonant frequency: 𝟏 𝒇𝒓 = 𝟐𝝅 𝑳𝑪 30-03-2024 Slide No. 17  RESONANT FREQUENCY Two important points to remember about this formula: At resonance XL = XC Any change in value of L or C will cause a change in resonant frequency. An increase in value of L or C, or both L and C, will lower resonant frequency. A decrease in value of L or C, or both L and C, will raise resonant frequency. 30-03-2024 Slide No. 18  PARALLEL RESONANCE Parallel LCR circuits can also be used as a resonant circuit. Resonant frequency of a parallel resonant circuit is calculated with same formula. 𝟏 𝒇𝒓 = 𝟐𝝅 𝑳𝑪 30-03-2024 Slide No. 19  TUNED CIRCUITS Each LCR circuit responds to resonant frequency differently than it does to any other frequency. An LCR circuit has the ability to separate frequencies. 30-03-2024 Slide No. 20  TUNED CIRCUITS A certain radio station broadcasts at a certain frequency (e.g. 693 kHz). ‘Tuner’ in radio is adjusted to ‘resonate’ at this frequency. Thus, the tuner selects the station you want and rejects all other stations. To select another station, ‘tuner’ is changed by adjusting resonant frequency. 30-03-2024 Slide No. 21  RESONANT CIRCUITS AS FILTERS Resonant circuits can be made to serve as filters. Series-LC circuits (resonant) offer: Minimum opposition to current flow at frequencies at or near resonant frequency Maximum opposition to current flow at all other frequencies 30-03-2024 Slide No. 22  RESONANT CIRCUITS AS FILTERS Parallel-LC circuits (resonant) offer: Maximum opposition to current flow at frequencies at or near resonant frequency Minimum opposition to current flow at all other frequencies Using these two concepts, BAND PASS and BAND STOP filters can be constructed 30-03-2024 Slide No. 23  BAND PASS FILTER Band pass filter – circuit which passes a narrow band of frequencies and attenuates all other frequencies higher or lower than desired band of frequencies. Greatest current flow exists at centre frequency (fr). Frequencies below lower cut-off (f1) and above upper cut-off (f2) drop are rejected. 30-03-2024 Slide No. 24  SIMPLE BAND PASS FILTER SERIES-LC circuit as a BAND PASS filter (A) It passes currents having frequencies at or near its resonant frequency, and opposes passage of all currents having frequencies outside this band. PARALLEL-LC circuit as a BAND PASS filter (B) If this circuit is tuned to same frequency as series-LC circuit, it will provide a path for all currents having frequencies outside limits of BAND PASS filter. 30-03-2024 Slide No. 25  SIMPLE BAND PASS FILTER Simplest type of band pass filter is formed by connecting the two LC circuits as shown. The upper and lower frequency limits of the filter action are filter cutoff points. 30-03-2024 Slide No. 26  BAND STOP FILTER Band stop filter circuit –used to block current for a narrow band of frequencies, while allowing current to flow at all frequencies above or below this band. Also known as a BAND- SUPPRESSION or BAND- REJECT filter. Minimum current flow exists at centre frequency (fr). Frequencies above lower cut-off (f1) & below upper cut-off (f2) drop are rejected. 30-03-2024 Slide No. 27  SIMPLE BAND STOP FILTER PARALLEL-LC circuit: SERIES-LC circuit (if tuned to same frequency as parallel circuit): Acts as a band-reject filter Acts as a bypass for the band of Blocks currents having rejected frequencies frequencies at or near resonant frequency, and Passes all currents having frequencies outside selected band 30-03-2024 Slide No. 28  SIMPLE BAND STOP FILTER The relative positions of resonant circuits in filter are interchanged. Purpose of band stop filter is directly opposite to that of a band pass filter. 30-03-2024 Slide No. 29  CONCLUSION Now that you have completed this topic, you should be able to: 3.16.1 Define the operation, construction and uses of the following filters: Low pass High pass Band pass Band stop 30-03-2024 Slide No. 30  This concludes: Module 3 Electrical Fundamentals II Topic 3.16: Filters