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Unformatted text preview: FILTERS The functions are :
1. To select the required frequencies from input signal. To eliminate the non-required q frequencies from the input signal. To choose certain required group q g p of frequencies from the input signal. 2. 3. Passband : certain group of frequencies freely able to pass the filter. Stopband : certain group of frequencies freely unable to pass th filt f the filter freely. l Transmission : Frequency transportation process. The amplitude of the frequency remains before (input) and after (output). In practice, -3 dB (decibel) is the point when the signal starts to weaken. Attenuation : The concept is opposite of transmission. Attenuation is when the signal amplitude starts to reduce to 3 dB compared to the initial signal. TYPES OF FILTER Low pass Filter (LPF) High pass Filter (HPF) Band pass Filter ( p (BPF) ) Band stop Filter (BSF) 1. Low pass Filter Consists of circuits allowing low frequency and disallow higher frequency q y g q y from passing through the filter. Designed to send frequency values from zero to cut-off frequency (fc) without attenuation. Frequencies higher than fc will be cut-off. 2. High Pass Filter Consists of circuit which allow only high frequencies to pass through the filter. filter Transmit all frequencies which is between fc and infinity. Frequency below fc will be eliminated. 3. Band Pass Filter Consists of circuits that allow only certain group of frequencies to pass g p q p through the filter. It is designed to send frequencies between 2 cut-off frequencies which are fc1 or fc2 ; whereas frequencies below belo fc1 o abo e fc2 will be or above ill eliminated. 4. Band Stop Filter This filter allows all frequency to pass through except for a group of specific frequency (stopband) (stopband). Also knows as bandattenuation/band elimination filter and the function is opposite BPF. It is designed to allow frequencies less than fc1 and bigger than fc2. a. Raw signal
Signal amplitude Frequency b. Low-pass filter Signal amplitude fc f OUTPUT FROM IDEAL FILTERS c. High-pass c High pass filter Signal amplitude fc f d. Band-pass filter Signal amplitude fc1 fc2 f e. Band-stop filter Signal amplitude fc1 fc2 f Signal amplitude Signal amplitude fc a. L Low-pass filt filter f fc b. High b Hi h pass filt filter f Signal amplitude Signal g amplitude fc1 fc2 f fc1 fc2 f c. Band-pass filter d. Band-stop filter OUTPUT FROM PRACTICAL FILTERS RC FILTERS This type of filter is a combination of resistors (R) and capacitors (C). Capable of allowing certain parts of signal to pass through the filter, filter besides disallowing unnecessary signal. RLC FILTERS
o This type of filter is a combination of a resistor ( ), inductor ( ) yp (R), (L) and capacitor (C ) 1. Band-Pass Filter (BPF) Vin C L R Vout o Filter allows signal at fr (resonant frequency) and frequency range below and above the fr value where fr is : fr = 1 2SLC o Bandwidth (BW) this filter is the frequency range when filter output is 3 dB. BW = f2 - f1 where, f1 : lower cut-off frequency f2 : upper cut-off frequency pp q y Vout
1 0.707 f1 fr
BW f2 f 2. . Band-Stop Filter (BSP) BandR C Vin L Vout o This filter eliminates the signal in between the upper cut-off frequency (f2) and lower cut-off frequency (f1) where : fr = 1 2SLC NOISE Generally, noise is defined as any unwanted signal that is not originated from/to - with input signal. Noise cannot be eliminated totally. There are two types of noise, Th f i which are internal and external. 1. External Noise Noise that originated from outside of system's component. Example : E l 1. Atmospheric Noise thunder, lightning li h i Man-made Noise from man-made instrument or system e g motor e.g. motor. 2. Internal Noise Noise that is originated from inside of system's component etc. Example : 1. Thermal Agitation Noise Noise N i produced by resistor component caused b d db i d by random motion of charged particle which is the electron in conductor. This type of noise can be found in all electronic circuits which use resistor. Continue.... 2. Shot-Noise Can be found in all amplifying C b f di ll lif i devices and is caused by the current flow in the electronic component i.e transistor. SIGNAL-TOSIGNAL-TO-NOISE ratio Use to compare noise and signal so that excessive noise will not occur. A good signal will have a high value of SNR. SNR = Signal Noise SIGNAL TRANSMISSION When a measurement variable need t b Wh t i bl d to be sent to other location, such as control room, transmission element is needed. Signal transmission elements can be categorized into:
1. Cable type Data is transformed through wire. 2. 2 Radio Frequency type Data is transferred using radio waves. Data Transmission Digital signal is compatible with the computer system where certain converter co e te is not sufficient. ot s fficie t Signal can be transmitted by parallel or series. A digital information which is transmit out/in or into computer system is called word (16 bits). Parallel Transmission Data is transmitted simultaneously. Information bits is send at the same time. Different transmission line is use ff for each bits in computer language. Example: 1011 ----> 1011 Series Transmission Information b I f i bytes will be send one by one ill b d b through a transmission line. Advantage : need only one transmission line. line Disadvantage : slower process compared to p parallel transmission. Example : 1011 -- 1 0 1 1 ...
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This note was uploaded on 05/09/2010 for the course CHE 3022 taught by Professor Abdirahman during the Fall '07 term at National Central University.
- Fall '07