INSTRUMENTATION AND CONTROL(MCT 3224)SIGNAL CONDITIONING : PASSIVE FILTERSNadzril SulaimanDepartment of Mechatronics Engineering,International Islamic University Malaysia

ContentConcept of filtersLow-pass filterHigh-pass filterBand-pass filterBand-reject filter

Objectives of topicExplain terms related to filterIdentify the circuit of different types of filtersExplain the characteristics of each type of filtersDesign different types of filters based on the design requirements

Soil sieving analogy

Introduction to filtersFilters pass, reject, and attenuate signals at various frequencies

Characteristics of filtersCommon types of filters:Low-pass - Allow low frequency signal; eliminate high frequency signal

High-pass - Allow high frequencies; eliminate low frequencies

Band-pass - Allow/Pass some particular range of frequencies; eliminate other frequencies outside that band

Band-rejection (Band-Stop) - Stop a range of frequencies; pass all other frequencies

Characteristics of filtersIdeal filters

Characteristics of filtersPractical filters

Important termsPass band frequencies passedStop band frequencies eliminated / attenuatedCutoff Frequency, fc (critical @ corner frequency) point in the stop band where frequencies have been attenuated by 3 dB (or frequency at which gain = 0.707)

Important termsBand width distance (in Hz) between 2 fc in Band-pass and Band-reject (Band-stop) filters

Passive filter circuitsLow-pass filter

Low-pass filterCharacteristics:

It blocks high frequencies and passes low frequencies All signals with frequency above some critical value are simply rejected

Design low-pass filterDesign Methods: 1) Find the cutoff frequency that will satisfy the design criteria 2) Select a standard value of C in F or pF 3) Calculate the value of R using equation of cutoff frequency * Choose another value of capacitor if the resistance value is below 1k or above 1M

Design low-pass filterCutoff / critical frequencyVoltage ratio / Magnitude of TF* f is the frequency of the signal under considerationResponse of the low-pass filter as a function of the frequency ratio ( f/fc ).

Design low-pass filterExample 1:

A student measures signal from a microphone. The measured signal contains noise at 0.5MHz. The desired information signal is at 400Hz.Develop a low-pass RC filter to attenuate the noise by 97%.

High-pass filterCharacteristics:

It blocks low frequencies and passes high frequencies All signals with frequency below some critical value are simply rejected

Design high-pass filterDesign Methods: 1) Find the cutoff frequency that will satisfy the design criteria 2) Select a standard value of C in F or pF 3) Calculate the value of R using equation of cutoff frequency * Choose another value of capacitor if the resistance value is below 1k or above 1M

Design high-pass filterCutoff / critical frequencyVoltage ratio / Magnitude of TF* f is the frequency of the signal under considerationResponse of the high-pass filter as a function of the frequency ratio ( f/fc ).

Design high-pass filterExample 2:

Pulses for a stepping motor are being transmitted at 2000 Hz, in which at this frequency the output is attenuated by 3dB . Design a filter to reduce the noise that present at 60Hz.

Band-pass filterCharacteristics:

A low-pass filter followed by a high-pass filter Blocks frequency below low limit and above high limit Frequencies between limits are passed

Design band-pass filterDesign Methods: 1) Find fL and fH that will satisfy the design criteria 2) Select a standard value of R that will satisfy the limit (RH / RL < 0.01) 3) Calculate the value of C using equation of cutoff frequency

* Choose another value of resistor if the capacitor value is not in the F or F range

Design band-pass filter* Resistor ratio, RH / RL must be kept below 0.01

Design band-pass filterExample 3:

A signal conditioning system use a frequency variation from 6 kHz to 60 kHz to carry measurement information.There is considerable noise at 120 Hz and at 1 MHz. Design a band-pass filter to reduce the noise by 90%.

Design band-reject filter (Twin-T)Design Method:

1) Find cutoff frequency that will satisfy the design criteria.

2) Select a standard value of C in F or pF 3) Calculate the value of R using equation of cutoff frequency

Design band-reject filter (Twin-T)Design Method:

4) Calculate the value of grounding components, R1 and C1 using the respective equation

Design band-reject filter (Twin-T)Example 4 :

A single line is multiplexed to carry sensor signal in a frequency range below 1kHz and communication signals ranging from 10kHz to 50kHz. There is a large noise component at 4.5kHz from a turbine in the plant. Design a twin-T notch filter to attenuate the 4.5kHz noise.

Filter-end