Lab 8

LOW-PASS FILTER  ( 3dB frequency and Bode plots )  

I. PURPOSE:

This laboratory session pursues two main objectives:

First, to build a low-pass RC filter and measure the output voltage (magnitude and phase) as a function of its frequency. A log-log plot (Bode plot) of the output signal vs frequency will help familiarize with the concepts of “3dB breaking point” and “decrease of output levels per octave and per decade.”

Second, to build active pass filters using operational amplifiers. Active filters display steeper frequency roll-off. They also allow building circuits that behave as RLC circuits without the need to use bulky L (inductor) components.  

II.  PASSIVE LOW PASS FILTERS

II.1 The scale of decibels

Comparison of signal amplitudes

Comparison of Power levels

II.2   Single-stage low–pass filter

Output voltage (magnitude and phase)

II.3   Bode-plots and the 3dB breakpoint

II.4 Voltage drop at high frequencies (in decibels) in single low-pass filter

II.4A Roll off per octave

II.4B Roll off per decade

II.5   Dual-stage low–pass filter

III.  ACTIVE FILTERS 

III.1 Capitalizing on the low output impedance and high input impedance of op-amps

III.2 Active pass filters: Filters with steeper roll-off