Exp_3_fa09 - Physics 3330 Experiment #3 Fall 2009 Filters...

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Physics 3330 Experiment #3 Fall 2009 Filters and Waveform Shaping Purpose The aim of this experiment is to study the frequency filtering properties of passive (R, C, and L) circuits for sine waves, and the transient response of the same circuits to square waves. Filters are important in experiments for enhancing signals of particular interest while suppressing unwanted background. The oscilloscope probe is introduced in this lab. Introduction A frequent problem in physical experiments is to detect an electronic signal when it is hidden in a background of noise and unwanted signals. The signal of interest may be at a particular frequency, as in an NMR experiment, or it may be an electrical pulse, as from a nuclear particle detector. The background generally contains thermal noise from the transducer and amplifier, 60 Hz power pick up, transients from machinery, radiation from radio and TV stations, cell phone radiation, and so forth. The purpose of filtering is to enhance the signal of interest by recognizing its characteristic time dependence and to reduce the unwanted background to the lowest possible level. In everyday life, your radio does this when you tune to a particular station, using a resonant circuit to recognize the characteristic frequency. The signal you want may be less than 10 -6 of the total radiation power at your antenna, yet you get a high quality signal from the selected station. A filter freely transmits electrical signals within a certain range of frequencies called the pass band , and suppresses signals at all other frequencies (the attenuation bands ). The boundary between a pass band and an attenuation band is called the cut-off frequency f c . We usually define f c to be the frequency at the half-power point or 3dB point, where the power transmitted is half the maximum power transmitted in the pass band. The output voltage amplitude at f = f c is 1/ 2 = 70.7% of the maximum amplitude. There are three basic types of filter. The high-pass filter transmits signals at frequencies above the cut-off f c and blocks them at lower frequencies. The low-pass filter transmits signals below f c and blocks higher frequencies. The band-pass filter, often taking the form of a resonant circuit, transmits a certain band of frequencies and blocks signals outside that band. For band-pass filters the bandwidth is the range of frequencies between the upper (f + ) and lower (f ) half power points: bandwidth f = f + –f . Many experiments require specific filters designed so that the signal from the phenomenon of interest lies in the pass-band of the filter, while the attenuation bands are chosen to suppress the background and noise. This experiment introduces you to the filtering properties of some widely used but simple circuits, employing only a resistor and capacitor for high- and low-pass filters and an LCR circuit for band-pass.
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Exp_3_fa09 - Physics 3330 Experiment #3 Fall 2009 Filters...

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