Phys 351 lab 2

# Phys 351 lab 2 - Lab 2: AC Filters Methods Section A:...

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Lab 2: AC Filters Methods Section A : Measurements of AC Signals Section A-1 , Frequency Measurements. In A-1, We used an oscilloscope screen to measure the frequency of a wave using a 1kHz sine wave. A small amount of waves were displayed on the oscilloscope and the amount was recorded. Then the position of the last peak was noted in the division of the scope it laid in. Lastly, the oscilloscopes horizontal sweep selector setting was read. From the three recorded pieces of information, a measure of frequency was found. However, and fortunately for us, on digital scopes, this information is measured by the scope and displayed. Section A-2 , Frequency from Fourier Transform. In A-2, we used another method to acquire the frequency. The waveform generator was used to set up a 2 Volt amplitude sine wave, at 1.25kHz and sent to channel one of the digital scope. The vertical sensitivity was set to 1V/div, and the Math button was pushed, and the FFT feature was selected. The sweep selector was rotated until the sample rate was approximately 2.5kHz per division. We then played around with the type of output and viewed any changes in the harmonics. Section A-3 , Phase Measurements. In A-3 the phase between two different inputs was measured. The phase difference was measured by counting the number of divisions between the zero crossing of the two waves, and then phase was calculated using the measured values of the division of lag and the total number of divisions per wavelength.

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The most accurate way of measuring phase difference or wavelength in the digital scope is using cursors. Section A-4 , Gain Measurements. In A-4, Gain was measured. Gain is the ratio of the output voltage over the input voltage. Measurements of Gain with a change in frequency are useful in finding the -3dB point, which is the half power point or “knee frequency”. This is found experimentally by graphing the frequency over a large range against the Gain of the function. Section B: Basic Filters Section B-1 , Low-Pass Filters. In section B-1, the circuit shown in figure 2.1, shown below, was used in order to measure the -3dB frequency. After the circuit was set up, the frequency, voltage input, and voltage output were displayed on the function generator. These values were recorded and gain was found using the relation discussed in section A-4, by taking the voltage out readings over the voltage in readings. The log of the frequency and the log of the gain were then graphed in order to find the -3dB reading. This value was found at the intersection of the “knee” of the graph. The measured value was then compared to the calculated value. Lastly, the phase shift of the circuit at 3 different frequencies was measured.
Figure 2.1 The low pass filter configuration used in section B-1 Section B-2 , High-Pass Filter. In section B-2, the high-pass circuit shown in figure 2.2 was built. The same procedure was performed as in section B-1. Figure

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## This note was uploaded on 11/03/2009 for the course PHYS 351 taught by Professor Staff during the Spring '08 term at UNC.

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Phys 351 lab 2 - Lab 2: AC Filters Methods Section A:...

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