Measuring AC Phasors in Electrical Engineering Laboratory, Lab Reports of Microelectronic Circuits

Download Measuring AC Phasors in Electrical Engineering Laboratory and more Lab Reports Microelectronic Circuits in PDF only on Docsity! Name: ___________________________ Lab Instructor: _________________ Date Performed: ________________ Date Due: _______________________ Lab Partner(s): ______________________________________________________ © 2008 Simon J. Tritschler. All Rights Reserved. 47 nF + VC − EE 304 Laboratory I Measuring Phasors The purpose of this laboratory assignment is to introduce basic AC signal measurement techniques using standard lab equipment. Unlike simple DC measurements in which we are strictly concerned with scalar quantities, AC signals often require the measurement of both magnitude and phase information; therefore, specific methods must be employed if the equipment that can do this automatically isn’t available (which is often the case due to prohibitive cost). Furthermore, although handheld DVM’s often have provisions for measuring AC voltages and currents, most are only accurate over a very narrow frequency range. Consider the following circuit: + 1. Build the circuit as shown. Generate a 1-VRMS sine wave with a frequency of 1 kHz. Make sure your function generator is properly terminated to avoid voltage errors. Connect channel one of your oscilloscope to the output of the circuit (VOUT) and connect channel two in parallel with the function generator to monitor the input signal. Displaying VOUT only, measure the peak-to-peak output voltage and convert it to its RMS value by the following expression (you may have to adjust triggering to display the waveform correctly): 22)( PKPK SINERMS VV −= . VOUT = __________ 1 kΩ 1 VRMS @ 1 kHz SINE + VOUT − + V1 − 1.5 kΩ © 2008 Simon J. Tritschler. All Rights Reserved. 2. Now display both channels simultaneously on the oscilloscope and observe how the signals differ. In addition to being attenuated in magnitude, the output of the circuit will be shown to be out-of-phase with the input. Measure the time difference between the two signals on the oscilloscope, paying careful attention to waveform position as any undesirable offset on the voltage axis will distort the time measurement. It is often useful to adjust the oscilloscope’s horizontal position so that the zero-crossing of the input waveform is aligned with a vertical line on the measuring grid, thus making it a simple task of finding the time difference between it and the zero-crossing of the output waveform. Remember, time-shifts to the left of the reference signal constitute a positive phase angle, while time-shifts to the right of the reference signal are negative. Give the phase angle in degrees using the following expression, noting the 1-kHz operating frequency (f): θ(DEG.) = 360 t f θOUT = __________ 3. Now show VOUT in phasor form by combining the voltage and phase information of (1) and (2), respectively (don’t forget the proper units): VOUT = _____ ∟ _____ Using an Unbalanced Oscilloscope to Measure Floating Voltage Phasors Most oscilloscopes found in electronics laboratories have unbalanced inputs. This means that their negative input terminals are internally connected to a common ground node. In the previous example, we were able to measure input and output voltages directly with an unbalanced oscilloscope because both signals were already referenced to a common ground as an inherent feature of the circuit. Unfortunately, it is not possible to directly measure signal across a “floating” component with an unbalanced oscilloscope because the grounded negative input terminal would cause an undesirable short- circuit. However, it is possible to determine the voltage across a floating circuit element by measuring the voltage phasor at each end with respect to ground and calculating their difference. [Oscilloscopes with floating balanced inputs are available, but at comparatively high cost, and although two- channel unbalanced oscilloscopes typically do have built-in circuitry for measuring the voltage difference between their channels, remember that for our application one channel of the oscilloscope must remain connected to the input signal in order to measure the relative phase angle at other nodes.]