RC Circuits Lab: Measuring Capacitance and Analyzing Digital Signal Distortion, Study Guides, Projects, Research of Humanities

Download RC Circuits Lab: Measuring Capacitance and Analyzing Digital Signal Distortion and more Study Guides, Projects, Research Humanities in PDF only on Docsity! ECE 35 Project 4 RC Circuits The objective of this lab is to measure the time characteristics of RC circuits and demonstrate how RC circuits can affect the transmission of digital information. NOTE: Your prelab will be checked before lab begins for completeness. Before starting the lab: 1) The switch is closed in the circuit below and the output waveform s(t) is observed across the capacitor. Using only the graph, determine the value of the capacitor C assuming it was initially uncharged. Do this by plotting ten values of the function ln(1-s(t)) spaced every 0.1 µs and fitting a straight line to the data points using a ruler. Explain how to determine the value of C from the slope of this line and then using this explanation, determine its value. Hint: What is the time constant? You can use other methods to arrive at the answer if you understand the process. 2) Design an experiment to measure the capacitance C replacing the DC voltage source and the switch in the circuit above with a function generator. Sketch the set-up and discuss what measurements are required to determine the capacitance and specifically how to set up the frequency and waveform of the function generator if the capacitance is not known. Note that you will use this procedure in lab. Hint: What should the input look like in comparison to the output? Experiment 1 – Measuring the capacitance from the RC time constant. The tutor will give you a capacitor with an unknown value. Using the sketch of the experimental set up done before you arriving at lab and the same procedure as used to determine C for the question before lab, determine the value of C, and have the tutor sign the book where you plotted your data and calculated the value. They will then measure the value independently to determine how close you came to the real value. Effect of RC circuits on digital signals In this part of the lab, we will qualitatively demonstrate the effect that an RC circuit has on a digital waveform. Our simplified digital communication system is the following: The signal generator, set on square wave, will act as our digital data source. The RC circuit acts as an integrator or a “low-pass” filter and represents the effect of a communication “channel” which distorts the waveform. The comparator acts as our receiver that attempts to reconstruct the original digital waveform from the distorted waveform. If Vin(t)>Vc , then Vout(t)=V+. If Vin(t)<Vc , then Vout(t)= V. You can think of Vc as a threshold voltage, in which voltages above it will go to a digital “high”, while voltages below it will go to a digital “low”. We will vary Vc and observe what effect this has on the output. The effect of the RC circuit on a digital waveform is shown via a simulation on the next page. The top sets of panels are ideal digital waveforms before transmission. The middle set of panels are the distorted waveforms before the comparator caused by the averaging effect of the RC circuit with the right panel being more distorted than the left panel owing more averaging caused by a larger RC time constant. The bottom set of curves are the recovered waveforms after the comparator. For the moderate distortion shown on the left set of panels, the comparator does a good job of recovering the original waveform with the width of the recovered pulses being nearly the same as the original data. However, when the RC time constant is large, the waveform is significantly distorted. The simple comparator then “misses” the second “one” sent at t=6 and a “zero” at t=30 causing transmission errors. In addition, the width of the recovered pulses does not always match the width of the original data producing duty cycle distortion. We will investigate the effects of RC circuits on digital waveforms in this part of lab.