EE 420L - Lab 1 - Review of Basic RC Circuits

Jonathan K DeBoy
deboyj@unlv.nevada.edu
30 January 2015

Pre-lab work

 

Introduction

This lab served as an introduction to the formats and procedures required for every lab this semester as well as a friendly refresher for RC circuit.

Experiment 1

Hand Calculations

Figure 1.1

This circuit is a low pass filter which will attenuate high frequency signals. By our calculations, a signal with an amplitude of 1V should attenuate down to 0.622V with a 51.4° phase shift.
 
 

Spice Simulations

Figure 1.2        Figure 1.3

Our LTSpice simulation showed exactly what we expected: a phase shifted signal with an amplitude of around 0.6V.
 
 

Experimental Data

Figure 1.4

I promise that this is not a simulation, rather the software and product by PicoScope. Looking at the measurements in the bottom, we can see an input sinusoidal amplitude of about 1V at 200Hz with an output voltage of 0.629V, matching what our hand calculations and simulations told us. The time difference between the peaks is 703µs. Using the period of a 200Hz signal (5ms), we can determine that 0.703/5*360= 50.6º of a phase shift, again matching our hand calculations.

 

Experiment 2

Hand Calculations

Figure 1.2

With the capacitor in parallel with the resistor, that impedance is less than before, so we should see a smaller attenuation the before. Our hand calculations show a different story, however we can blame plugging in numbers in different stages rather than all at the end. This circuit will also cause less of a phase shift than the previous one.
 
 

Spice Simulations

Figure 2.2          Figure 2.3

LTSpice confirms the assumption that this circuit will attenuate the same signal less since the magnitude of this waveform is around 0.67V instead of 0.62V. The phase shift is minimal if not non-existent.
 
 

Experimental Data

Figure 2.4

From our measurement in the lab, we can actually see a small phase shift measured by the time delay between peaks: 117µs. This information allow us to determine that there was an 8.4º phase shift. The magnitude of the output voltage is 0.69V instead of the 0.62 from last time, backing up our original assumptions.

 

Experiment 3

Spice Simulations

Figure 3.2          Figure 3.3

The simulation shows how the voltage across the capacitor increases and discharges in an exponential fashion with a time constand dependent on the values of R and C.
 
 

Experimental Data

Figure 3.4     Figure 3.5

The Picoscope that I was using as both an oscilloscope and as a function generator is powered through USB, which means it is limited in the amount of current it can drive with a given load, especially when creating a square wave. To the right is the waveform of the lab oscilloscope (better square wave under load). We can see that the capacitor discharges in an e^(-t/RC) fashion unit the next pulse arrives.

   

Conclusion

This was a simple refresher for RC circuits and an introduciton to the different policies implimented in this lab versus previous ones.
 
 
 
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