ANALOG & DIGITAL ELECTRONICS

LAB 3: Basic AC circuits – Feb. 22 (due Feb. 26)

Equipment

Function generator, oscilloscope, multimeter, breadboard, resistors and capacitors.

Purpose

I want you to measure the RC time constant and transfer function of a simple filter and develop a feel for how it works. Experiment

Part I

Construct a RC like the one shown at the right. Choose R and C so that the RC time constant is about 160s. Measure the resistor and capacitor on your meters and record the measured values. (The 33k resistor shown is just a suggestion, but you should choose 10k<R<100k.) Then measure the time constant with the oscilloscope using a square wave of about6Vpeak-peak for the input. Note that you probably want the frequency of the square wave somewhat less than 1kHz. (See what works best.) Sketch the input and output waveforms, showing the scale for time and voltage. Show me your circuit at this point.

Part II

Now use a square wave of about 2kHz for the input and sketch the output and input like you did in Part I above. Repeat this using a square wave of frequency 20kHz. Show me your circuit at this point. Does it look like it is integrating the 20kHz signal?

Part III

Measure the frequency response of the filter using a sine wave input over the range of 30 Hz to 300 kHz by measuring both the input and output amplitudes on the scope. Use the oscilloscope to do this. (You really only need two points per decade, e.g. 30Hz, 100 Hz, 300 Hz, 1 kHz, 3kHz, etc. Use an input amplitude of about 10Vp-p.) Does this really look like a low pass filter?

I want you to find the -3dB point, i.e. where the output amplitude is 70% of the input amplitude. Make a Bode plot of the results (20 times the log of the ratio of the magnitude of Vout to Vin vs log frequency). Measure the phase shift at 100 Hz and 10 kHz. You only need to measure the phase shift to the nearest 10 degrees. Show me your circuit at this point. Are these phase shifts what you would expect?

You have three different measurements of the RC =  = 1/(2fc) time constant in this lab. One is from the measurement of R and C, the second is the time constant from the charging curve and the third is the frequency of -3dB point for the transfer function measured in part III. Are these consistent with each other? (Compare them quantitatively.)