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Consider the ring oscillator circuit below: A ring oscillator is a cascade of an odd number of inverters with the final invert output fed back to the input of the first inverter. The circuit is self-oscillating, and has no input from another circuit.

Assume a constant finite propagation delay t P for the inverters. Derive an equation that describes the oscillation frequency of the ring oscillator in terms of t P and the number of inverters N. Explain how this equation could be used to measure propagation delay. Develop a method using an oscilloscope and a ohm resistor that would allow you to display the dynamic supply current waveform i DD. Explain your technique and draw a circuit diagram.

Also remember that oscilloscopes measure voltage, not current. Apply this signal to a single inverter the remaining inverter inputs should be tied low. Monitor both the inverter input and output with the oscilloscope. Adjust the oscilloscope output waveform display to maximize use of the screen in the vicinity of a rising edge on the inverter output.

Note the positions of marker lines, and verify that you understand how this relates to your Prelab Step 1. Discuss the impact of capacitive loading on rise and fall time. Allow the inputs on the right side of the chip to float, rather than being grounded.

What effect do floating inputs have on the output waveform? Seek help from your instructor if you are in doubt about your method! Drive one inverter with your 1 MHz squarewave signal the other inverters should be tied low. Show the output of the inverter on the other probe of your scope. Record the waveform in your lab book and measure the numerical value of the peak current from your waveform. Next, drive two inverters with the same squarewave input signal the two inverter inputs would be tied together at this point.

Record the waveform and measure the peak current. Repeat the process by driving three inverters, then four, and so forth, each time recording the waveform and measuring the peak current. Plot peak current as a function of the number of simultaneously switched inverters. Discuss any trends in your data, and propose an explanation for the trend. Now that all six inverters are switching simultaneously, remove the ohm resistor , then look at the dynamic supply voltage v DD at pin Record the waveform and measure the peak deviation from the nominal value V DD.

Connect a 0. Make sure that the capacitor is physically close to the package; you may even want to cut the leads a bit in order to minimize lead length. Repeat Step 6. Do a percentage change calculation. Use inverters 1 to 5 of the 74HC04 hex inverter for the ring oscillator. Use the remaining inverter as a buffer between the ring oscillator and the instrumentation frequency counter on the oscilloscope.

That is, select one of the inverter outputs from the ring oscillator, and apply this to the input of the sixth inverter. Measure the output of the sixth inverter. Once you have a stable reading of frequency, try probing inside the ring with your other oscilloscope probe. Note any differences in waveform quality, and note the degree to which the oscilloscope probe alters the measured frequency.

What is the effective capacitance of the probe? Hint: look at the probe connection to the oscilloscope. Use the equation you derived in the prelab to estimate the propagation delay t P for a single inverter, and compare to the published specification and as well as your direct measurement results. Use the digital probe pod and digital waveform display to create a measured timing diagram of all five inverter outputs.

Compare to your prelab prediction. All Done! Clean up your work area Remember to submit your lab notebook for grading at the beginning of next week's lab.


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