User:Stephen K. Martinez/Notebook/Junior Lab/2008/09/10

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Oscilloscope Lab


Basic Waveform Measuring
Started by turning on both the oscilloscope, and the function generator. Then I ran a BNC cable between the ch1 on the scope and the output connector on the generator. I set the frequency of the generator to a sine wave at 200 (+- 1) Hz. Both ends of the cable were capped by terminators to match impedance and disallow an RF signal from sending a signal back terminator. I made adjustments to the Volts/Div and Sec/Div buttons to create a larger (appearing) amplitude and longer (appearing) period respectively.

First I measure the voltage by observing the volts/div setting I was using was 500mV then counted the divisions approximately 4 to calculate that the amplitude of my wave was 2 Volts. I used the same reasoning from the indication that the sec/div was set to 1ms and counted the divisions between peaks to obtain approximately 5 corresponding to 5ms.

Next used the cursors tab of type Voltage and put the two lines cursor 1 at the peak and cursor 2 at the trough to measure the voltage indicated by the delta as 2.02V. Then changes the type to time and put the cursors at the two peaks and obtained a delta of 5ms or 200Hz.

Finally measured the Peak to Peak Voltage of the sine wave by pressing the measure button and then pk-pk selection obtaining a value of 2.02V. Then I changed that selection to type Period and obtained a value of 5.000 (+-.08) ms, which is using 1/T=f frequency of 200Hz.

Then I increased the amplitude by the output level button, and using the measure function obtained a value of 5.08V?. Most likely this result is due to the fact that I hadn't yet increased the the volts/Div range to suit my waveform. Upon raising the volts per division I obtained a measurement of 7.2V (at 1V per division) - I believe that for the 500mv/div setting the maximum is therefore 5.08V and that each V/D setting will in fact have a maximum based on the total output it can put on screen. When I returned my output level to the previous setting I got a new measurement of 2.04 a .02 increase from before which indicates that the recording of the amplitude decreases in accuracy at higher V/d. I increase the DC offset and as I did so the waveform bounced around, at the highest setting the tops of the sine waves were cropped off. DC offset is the mean amplitude of the waveform, it occupies headroom of the signal - therefore the cropping, the solution is to use a high-pass filter to exclude the average from the signal DC offset.

Triggering
Dr. Koch explained Triggering. Triggering tells the sweep to keep going across the screen. Continued by examining certain effects of the trigger function - when turned in either direction to a certain degree resulted in a drifting to the left or right of the signal which occurs because the scope is no longer triggering off the input signals time base. We are currently using edge triggering on a rising slope which means that after the scope has completed one sketch from left to right it wont begin again until it reads a rising voltage. The other types of trigger are video and pulse which drifted across screen and was triggered by an inverted polarity for video and it seems like pulse works by triggering whenever there is a non-zero voltage. Triggering defines the time as zero, and if we switch from rising to falling then the waveform changes in phase by 180.

High/Low Pass Filters
Dr. Koch explained Filters. High and Low pass filters are the same circuit - an alternating voltage source, a resister and a capacitor in series, only the placement of the the scope leads makes the difference. For a high-pass filter the scope reads across the resistor which has the effect that when their is a sharp spike in voltage as in a square wave the majority of the current goes through the resistor as the capacitor charges, therefore the fastest (highest frequency) signals making up the shape are read in the scope whereas the slower signals are cut out as less and less current goes through the resistor. The low-pass filter works on exactly the same principle as the high-pass except the read is over the lagging slow frequencies.

AC Coupling
Dr. Koch explained some uses for these two coupling examples. Ac coupling therefore acts as a high-pass filter and dc as a low-pass. Examining the difference between ac and dc coupling, I found that for examining a normal dc signal or logic signal a dc coupling would be most useful because it only measures the voltage output because a direct current is stopped and charged at a capacitor, whereas for the logic signal the ac coupling behaved as an oscillating circuit because of the capacitor, it rose and fell at the same voltage but with attenuation due to the oscillating. By the same accord when measuring a sinusoidal or other periodic signal the dc couple did show a low voltage time attenuated waveform that needed to be adjusted by the trigger, whereas by using the ac couple we receive the normal signal by cutting the d current out.

Next I attempted to measure the fall time by setting the signal to square wave, the dc offset to zero and amplitude of 8.56V. I had trouble getting the waveform to the appearance on the website, but Mr. Gragossian helped me adjust the necessary levels to obtain the observation necessary.

Then using the cursor option I measured using the first cursor the peak of the wave, and then using the second cursor to the lowest trough, then using the difference between those two fount approximately 10% of the maximum voltage and set the firs time cursor there and the other at the originating point to find the fall time as 49.6ms. Using the measure tab then I received a value for the fall time as 82.23(+-.12)ms which is quite far from my measured value.

The RC value is therefor 49.6ms because of the relation RC = tau. Thanks to Michael for his tip.


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