User:Thomas S. Mahony/Notebook/Physics 307L/2009/08/24

Oscilloscope Lab

^{SJK 00:34, 15 September 2009 (EDT)}

Note

I couldn't figure out how to use the references, like so many wikipedia pages have. I tried using the same code for the little numbers as listed on the help page, but for some reason it isn't working yet.

Steve Koch 00:24, 15 September 2009 (EDT): Check out Der's formal report from last year. He uses an extension for MediaWiki and I loved the way it worked for him. Particularly cool was the ability to add comments to references. Here's the extension page (I think): http://openwetware.org/wiki/OpenWetWare:Biblio Also, I am almost surely going to require all students to do either this extension or something like it, since it's a very useful skill to learn. So, you're way ahead of the game. Here's some other tools I like for reference managing:

http:\\citeulike.org easily add citations to your "library" on citeulike's servers
http:\\mendeley.com easily add citations to your locally stored or server-stored library

There are lots of others people like, including some that cost money.

Equipment

^{SJK 00:25, 15 September 2009 (EDT)}

00:25, 15 September 2009 (EDT)
Good job including exact model numbers. Plus, I love the photo! Funny, but more than that, photos are a great addition to lab notebooks in terms of conveying a whole slew of information easily.

Tektronix TDS 1002 Oscilloscope
Wavetek 180 Function Generator
BNC cable

Setup

We plugged a BNC cable from the Channel 1 port on the oscilloscope to the Lo Output on our function generator. After playing with the settings for a little bit to make sure both were working correctly, we started the lab.

Basic waveform measurement

We set the function generator to 20 Hz which was easily observed on the oscilloscope.Using the lines on the screen, we estimated the signal had an amplitude of 280 mV. We then used to the cursor to get the more precise measurement of 284 mV. We also used the measure function and got the same value of 284 mV. We then followed this same procedure for several other waves:

Data:

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Triggering

^{SJK 00:27, 15 September 2009 (EDT)}

reference- http://en.wikipedia.org/wiki/Oscilloscope#Triggered_sweeps

Triggering is the way in which an oscilloscope figures out how to correctly display a signal. The one we worked with had 3 modes, but they essentially follow the same basic principals. A trigger works by sending pulses in response to some type of stimulus from the signal. The oscilloscope then measures this time scale and syncs it with the display. The three triggers we had were:

Edge Trigger- This trigger causes a pulse every time the voltage hits a specified threshold. This can be set to trigger for rising voltages or falling voltages.
Video Trigger- This trigger extracts pulse information from a video feed (such as a PAL or NTSC format).
Pulse Trigger- This trigger allows the user to specify a pulse width, causing the trigger's pulses to synchronize with the signal's pulses.

AC Coupling

AC coupling takes a signal and removes the DC component by using a capacitor. This method is useful for looking at the AC portion of a signal that may have a large bias voltage. It is, however problematic when used on a signal containing a wide range of harmonics, or very low frequencies. When such is the case, the signal gets distorted.^[1] This distortion has a useful purpose, however. Like everything in the real world, the oscilloscope has some resistance and capacitance (especially as a capacitor is used in AC coupling). These quantities are multipled together to get the RC constant, which has many uses. To find the RC constant, we supplied a square wave with a large DC offset to the oscilloscope and turned on the AC coupling. We got a signal similar to one shown:

We then measured the fall time, or time it takes for the voltage to go from the peak to 10%, to be 56.0 ms. Since a capacitor charges and discharges at the same rate, the fall time is equivalent with the rise time. Using the formula:2

[math]\displaystyle{ RC \cong \frac{t_{r}}{2.197} }[/math]

where [math]\displaystyle{ t_{r} }[/math] is the rise time we calculated the RC constant to be 25.5 ms.

This can be compared to the rise time listed in the oscilloscope's manual of <5.8 ns, which I found on Kyle Martin's lab page (Thanks Kyle!). ^{SJK 00:30, 15 September 2009 (EDT)}

00:30, 15 September 2009 (EDT)
Very nice citation of Kyle's page! I think, however, that the 5 ns is referring to a different rise/fall time. Most likely having to do with the low-pass bandwidth limit of the scope?

↑ http://www.allaboutcircuits.com/vol_3/chpt_4/11.html

1 ref: http://www.allaboutcircuits.com/vol_3/chpt_4/11.html

2 ref: from wikipedia: http://en.wikipedia.org/wiki/Rise_time

FFT

Though I didn't have time to get to the FFT part of the part, I can still explain what it does. A FFT or Fast Fourier Transform is a method for taking the fourier transform of a signal. It basically maps a signal in the time domain to the frequency domain, breaking signals containing many frequencies into delta functions whose height corresponds to the amplitude of a particular frequency. Since the signals we put into the oscilloscope were coming from a function generator, I would expect to only see a single delta function when switching to FFT mode if we were putting in a sinusoidal signal. Sawtooth would be multiple frequencies, although there would be one dominant one, and square waves would have yet even more frequencies.