User:Darrell Bonn/Notebook/307L Lab book/OScope

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Lab 1: Oscilloscope

Osciloscope Lab Summary

SJK 02:57, 17 September 2008 (EDT)
02:57, 17 September 2008 (EDT)
This is a good lab summary. I am still on the fence as to whether I'd strongly prefer it to be on a separate page, but that's not a big deal now as this works. Maybe will be more important for subsequent labs which will span multiple days. I think you do a good job of summarizing and importantly, you report your "final result" with meaningful uncertainty and explanation as to how you arose at that value. In your subsequent labs, your summaries will probably be longer, simply because there will be more to report and discuss.

I used an oscilloscope and a function generator to get some practical experience measuring signals with an oscilloscope. Using the function generator to supply sine, square and triangle waves, primarily at 500Hz with 6v amplitude, I measured these by eye, with the cursors and with the automatic measuring functions. Then I played with the trigger functions, going through available trigger settings to see the effects the triggers had on data display. Then I measured the time time constant of the oscilloscope by setting a low frequency (70Hz) with AC coupling and measuring the time it took the signal to fall to 10% of it's original signal. This drop off is the charging of the capacitor in the RC coupling circuit. I found this to be between 61.5 and 62ms. The time constant of the RC circuit is considered to be 2.197 times this fall off time (see [| rise time article]. Thus for this scope the time constant is 28.2ms (+/- about .3ms based on the observed variation in time measurement)

I learned the difference between AC and DC coupling both from a user standpoint and as to what the functional circuit within the scope is. This was something I'd run into in the past and had never actually understood.

The only improvement to this lab from my standpoint would be to put more emphasis on math functions and storing wave forms. These would need a few introductory comments on how to save and recall a wave form as punching buttons till you get it is a bit slow for the time alloted. SJK 02:45, 17 September 2008 (EDT)
02:45, 17 September 2008 (EDT)
Good idea. I am definitely going to expand this lab to two days (including the very first day of class) next year, as it seems uniformly true that students wanted more time to investigate things.

Data and Procedures

SJK 02:54, 17 September 2008 (EDT)
02:54, 17 September 2008 (EDT)
Excellent raw data notebook! You do a really great job of recording the important information. One of the tests of a great lab notebook would be the likelihood of someone else begin able to replicate your work a year from now. I think you're doing very well on that test. Great work!

SJK 02:47, 17 September 2008 (EDT)
02:47, 17 September 2008 (EDT)
Great job recording the details of the instrumentation! This is a pretty basic setup, but to be nit-picky, some description of how things were connected, cable type etc. couldn't hurt. A picture would be an easy way to describe it. But getting these model numbers down is a major part.

Instruments Used

  • BK Precision 4017A Function Generator
  • Tektronix TDS 1002 60MHz, 1GS/s Digtal O-Scope

Measuring Basic Waveform Data

Measuring sine wave in from the function generator 4017A Settings: 500 Hz, Level? No readout for output level is available. TDS1002: Channel 1 input, DC Coupling, 1V/Division, 1ms/division

Turned function generator output level full down, connected to the oscope Slowly increased output till I found a good sine wave. Played with coupling a bit - AC/DC until a good display was available either way

Measuring by mark 1 eyeball Able to clearly see 5 cycles taking the expected .5ms each Power level: 6v peak to trough

Measuring with cursors Voltage is 6.04 peak to peak (cursor resolution is .04V). Period is 2ms (freq = 500Hz) (cursor resolution is .04ms). Measurement made across 4 cycles and across 1 cycle (no variation)

Measuring with automatic functions

Frequency ranges from 501 to 502Hz, (Period of 1.992 - 2.000ms) Peak to Peak 6.04V to 6.08V

Repeating measurements with a square wave - all other settings on function generator unchanged Changed oscope display to 250us resolution

By eye: Period is about 1.95 ms (line just off the 2ms line). Voltage is 6.1V By Cursor: 1.980ms, 505.1Hz, 6.28V peak to peak By Auto Measure: 503.8-504.3 Hz, 1.983 - 1.985ms, 6.28VPeak to Peak

repeat with saw tooth, no instrument changes By eye: Period is right at 2ms, 500Hz; Amplitude is 5.95V By Cursor: Period is 2ms, 500Hz; Amplitude is 5.92V By Auto Measure: Period is Frequency is 501.03-501.08Hz, Amplitude is 5.96V peak to peak

Triggering: Rising edge means that the trigger occurs as the signal passes the threshold on the way up Falling edge means it looks for the threshold going down With a constant signal this will shift the phase by 180 degrees.

AC/DC Coupling

From lecture: Input goes through a RC circuit AC coupling is across the resistor, DC coupling is across the capacitor.

Thus AC coupling is effectively through a high pass filter while DC coupling is through a low pass filter. SJK 02:50, 17 September 2008 (EDT)
02:50, 17 September 2008 (EDT)
My description was a simplification of things I don't fully understand, which I'm guessing you know, but don't want to mislead you. In DC case, it is effectively a low pass filter, and this would be due to the bandwidth limit of the oscilloscope. I think 60 MHz in our case. So, it wouldn't be the same capacitance as in the AC coupled like I drew it on the board.

Data: Applied a large (10V) signal to the scope AC coupling reveals ripples and uneven qualities in the waveform DC coupling removes the small changes showing a stable square wave Using DC coupling on a fast signal revealed the time constant of the RC coupling circuit

Measuring the time constant

Set function generator to 40Hz, square wave, amplitude about 8.6v all measured with DC coupling to confirm Switched to AC coupling. Measured the Peak of the waveform at 8.48V Measured the 3db drop off (4.24V) at 16ms Measured to the 90% drop off: .424V (.400 was limit of cursor value) at about 72ms

Using measure key I get 115.6ms (that's all the way to the end, not the 10% point) This was in error - the fall time measurement was innacurate and flagged so with a question mark

Adjusting the frequency up to 70 Hz allowed for a fall time that was right at 90%. This measured a fall time of 61.5-62msSJK 03:01, 17 September 2008 (EDT)
03:01, 17 September 2008 (EDT)
Your numbers are about 10 ms higher than most of the other students. Could possibly be the o-scope, but I'm wondering if it's a quirk of your method for measuring it. Not a big deal though, just wanted to point that out.

From wikipedia page [| rise time] rise time (or fall time in this instant, they are symetric in this situation) rise time = approximately 2.197 tau tau = RC so rise time of 62ms indicates an RC time constant of approximately 28.2

Math Functions

SJK 02:52, 17 September 2008 (EDT)
02:52, 17 September 2008 (EDT)
Excellent way of testing out the FFT feature and reporting some meaningful measurements

Used the FFT to check signal purity on the function generator sine wave. Primary signal is stable, 2'nd frequency is down 32dB, most noise below 45dB