# Physics307L F07:People/Mondragon/Notebook/070829

see comment

Steven J. Koch 01:45, 5 September 2007 (EDT):Excellent lab notebook!

## Purpose

The purpose of this exercise is to become familiar with use of an oscilloscope.

## Equipment used

see comment

Steven J. Koch 01:38, 5 September 2007 (EDT):It is great that you recorded the model numbers and details!
• Tektronix TDS 1012 (UNM OPTICS LAB 001)
• Heathkit ET-1000 Circuit Design Trainer
• Oscilloscope probe (E Z Hook Arcadia CA. RG-58C/U b7y Belden 70903 JV)

## Goal 1

Display a ~200 Hz sine wave on the oscilloscope

### Set up

see comment

Steven J. Koch 01:40, 5 September 2007 (EDT):Thank you once again for being willing to use the Heathkit instead of the function generator!

Because I was using the Heathkit Circuit Designer instead of a function generator, I had to use a non-standard set up. The Heathkit does have a built-in function generator, but due to its outputs I had to use an oscilloscope probe instead of a standard BNC cable. I hooked the ground terminal of the probe to the Heathkit's ground and the other terminal to the the Heathkit's function generator output. I adjusted the function generator's frequency and frequency multiplier knobs to output a sine wave of about 200Hz.

#### Result

The oscilloscope was already set up in a way that would display the sine wave coming from the generator. Its horizontal scaling was set up to be 10 ms/div and the vertical was 500mV/div. The trigger was set to be a downslope at -180 mV. 15 peaks were visible on the screen.

As requested by Koch, I made sure the input and the trigger were on DC coupling. They both were on AC, so I switched them.

The display was kind of cluttered do to the timebase, so I adjusted the time/div to 2.50ms

### Measurements

see comment

Steven J. Koch 01:41, 5 September 2007 (EDT): You do an excellent job of reporting all measurements with error bars! This is a great habit to be in for the rest of your research career. We will discuss more as the semester progresses on how to report errors for various kinds of measurements, and you are off to an excellent start.

The Heathkit does not appear to have a voltage control for its function generator. So, what is its peak to peak voltage output?

• Using the grid on the screen, I measure the peak to peak voltage to be 2.75 +/- 0.05 V
• Using the cursors, I measure the peak to peak voltage to be 2.66 +/- 0.04 V
• Using the O-scope's measure function, I measure the peak to peak voltage to be 2.74 +/- 0.04 V

### Performance with different wave forms and amplitudes

#### Triangle wave

I adjusted the Heathkit to output a triangle wave with the same frequency. The peak to peak voltage appears to be bigger

##### Measurements
• Using the grid on the screen, I measure the peak to peak voltage to be 3.25 +/- 0.03 V
• Using the cursors, I measure the peak to peak voltage to be 3.19 +/- 0.09 V
• Using the O-scope's measure function, I measure the peak to peak voltage to be 3.23 +/- 0.01 V

#### Square wave

I changed the O-scope probe to measure output from the Heathkit's square wave output. I had to change the volts/div to 650mV because the peak to peak voltage was much bigger. I had to adjust the trigger level to 2.06V because the square wave had a DC offset.

##### Measurements
• Using the grid on the screen, I measure the peak to peak voltage to be 4.25 +/- 0.04 V
• Using the cursors, I measure the peak to peak voltage to be 4.26 +/- 0.05 V
• Using the O-scope's measure function, I measure the peak to peak voltage to be 4.32 +/- 0.01 V

#### Sine wave with lower amplitude

I changed the set up so that the function generator fed its signal through the Heathkit's 100K potentiometer, though the oscilloscope probe, then to ground. I returned the volts/div setting to 500mV and the trigger to -180mV.

##### Measurements
• Using the grid on the screen, I measure the peak to peak voltage to be 2.50 +/- 0.04 V
• Using the cursors, I measure the peak to peak voltage to be 2.50 +/- 0.05 V
• Using the O-scope's measure function, I measure the peak to peak voltage to be 2.54 +/- 0.01 V

### AC Coupling fall time

see comment

Steven J. Koch 01:44, 5 September 2007 (EDT):Great work here and very good use of the "math" wiki editing. Most people were getting about 50 ms instead of 32 ms, so I think your measurements may be off a bit. The reason shouldn't be because of using the heathkit (the time constant only depends on the o-scope), so I am not sure what the problem is.

To measure the O-scope's AC coupling fall time, I fed a low frequency square wave to the oscilloscope an set the input channels input mode into AC coupling. Due to the way AC coupling mode works, instead of a square wave the O-scope will measure a sharp spike in voltage corresponding to the leading edge of the square wave, followed by an exponential decline.
The voltage will fall of according to the formula
$\displaystyle{ V(t)=V_o e^{-t/t_f} }$
where $\displaystyle{ t\,\! }$ is the elapsed time after the voltage peaks at $\displaystyle{ V_o\,\! }$ and $\displaystyle{ t_f\,\! }$ is the oscilloscope's characteristic fall time.
If the voltage has decayed to $\displaystyle{ \frac{1}{10} V_o }$ at time $\displaystyle{ t=t_d\,\! }$, then the fall time is given by the formula $\displaystyle{ t_f=-\cfrac{t_d}{\ln \tfrac{1}{10}}\approx\cfrac{t_d}{2.3} }$
Using the cursors, I measured $\displaystyle{ t=t_d\,\! }$ to be around 32ms, so the oscilloscope's fall time was $\displaystyle{ \approx 14\mbox{ms} }$