# Physics307L F08:People/Mondragon/Notebook/070829

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## Purpose

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

## Equipment used

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- 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

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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.

### Adjustments

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

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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

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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

[math]\displaystyle{ V(t)=V_o e^{-t/t_f} }[/math]

where [math]\displaystyle{ t\,\! }[/math] is the elapsed time after the voltage peaks at [math]\displaystyle{ V_o\,\! }[/math] and [math]\displaystyle{ t_f\,\! }[/math] is the oscilloscope's characteristic fall time.

If the voltage has decayed to [math]\displaystyle{ \frac{1}{10} V_o }[/math] at time [math]\displaystyle{ t=t_d\,\! }[/math], then the fall time is given by the formula [math]\displaystyle{ t_f=-\cfrac{t_d}{\ln \tfrac{1}{10}}\approx\cfrac{t_d}{2.3} }[/math]

Using the cursors, I measured [math]\displaystyle{ t=t_d\,\! }[/math] to be around 32ms, so the oscilloscope's fall time was [math]\displaystyle{ \approx 14\mbox{ms} }[/math]