Rev 1.0, Jan 2004

ECE 2006 LABORATORY 3

DIGITAL OSCILLOSCOPE SWEEP CONTROLS

PRE-LAB CALCULATIONS

Complete Table 2, Theoretical Frequency Response Calculations, prior to performing lab experiments. See “RC Circuit Equations” below for help.

OBJECTIVES

The learning objectives in this laboratory give the student the ability to:

  • Use the oscilloscope internal sweep generator.
  • Use the oscilloscope trigger level and slope controls.
  • Use the oscilloscope to display the difference of two signals.
  • Use the oscilloscope for frequency, phase angle and voltage measurements.

REFERENCES

TDS 3012 B Series User Manual

Background

Oscilloscope time mode:

In the last lab you learned how to use the oscilloscope in X-Y mode. In this lab you will use the oscilloscope in time-based. In time (or sweep) mode the oscilloscope samples the voltage levels of the input channels over time. The voltage on the input channels causes the display to deflect vertically and the horizontal axis represents the time domain. You can adjust the seconds per division to display more or less of the waveform and you can adjust the vertical sensitivity to change the vertical volts per division.

Oscilloscope math operations:

This generation of oscilloscopes allows you to do mathematical operations on the input waveforms. You can then display the results of these mathematical operations as another input waveform. This allows you to observe waveforms that you could not measure directly.

Oscilloscope trigger:

The oscilloscope is used to observe repetitive signals. If the screen is redrawn with the display signal in a different part of its cycle each time the oscilloscope screen will show a drifting wave shape. A trigger is used to catch the displayed signal in the same location every time the screen is redrawn. The trigger controls include source selection, level selection, slope selection, and time delay. The source can be set to channel 1, channel 2, line (60 Hz power line), or external. The level control adjusts the voltage level of the source at which the waveform will be drawn from the center of the screen. The slope switch determines if the trigger will on the signals positive slope or the negative slope. It is easy to see where the oscilloscope will trigger the waveform by looking at the trigger level indicators on the screen.

Oscilloscope phase angle measurements:

Phase angle measurements include an angle and a sign. The phase angle can be measured manually by reading off the screen or automatically by the oscilloscope. A phase angle difference includes a sign as well as a magnitude. If Vr occurs, say 30o, earlier in time than Vs, then Vr leads Vs by 30o or Vs lags Vr by 30o, and the phase angle difference is Vr - Vs = +30o. If Vc occurs, say 30o, later in time than Vs, then Vs leads Vc by 30o or Vc lags Vs by 30o, and the phase angle difference is Vc- Vs = -30o.

RC CIRCUIT EQUATIONS

For the RC circuit in Figure 1, the phase difference between Vr and Vs is

Vr - Vs = 90 o-tan-1(CR)

where

C is the capacitance in Farads, and  = 2f, 0 o tan-1(CR)  90 o

and the phase difference between Vc and Vs is

Vc- Vs = -tan-1(CR)

For the RC circuit in Figure 1, the magnitude ratio Vr/Vscan be found by using the voltage divider equation given in class, Vr = Vs * R1 / (R1+R2). In this case, R1 is the value of the resistor in the circuit, R. R2 is the impedance of the capacitor, (jωC), which is dependent on the fruequency of excitation. The voltage divider can be algebraically re-written to give the following result:

Likewise, the magnitude ratio Vc/Vs can be written as:

EQUIPMENT

Digital Oscilloscope TDS 3000 B Series

Function Generator

Resistor

Capacitor

PROCEDURE

This lab deals with the time based mode of the oscilloscope. Last lab you learned how to use the X, Y axes mode on the oscilloscope. Now both channel 1 and channel 2 will represent vertical voltage deflections versus time.

Initial Set-up

  • Set the oscilloscope to time mode:
  • (Display > X Y Display > Off (YT))
  • Turn on channel 1.
  • Set the channel 1 to ground reference and set level to center graticule line.
  • (Ch1 > Coupling > GND)
  • Set channel 1 back to DC mode.
  • (Ch1 > Coupling > DC)

1. Trigger

1.1Untriggered

  • Connect the function generator to channel 1.
  • Set the function generator to 3 Vpp, Square Wave, 100 Hz.
  • Set the triggering mode of the oscilloscope to external.
  • (Trigger Menu > Source > Ext
  • Set sweep speed to 4mS/div.
  • (Turn Horizontal scale knob)
  • Have the lab instructor observe this response.

Lab Instructor ______

1.2Triggering Automatic

  • Now adjust the trigger to channel 1
  • (Trigger Menu > Source > Ch1)
  • Press the (Set to 50%) button

This will automatically set the trigger level. Notice how the waveform stops drifting. The scope is now centering the waveform at the same point every time.

  • Have the lab instructor observe this response and save to disk.

Lab Instructor ______

1.3Triggering Manual Levels

Observe the change in the triggering level on the center vertical graticule line.

  • Adjust the function generator to a 6 volt peak to peak 100 Hz sinusoidal wave.
  • Set the trigger to channel 1.
  • Set the trigger slope to trigger on the positive (rising) slope.
  • (Trigger Menu > Slope > Rising Edge )
  • Adjust the trigger level to 0 volts.
  • (Turn trigger level knob)
  • Adjust the trigger level to +1 volts.
  • Adjust the trigger level to +2 volts.
  • Adjust the trigger level to +4 volts.
  • Set the trigger slope to trigger on the negative (falling) slope.
  • (Trigger Menu > Slope > Falling Edge )
  • Adjust the trigger voltage to +1 volts.
  • Capture this output to disk.(SCREENSHOT)
  • Adjust the trigger voltage to +2 volts.
  • Adjust the trigger voltage to +4 volts.
  • Have the lab instructor observe this result.

Lab Instructor ______

2.Frequency Measurement

2.1Set the function generator to 150 Hz sinusoidal wave

2.2Period Measurement (Manaul)

  • Adjust the horizontal scale to obtain approximately one period on the oscilloscope screen.
  • Count the number of divisions (vertical graticules) for one period.
  • Multiply this count by the number of seconds per division. Number sec/div is given as M__#__ on the screen. The product is the period. Record in Table 1.
  • Your frequency is 1/T. Record in Table 1.

2.3Period Measurement (Automatic)

  • (Ch 1 > Measure > Select Measurements for Ch 1 > Period)
  • (Ch 1 > Measure > Select Measurements for Ch 1 > Frequency)

Note: You may need to hit “-more-” if these options are not shown on the right hand side of the display.

  • Record the automatic measurements shown on the screen in Table 1.
  • Save the output to disk. (SCREENSHOT)
  • Find the percent error of your period measurement versus the automatic measurement and record in Table 1.

2.4Repeat steps 2.2 and 2.3 for frequencies 400 Hz, 900 Hz, and 2400 Hz.

TABLE 1

PERIOD MEASUREMENT ANDFREQUENCY CALCULATION

Generator
Frequency
Hz / Manual Measurements / Automatic Scope Measurements / % Error
Period
msec / Calculated
Frequency
Hz / Period
msec / Frequency
Hz / Error in period measurement
150
400
900
2400

3.Frequency Response

3.1Theoretical

  • Calculate the phase angle and magnitude ratio for each of the frequencies in Table 2. Record your results in Table 2.

TABLE 2

RC CIRCUIT FREQUENCY RESPONSE (THEORETICAL)

Frequency / Magnitude / Phase angle
Hz / Ratio / Degrees
f / Vr/Vs / Vc/Vs / Vr-Vs / Vc-Vs
200
350
500
700
1000
1500
2000

3.2Measurement Setup

  • Connect the circuit in Figure 1
  • Adjust the function generator for a

sinusoidal waveform with a magnitude

of about 3 volts peak to neutral.

  • Display both channel 1 and channel 2.
  • Set triggering to channel 1 50%.
  • Ground both channels to zero divisions
  • Set both channels to DC.

3.3Display Vc

Vc = Ch 1 – Ch 2

  • Hit the (Math) button
  • Set 1st source to channel 1.
  • Set 2nd source to channel 2.
  • Set operator to minus. Now math operation is shown as red on the oscilloscope screen and can be treated like channel 1 & 2 waveforms.

3.4Magnitude Measurements

For each frequency in Table 3 measure Vs, Vr, and Vc.

  • Manually measure the voltage magnitudes for 200 Hz and record in Table 3.
  • Use the automatic measurements to measure magnitude.
  • (Measure > Ch 1 > Max for channel 1)
  • Repeat for Ch 2 and Math
  • Record the values shown on the screen to Table 4. (Save display for 200 Hz.)
  • Repeat automatic measurements for each frequency.
  • Have the lab instructor verify your technique

Lab Instructor ______

3.5Phase Measurement (Manual)

At 200 Hz you will measure the phase angle differences Vr-Vs and Vc-Vs.

  • Find the number of degrees per division
  • Find the number of divisions between the same points of the two waveforms.
  • Phase difference is (deg/div )* (# of divisions).
  • For the sign, remember the leading waveform peaks first. (Leading wave – trailing wave is positive sign; otherwise, negative).
  • Record measurements in Table 3
  • Have the lab instructor verify your technique

Lab Instructor ______

TABLE 3

RC CIRCUIT FREQUENCY RESPONSE (MANUAL DATA)

Frequency / Magnitude / Phase angle
Hz / Volts / Ratio / Degrees
f / Vs / Vr / Vc / Vr/Vs / Vc/Vs / Vr-Vs / Vc-Vs
200

3.6Phase Measurment (Automatic)

Press button of waveform to measure to (Ch 1, Ch 2, or Math).

  • Select Phase measurement from measurement menu.
  • Press button of waveform to measure from (Ch 1, Ch 2, or Math).
  • Select “OK Create Measurement” to show phase angle differences.
  • Repeat automatic measurement for each frequency in Table 4.
  • Save display to disk for 200 Hz.(SCREENSHOT)

TABLE 4

RC CIRCUIT FREQUENCY RESPONSE (AUTOMATIC DATA)

Frequency / Magnitude / Phase angle
Hz / Volts / ratio / Degrees
f / Vs / Vr / Vc / Vr/Vs / Vc/Vs / Vr-Vs / Vc-Vs
200
350
500
700
1000
1500
2000

CONCLUSIONS

Remember to save display to disk for every section that requires it and include screen shots in your lab report.