Waveform Generators and Oscilloscopes. Lab 6

Transcription

1 Waveform Generators and Oscilloscopes Lab 6 1

2 Equipment List WFG TEK DPO 4032A (or MDO3012) Resistors: 10kΩ, 1kΩ Capacitors: 0.01uF 2

3 Waveform Generators (WFG) The WFG supplies a variety of timevarying voltages at its output terminals There are two terminals on the WFG - sync and output Always use only the output terminal. The 202 class does not need the sync terminal - ever! Note the maximum voltage rating on your grading sheet 3

4 WFG Switch the WFG on Press Output button. Note: Make sure that you press Output button, every time switch the WFG on If Output button is off, status message Output Off is shown in the upper-right corner of the display It should automatically deliver a 100 millivolt peak-to-peak sine with a frequency of 1 khz Press the Freq softkey to see frequency Press the Ampl softkey to see the peak to peak voltage 4

5 Setting Parameters To change the frequency of the signal generated, first display the frequency by pressing the Freq softkey Enter the magnitude of the desired frequency, using numeric keypad In our case, enter the value 2000 softkey Select the desired units In our case, press Hz softkey 5

6 Setting Parameters To set the peak to peak voltage press Ampl softkey Enter the magnitude of the desired amplitude, using numeric keypad In our case, enter the value 1 softkey Select the desired units In our case, press Vpp softkey V pp 6

7 Setting Parameters Note the other units in amplitude and frequency You could use the knob to change values of the amplitude or frequency, too You could also use Graph button, to view a graphical representation of the waveform parameters To turn off the Graph Mode, press Graph button again Combined with the previous settings, you should be generating a 1V pp signal with a frequency of 2kHz 2000Hz 7

8 Old vs. New The new scopes have LCD screens whereas the old ones had CRTs CRT stands for cathode ray tubes The new scopes are much more accurate and have much higher resolution Sometimes, too sensitive to input noise Built in statistical measurements can jump around 8

9 Oscilloscope The oscilloscope has 3 input terminals - Channel 1, Channel 2 and Trigger Use only Channel 1 and Channel 2, NOT Auxiliary input Check the front of the scope just above the terminals for the specifications Record the input impedance Record the maximum input voltages that can be applied 9

10 Oscilloscope Resolution Resolution is the ability to tell two points apart What is the minimum voltage and time that can be displayed by the oscilloscope? Is it the same as resolution? 10

11 Oscilloscope Connect the WFG (Wave Function Generator) output up to the input of channel 1 of the scope Use the adapters - one on the WFG and one on the scope Red terminal is positive, black is negative Remember - black terminal ALWAYS goes on the ground side of the circuit Push the Autoset button Push the Menu off button to get rid of the menus 11

12 Reading the Oscilloscope Display Each division is one FULL box The horizontal axis is time. The display will tell you how much time is represented by 1 division of the screen. This is done along the bottom of the display just right of centre The vertical axis is voltage. The volts per division is displayed in the bottom left corner for each channel 12

13 Changing Scope Display To manipulate the display of the signal on the screen use the Scale knob for the Horizontal axis and the Scale dial associated with that particular channel Note what happens to the scale and the corresponding display of the signal when you turn either of the dials 13

14 Reading the Scope Display With each change you make to the display, make sure you understand how to measure the time period and the peak to peak voltage Does it match the display on the WFG? 14

15 Information mismatch Compare the sine wave voltage and period from the oscilloscope display with the sine wave voltage and frequency displayed on the wave function generator The chip inside the WFG made an assumption about the devices connected to it - which is a very poor model for the oscilloscope 15

16 Computer Controlled Equipment In the case of the WFG for example, many different signals can be set using the keypad The WFG display displays is driven by the chip that interprets the keypad inputs The internal computer then adjusts internal voltages and currents to levels corresponding to the desired output 16

17 Circuit Model The value of I depends on the external device Therefore, the chip inside the WFG must use an appropriate model for the external device to relate the value of I to the desired output + I External Device Wave Function Generator 17

18 Mismatch explained The chip in the WFG assumed that it would be connected to a 50Ω device However, the input impedance of the oscilloscope is 1MΩ This erroneous model used by the WFG, causes the discrepancy between the WFG display and the oscilloscope 18

19 Matching the Equipment Press Utility button Navigate the menu to set the output termination Press Output Setup softkey Press Load softkey to choose High Z Press Done softkey Notice the status message High Z Load is shown in the upper-right corner of the display Note that you could select any other desired load impedance, but we will stick to High Z for all EEE202 experiments 19

20 Using Oscilloscope for measurements Always use the oscilloscope in high-resolution mode. Acquire Mode Hi-Res Use the oscilloscope to display various statistics about the signal such as peak to peak voltage, frequency, period etc. Press the Measure button on the oscilloscope On the bottom of the screen, press the Measurements softkey On the right side of the screen, cycle through the menu and choose Frequency Similarly, measure, time period, peak to peak voltage (use Amplitude on the oscilloscope) 20

21 Taking Measurements Build a simple voltage divider as shown: 21

22 Taking Measurements Connect the oscilloscope across the resistor with the black terminal of the adaptor always being connected on the ground side of the resistor Calculate the voltage across each resistor by reading the oscilloscope display Record the voltage drop across R 1 on your grading sheet 22

23 Measuring Current Oscilloscopes are not equipped to measure currents directly To measure current using the scope, first measure the voltage across a given resistance, and then calculate the current using Ohm s law What is the current through the 1k resistor? Record your answer on the grading sheet 23

24 Multiple Measurements Next, build this circuit on the breadboard (the source is the WFG): Set the WFG to supply a square waveform of 20kHz with a peak to peak voltage of 10V 24

25 Multiple Measurements Display the source voltage on Channel 1 Display the voltage across the capacitance on Channel 2 Record this display on graphing paper Explain your observations below it 25

26 The VPULSE Source period V2 t-rise pulse width t-fall V1 Time 26

27 Build this circuit Pulse Width Edit the attributes of PULSE as shown above. In LTspice, µ is represented by the letter u 27

28 VPulse Simulation In the Transient dialog box, set Final time to 50µs. Plot the source voltage, and the voltage across the capacitor 28

29 Filtering The previous circuit basically smoothed the input out Instead of having near instantaneous jumps, the rise to peak voltage is much more gradual This is also referred to as filtering, and will be dealt with later 29