1 Waveform Generators and Oscilloscopes Lab 42 Equipment List • WFG • TEK DPO 4032A • Resistors: 10kΩ, 1kΩ • Capacitors: 0.01uF3 Waveform Generators (WFG) • The WFG supplies a variety of time-varying 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 sheet4 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 voltage5 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” softkey6 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 Vpp7 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 1Vpp signal with a frequency of 2kHz “2000Hz”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 around9 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 applied10 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?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 menus12 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 channel13 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 dials14 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?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 oscilloscope16 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 output17 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 + – External Device I Wave Function Generator18 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 oscilloscope19 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 experiments20 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)21 Taking Measurements • Build a simple voltage divider as shown: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