ELEC 242 Lab

Experiment 3.2

PC Instruments

Equipment and Components

Labview Instruments

Much of what we've done in lab so far involves setting a voltage (or current, or frequency) to a particular value, measuring another voltage (or current, or frequency), writing it down, repeating the process, and when it's all done, plotting the result. The data acquisition (DAQ) card in the Lab PC can generate and measure voltages (but not currents), and if we could program it to make and record our measurements, we could speed up many of our experiments.

The DAQ card is interfaced to a program called "Labview" which allows us to take a signal, convert it to a sequence of samples, perform mathematical operations on them, and display the results on the PC screen. We can also generate samples of a function by computing their values and convert them to voltages to form an output signal.

Well, this is just what our lab instruments do, only with continuous functions rather than samples. This difference becomes significant when the frequency of the sampling is less than twice the bandwidth of the signal. In this lab we'll be dealing with DC and low frequency signals, so we'll quietly ignore this difference.

Part 1: The Lab PC as a DMM



Step 1:

 Connect the 50 pin ribbon cable from the DAQ card to P12 on the back of the interface board. Be sure that it goes all the way in so that the ejector latches snap into place.

Step 2:

 Disconnect the BNC patch cable from the function generator and reconnect it to the 0-6 V power supply.

Step 3:

 Adjust to give a value of of 7 V.

Step 4:

 The input to the PC DMM is connected to A/D input 0 on the DAQ card. This in turn is connected to pin 1 on the bottom interface board socket connector.

Strip a piece of wire about 6 inches long to use as a probe. Plug one end of it into pin 1 of the interface board socket connector. Connect the other end of it to the collector of the transistor ( ) in your circuit.

Step 5:

 Load the "DMM" program from the Start menu by following the path Programs -> Labview -> 242 -> DMM. It should look like this:


Step 6:

 Press the DC Volts button on the panel.

Step 7:

 Start the instrument by selecting Run from the Operate menu, or by pressing the run button (the small arrow just below the menu bar), or by pressing CTRL-R on the keyboard with the cursor over the panel. The output should display the voltage of the signal (about 7 V).

Step 8:

 Use the PC DMM to measure the other voltages in the circuit ( , , and ). Do they agree with the voltages you measured with the DMM (for = 7 V)?

Step 9:

 Stop the DMM program by pressing the red STOP button.

Part 2: Multivoltmeter

Since the DAQ card has 8 input channels, it can measure up to 8 voltages at once. In particular, it can measure all four voltages in our circuit simultaneously. Since the PC can also perform calculation on the measurements, it can also compute , , and for us.


Step 1:

 Connect A/D channels 0-3 to your circuit as shown in the following diagram. The numbers on the connector symbols ( ) are the pin numbers on the interface connector socket strip (P11).


Step 2:

 Also add the 1 nF capacitor shown in the diagram. This capacitor is a bit of magic to avoid anomolous behavior.

Step 3:

 Load the "CE Voltmeter" program from the Start menu by following the path Programs -> Labview -> 242 -> CE Voltmeter. It should look like this:


Step 4:

 Start the program by pressing the Run button or CTRL-R.

Step 5:

 If necessary, adjust so that is 7 V. Do the other voltages match your previous measurements?

Step 6:

 The program also calculates the base and collector currents and beta. Do these agree with your earlier values?

Step 7:

 Repeat the transfer characteristic measurements you made in Part 2 of Experiment 3.1. Plot vs. and compare with your previous curve.

Step 8:

 Stop the program by pressing the red STOP button.

Part 3: Automatic Curve Tracer

Since the PC can generate voltages, measure voltages, and make calculations and plots, why not let it do the whole job for us?


Step 1:

 Disconnect from the 0-6 V supply and connect it to the D/A output channel 0 (pin 10 on the interface board socket strip). Also disconnect the wires to A/D Channels 1 and 3. Here's the circuit:


Step 2:

 Load the "CE Curve" program from the Start menu by following the path Programs -> Labview -> 242 -> CE Curve.

Step 3:

 Start the program by pressing the Run button or CTRL-R. The value displayed for "Vin" on the panel should begin to increase in 0.1 V steps. You should be able to verify this from the scope. The program also displays the corresponding value of "Vout".

Step 4:

 After 25 steps, the program will display the plot of vs. and stop. Make a printout of this plot by selecting "Print Window..." from the "File" menu. Include this plot in your lab notebook. How does it compare with your manual plot?

Step 5:

 Close the various PC Instruments by clicking the "X" box in the upper right hand corner of each window. Answer "No" if the question "Save changes to xxx.vi" appears. Disconnect the ribbon cable from the interface board by using the ejector levers. Do not try to remove it by pulling on the cable.