ELEC 241 Lab

Experiment 6.3

Digital Instruments

The spectrum analyzer we used in the previous lab is an example of a PC based, digital instrument. Analog spectrum analyzers are available, but they are quite expensive (costing considerably more than the entire Lab PC setup) and only perform a single function. If we restrict ourselves to signals which satisfy the Nyquist criterion for the maximum sampling rate and which are large compared to the quantization step of the data acquisition card, we can use Labview to create a whole collection of software lab instruments.

Part 1: PC Function Generator and Oscilloscope

Let's try to duplicate two of our conventional instruments: the function generator and the scope.


Step 1:

 You should still have D/A output 0 connected to CH2 of the scope. If not, reconnect it.

Step 2:

 Load and start the Labview program "Function Generator".

Step 3:

 Note that the waveform appears to be severely quantized. This is because the generator only produces frequencies which are integer multiples of the buffer size, hence the sine function takes on the same value for the same sample of each cycle.

To see this, switch to a lower frequency range (e.g. 100) and see that the number of steps per cycle increases. Also note that there is a significant lag between operating the control and the corresponding change in the output.

Step 4:

 Experiment with the various controls and see how the output responds. Note any limitations on the waveform produced.

Step 5:

 Stop and exit the function generator program.

Step 6:

 Connect the (old-fashioned, analog) function generator output to the third ($v_{sound}$ ) input to the mixer. Unplug the dynamic microphone from J1-6. Verify that the function generator signal appears at the mixer output (and A/D input 4).

Here's what you should have:

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Step 7:

 Load and start the Labview program "Oscilloscope". Experiment with the various controls and compare its behavior to the analog scope.

Step 8:

 Reduce the function generator AMPLITUDE to zero and plug the dynamic microphone back into J1-6. Adjust the PC Oscilloscope controls to give a good display of speech waveforms. Try turning the Acquire ON button to OFF and using the Acquire ONCE button to get "single shot" waveforms.

Question 7:

 Compare the capabilities, advantages, and disadvantages of the PC function generator and scope with their analog counterparts.