Lab 13

Lab 13 - Analog to Digital Conversion and Aliasing

Objective:

To give the student experiment in the generation and recording of Analog signals using a Data Acquisition Board.

Procedure:

Testing the DAQ board. - If you have not already done Exercise 13, you should do it now to make sure that the DAQ board is working.
Aliasing

Using a fixed output frequency (1KHz) and amplitude (2Volts) on the function generator, create a VI which digitizes analog input channel 0. The VI should contain the following:

An x-y plot of the digitized waveform (Voltage versus time).
A fourier transform spectum of the digitized waveform. (voltage amplitude versus frequency)
The time axis of the x-y plot should be fixed at 0 to 10ms. This corresponds to 10 cycles of a 1KHz sine wave.
The frequency axis of the Fourier spectrum should be from 0 to 5KHz.
The user should be able to change the digitizing rate of the input channel.
As the user changes the digitization rate, the size of the waveform buffer should also change such that 4ms of data is digitized. (eg. for a digitization rate of 100KHz [1 point every 10m s], the buffer size should be 1000 points. For a rate of 500Hz [1 point every 2ms], this corresponds to 5 points.

Once you have the VI constructed, use the VI to measure the 1KHz waveform at digitizing rates of 70KHz, 50KHz, 25KHz, 10KHz, 5KHz, 4KHz, 3KHz, 1500Hz, 800Hz, 400Hz. For each of the digitizing rates, CUT and PASTE the resulting plot x-y plot and Fourier Spectrum from LABVIEW into a word file to be included in your lab report.

Additional information and a copy of this lab is located at:

http://web.njit.edu/~gnita/VI/OPSE310/OPSE310lab_13.htm

Submit the created VI or VIs by attaching them to an e-mail document that contains the answers to the discussion questions below as well as the PLOTS from Labview as required in Part II.C. (Note that you can either attach the VIs or zip the VIs by using WinZIP). E-mail the answers to the discussion questions and the files to:

gnita@njit.edu

III. Discussion:

In a WORD file, paste the voltage versus time and voltage versus frequency plots.

For Part II.C, at which digitization rate do you start to see aliasing? WHY?

How does your answer to question 2 compare to what you would expect from the Nyquist theorem? Is there a discrepancy? WHY or WHY NOT?

Can this simple VI be used as the basis for a computer based oscilloscope? HINT: What if the input signal has only a 5mV maximum amplitude? Can it digitize a 10MHz signal? To answer this question, you will need technical details of the DAQ board. Look at the National Instruments web page (www.ni.com).

Do the phases of the digitized waveforms match? (eg. when digitizing at rates of 100KHz and 50KHz, are the phases of the digitized waveforms the same?) Do you expect them to?