Pulse Amplitude Modulation
Pulse Amplitude Modulation
Objectives
1-To gain familiarity with Pulse Amplitude Modulation (PAM) techniques
2-To evaluate a simple PAM modulator and a demodulator
3-To understand and evaluate the effects of aliasing distortion of sampled signals.
Reference: Refer to Chapter -7 in your textbook.
Introduction.
In this experiment, PAM modulation and demodulation along with the distortion due to aliasing are introduced. PAM is a type of modulation used in the transmission of digital signals. A test tone ( a sine wave) is PAM sampled at the carrier frequency’s rate. Thus, the number of samples per cycle is equal to the ratio of fs to ff Where fs is the sampling (carrier) frequency and fi is the information frequency.
It should be noted that since noise if of the additive form, the PAM is the least desirable type of pulse modulation. One way to solve this problem (as typically adopted) is that the pulses are converted to digital bits. This type of modulation is then referred to as PAM – PCM ( pulse amplitude modulation – Pulse Code Modulation / demodulation)
We will visit other types of pulse modulation techniques in future experiments.
To generate PAM, an information signal is multiplied by a pulse train. This will be apparent in the experiment that follows.
Experiment:
A- Basics
1- Design a simple PAM generator as shown. Set the values as explained below:
Modulator:
a- the square wave amplitude =1 and the offset =1/ The sampling information is as shown.
b- the information signal has an amplitude of 1.5 and an offset of 2V. The frequency is an INTEGER type that goes from 0 to 120 Hz. ( Knob)
The sampling information is as shown.
c- place two graph indicators ( sampled signal and intelligence) so that the time range is 0 to 0.2 seconds. Remove the auto scale on x- axis
d- multiply the two signals as shown and add a third graph indicator. Call it PAM. Likewise, the time range is from 0 to 0.2 seconds.
e- Organize the items on the front panel so that the information signal is on top, below it the square wave and below that the PAM. All have similar dimensions.
Demodulator
f- ad a low pass filter (express>Signal Analysis>Filter)
set the parameters as shown
Notice, the cut off frequency is set to near the frequency of the information signal.
g- Add a Spectrum analyzer. Express>Signal Analysis>Spectral and set its parameters as shown
Test:
a- On the front panel, set the intelligence frequency to 20 Hz. To get the exact value, then on the frequency knob, right click and select “digital display” then write 20 in the digital display.
b- How many samples per cycle do you see in the PAM signal? N = ? Does theory agree with this result? Show calculations.
c- place your plots here
d- in the spectrum you should see something like this. Explain it well
f- display the resulting filtered signal.
g- Does the shape of the demodulated signal sufficiently represent the original signal ?
h- While the VI is running, change the frequency of the intelligence signal to 40 Hz.
Write your observations on:
-Number of samples per cycle
-The spectrum
-The demodulated output (note, the output is still acceptable)
Aliasing:
a- While monitoring the spectrum (since this gives the clearest representation of the issue of aliasing) slowly increase the frequency if the intelligence signal until the spectrum of the baseband signal seems to overlap the neighboring spectrum. Here, aliasing just occurred.
Place an image of the spectrum, the PAM, and the output.
b- does the resulting demodulated signal represent the original signal.?
c- What is the frequency of the intelligence signal ?
d- Research for a definition of aliasing. Then, using your OWN words, explain it.
Place an image of the front panel and of the block diagram. All must be neat and well organized.
B- Audio Test and Implementation.
In this section, two steps will be taken.
1- Simple I/O audio test.
2- PAM modulation/demodulation
Note, due to RAM and resources, one may run into data overflow/underflow errors. This is not so much a programming issue as much as buffering error.
1- Simple audio I/O
1-Conenct the myDAQ to the Pc
2-build the following block diagram and front panel
3-set the parametres as hown (
4- On the block diagram, place a DAQ assistant to acquire a signal>Analog>Voltage then hold control key down and select both audio Left and audio right.
Set the voltage ranges and the sampling information for both channels as shown.
For the generation, place a DAQ assistant to generate audio left and audio right.
DAQ Assistant:
DAQ Assistant 2
The Spectral Measurements is as was done above (in the previous section)
5- Set the volume control mid-range.
Play an audio signal though myDAQ and listen to it on the ear piece. Verify results to instructor.
Once all is working fine, place your front panel and blo9ack diagram below
Again, verify operation to instructor.
2- PAM – Audio
This part may cause data overflow/underflow errors. Will deal with it as it comes up. Shut down any other items running on your PC.
Here, we will sample the audio signal and demodulate it.
1- Build the block diagram and front panel as shown. Information on the DAQ data and filter are provided below. Notice, the DAQ assistants are still set to stereo (audio Left and audio Right)
DAQ Assistant
DAQ Assistant 2
Filter
After connecting all, verify operation, watch the PAM and the resulting signal
Demonstrate a successful lab to instructor.
Submit on D2L