Sampling and Time Division Multiplex
Sampling and Time Division Multiplex
Objectives

To understand the concept of sampling a continuous analogue waveform

To investigate sampling a waveform using an analogue to digital converter

To investigate the effects of sampling rate and to understand the concept of aliasing

To appreciate the Nyquist limit applied to sampling rate

To investigate time division multiplexing of signals
Preparation:
login the PC with username: lab and password: lab
Switch on the “Feedback Rat 92-200” box.
Double-clicking the “Modulation and Coding...” icon:
Click on the “Sampling and Time Division Multiplex” block
Click on Practical 1 on the right panel.
Practical 1: Sampling Analogue Signals
In this practical you will investigate the effects of sampling a continuous analogue signal
using an analogue to digital converter.
Procedure
1. Click Make Connections, follow it to complete all connections on the board.
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2. Identify the Micro Controller and A/D – D/A circuit block, located towards the top,
left-hand corner of the board.
3. Set the A/D 1 Offset, the A/D 1 Amplitude and the D/A 1 Offset to mid position.
4. Set the Function Generator to Slow. Set the Signal Level Control for maximum
output.
5. Open the frequency counter
and set the Frequency (in the Function
Generator block) to approximately 400 Hz. Close the frequency counter and open the
oscilloscope. In the Function Generator block, use the waveform selector to select a
sine wave output.
6. Increase the size of the oscilloscope so you can see the waveforms more easily.
Change the resolution from 8 bit, 4 bit to 2 bit and note the outputsignal (blue) and
input signal (yellow). Explain your observations.
7. Try the different resolutions and also adjust the amplitude of the signal using the
Signal Level Control. Explain your observations.
8. With 2 bit resolution, change from sine to triangle waveform. Is there any difference
on the output signal?.
9. Return to a sine wave and select 8 bit resolution and maximum amplitude. Now
increase the frequency of the function generator gradually. You will need to increase
the timebase speed on the oscilloscope so you can see only a cycle or two to see what
is happening. Explain your observations.
Practical 2: Aliasing
In this practical you will investigate the effect of sampling an analogue signal at sample rates
near to and below its frequency.
Procedures
1. Click Make Connections, follow it to complete all connections on the board. In this
Practical you will only be using 8 bit resolution.
2. Set the A/D 1 Amplitude, the A/D 1 Offset and the D/A 1 Offset to mid position.
3. Set the Function Generator to Fast. Set the Signal Level Control for maximum
output.
4. Open the frequency counter and set the Frequency to approximately 2 kHz.
5. Open the oscilloscope. In the Function Generator block, select a sine wave.
6. Record and comment on your observations.
7. Increase the signal frequency and note the changes in the output signal. Note what’s
happening when the input signal frequency is near to the 10 kHz. Explain your
observations.
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8. Set the frequency to about 9.5 kHz. Move the frequency counter probe to the output of
the D/A Converter (monitor point 2). Now, slowly raise the frequency. Record the
input signal frequency and output signal frequency. Explain your observations.
9. Set the input signal frequency to approximately 15 kHz. Note the result on the
oscilloscope. Move the counter back to the sampled output signal and measure the
frequency. How do you think it is related to the input frequency?
10. Increase the signal frequency above the sampling frequency (20 kHz) and investigate
the effects.
Practical 3: Time Division Multiplex
In this practical you will investigate time division multiplex using two A/D converters and a
single D/A converter.
Two analogue signals: one a sinusoid and the other a variable dc voltage are fed into the two
a/d converters. The microprocessor samples the two alternatively at 20 kHz. The multiplexed
signal is passed to a D/A and you can see it on the oscilloscope.
Procedures
1. Click Make Connections, follow it to complete all connections on the board.
2. Set the A/D 1 Amplitude and A/D 2 Amplitude to maximum. Set the A/D 1 Offset,
the A/D 2 Offset and the D/A 1 Offset to mid position.
3. Set the Function Generator to Slow. Set the Signal Level Control for maximum
output.
4. Open the frequency Counter and set the Function Generator Frequency to
approximately 1 kHz.
5. Open the voltmeter and use it to set the variable dc Source to approximately zero.
6. Open the oscilloscope. On the Function Generator block, select a sine wave output.
7. Note the signal on the upper trace, containing samples of the sine wave alternating
above and below the dc voltage. Adjust the dc Source voltage and note the changes.
8. Increase the frequency of the sine wave and find the Nyquist limit.
References:
Feedback software help files.
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