DEPARTMENT OF ELECTRICAL AND CONTROL ENGINEERING
Objectives
1. To reconstruct the starting analog signal through filtering
2. To observe the aliasing
Instruments
• MCM30/EV Module
• Oscilloscope
Theory
The analog signal reconstruction starting from the samples is made using a low-pass filter. When the sampling frequency is equal to twice the theoretical one, an ideal low pass filter with band pass F/2, can perfectly extract the same spectrum of the starting signal.
If the sampling frequency is increased, the filtering is easier, as the repetitions of the spectrum of the signal s(t) are separated between them.
If the signal is sampled with a frequency lower than the theoretical one, or a filter with band not limited enough is used, a phenomenon occurs called aliasing.
Procedure
Signal reconstruction by filtering
• Set the circuit to PAM-1 channel mode, with flat sampling. Set SW1=flat, SW2=1CH, SW3=2; insert J2=d
• Apply 1kHZ-2Vpp to the analog input of the modulator (connect TP24 to TP1 and adjust the 2Vpp signal level)
• Connect the modulator output to the input of the 3.4 kHz low pass filter (connect TP11 with EXT IN). Set LEVEL Fl to maximum position
• Change the PAM pulse width (set SW3=1)
Exercise
i. Sketch all the waveforms.
ii. How does the reconstructed signal change?
Aliasing
• Set the circuit to PAM-1 channel mode, with flat sampling (SW1=flat, SW2=1 CH, SW3=2; J2=d)
• Connect 5kHZ to the analog input of the PAM modulator (connect TP26 to TP1)
• Connect the modulator output to the input of the 3.4 kHz filter (connect TP11 to EXT IN). turn LEVEL Fl to maximum position
• Examine the transmitted analog signal (TP4), the sampling pulses (TP6) and the PAM signal (TP11)
Exercise
i. What can you deduce from the waveforms?
ii. Examine the waveform of the reconstructed signal in TP21 and explain its shape.