Datasheet

Thursday, 17 July 2003 MiniProject: Design Aspects Colin K McCord
EEE512J2 – Electronic Product Design Page 7 Chapter 2: Fundamentals
2.6. Aliasing
Aliasing is an undesirable effect that can occur when digital sampling analogue voltages. This is the display
of an apparent signal which does not actually exist, usually caused by under-sampling.
Many samples should be taken per cycle (Nyquist theorem states that “to define a sine wave, a sampling
system must take more than two samples per cycle”.) to ensure an accurate representation of an analogue
signal in a digital memory. If only one sample is taken per cycle, or one sample per several cycles, then
aliasing occurs. For example say a waveform is being sampled every three cycles, these samples may form
together, particularly when using pulse interpolation (join the dots), to look like a valid waveform.
S1 S2 S3 S4 S5 S6 S7 S8 S9 S10
Figure 2.6a. Demonstrating aliasing, red is the real waveform, while blue is an alias.
Figure 2.6a clearly demonstrates how false signals (aliasing) are created. The red waveform is the real
waveform, notice that the waveform is under sampled (see green arrows for sample points). The black dots
shows were the real waveform (red) has been sampled, by joining the dots, it is clear that a perfect sine-
wave is created (blue), which is an alias of the original signal. Note that it is impossible to tell that the blue
signal is an alias.
There is nothing that can be done after sampling to correct aliasing; hence the solution is to filter out high
frequencies by sending the input signal through a low-pass filter. Ideally all frequencies above half the
sample rate should be filtered out.
2.7. The PIC Microcontroller
A PIC (Peripheral Interface Controller) microcontroller is an IC manufactured by Microchip.
These ICs are complete computers in a single package. The only external components necessary are
whatever is required by the I/O devices that are connected to the PIC.
The traditional Von-Neumann Architecture (Used in: 80X86, 8051, 6800, 68000, etc…) is illustrated in Figure
2.7a. Data and program memory share the same memory and must be the same width.