Page 378 - Analog and Digital Filter Design

375
Introduction to Digital Filters

So, Why Use a Digital Filter?

Some advantages of a digital filter are: (1) reproducible response; (2) not tem-
perature sensitive; (3) programmable. In a few cases the signal may be digitally
processed in some way, then filtered. then digitally processed some more; digital
filtering is the most obvious technique to use. Digital filters can have a sharp
cutoff in the frequency domain, combined with linear phase in the time domain.

Some disadvantages of a digital filter are: (1) unable to pass power; (2) sampling
effects; (3) requires a power supply: and (4) frequency range limitations.

The advantages are somewhat obvious: for example, logic levels are unaffected
by temperature. The disadvantages are perhaps more subtle. Many passive filter
designs are used to remove noise from power supplies-no one would consider
using active analog or digital filters for this. Sampling effects include aliasing.
where out-of-band signals are frequency shifted and appear in the passband. A
digital filter, like an active analog filter, requires a power supply. In applications
where POW power consumption is important, passive filiers are often used.
The frequency range of the filter will depend upon the resolution needed to
give us a high dynamic range, for example, whether we need 8-bit or 16-bit
performance.

Passive filters are usually used at radio frequencies, although the need for them
is decreasing. High-speed analog-to-digital converters are being produced with
ever increasing sampling rates. These can take their input signals direct from the
RF stage of a radio receiver. without the need for further filtering and demod-
ulation stages.

Reference

1. Rorabaugh, C. Britton. Digital Filter Designer's Hcindbook. New York:
McGraw-Hill. 1993.

Exercises

15.1 What are the basic components of a digital filter?

15.2 Is the response of a digital filter defined in the time or frequency
domain? How does this compare with the definition of an analog
filter?
15.3 What is the fundamental difference in architecture between that of an
IIR filter and an FIR filter?