Page 288 - Analog and Digital Filter Design

10
CHAPTER

SELECTING COMPONENTS FOR

ANALOG FILTERS

This chapter is very practical in orientation. It describes how different mate-
rials and component types can affect the performance of filters. In detail, it
shows how the construction of components could affect performance. Qpera-
tional amplifiers (op-amps) are also described. Amplifier parameters can have
a signilicant effect; their most significant parameter is the gain bandwidth
product.

Generally speaking, active filters are only used at low frequencies because of the
demands placed on the op-amp. A typical limit for an active filter is a cutoff
frequency of 100 kHz, although current mode devices can work at much higher
frequencies, perhaps 10MHz or more. Passive filters are used up to a few
hundred MHz. Above about 200MHz there are other more suitable filters, suck
as helical resonant cavity, surface acoustic wave (SAW), and stripline (tracks on
a printed circuit board).

Capacitors

Capacitors are constructed from two conducting surfaces (known as plates)
separated by an insulator (known as a dielectric). The metal plates are made
from a thin metal film that has been deposited onto the insulation material. The
dielectric can be a number of materials including ceramic, mica, and plastic film.
The capacitor type is usually known by the dielectric, thus they are "ceramic"
capacitors and "polyester" capacitors.
Ceramic and mica capacitors are made using flat dielectric sheets; the simplest
construction uses just one insulating layer with a conducting plate on either side.
Higher valued devices use several insulating layers with interleaving layers of
metal film. The metal film layers are bonded alternatively to side A, side €3, side
A, side B, and so on.