Page 289 - Analog and Digital Filter Design

286 Analog and Digital Filter Design

Plastic film capacitors, such as polyester, use two layers of metalized plastic film.
One form of construction is identical to that of ceramic capacitors, where flat
sheets of metalized film are used.

Another form of construction for plastic film capacitors uses rolled films. The
two metalized layers are placed one above the other and then rolled, so that
the two conductors spiral around each other with insulating layers in between.
The films are laterally offset from one another so that the conductor of “side
A” protrudes from one side, and the conductor of “side B” protrudes from the
other side (this technique is sometimes known as extended foil). It is then rela-
tively easy to bond lead wires to the ends of the resulting cylindrical body. The
rolled form of construction provides a metal film around the body of the capac-
itor; this can be connected to earth or the “earthy” side of a circuit to reduce
external electrical field pickup. The outer foil is marked on the outer case of
some film capacitors.

A capacitor’s behavior is almost ideal, compared with other types of compo-
nent. Capacitors are formed from two conducting layers separated by an insu-
lator. Every capacitor will have some series inductance, which is due to the plate
conductors and the lead wires attached to them. Each capacitor will also have
series resistance due to both the conductors and the dielectric of the insulator,
this is known as equivalent series resistance or ESR. These imperfections become
more noticeable at high frequencies.

Generally, ESR is more of a problem with aluminum or tantalum electrolytic
capacitors that are rarely used in filter designs (tantalum may sometimes be
used in active filters). These types of capacitors are normally used to decouple
power supplies. Digital circuit designers have become accustomed to connect-
ing lOnF ceramic capacitors across tantalum devices used for power supply
decoupling. This is because the higher value tantalum capacitor absorbs low-
frequency transient currents, while the ceramic absorbs the high-frequency tran-
sient currents.

It has been known for ceramic capacitors to be destroyed by passing a high level
of RF power through them. The heat generated by the effective internal resist-
ance, mainly due to the dielectric, can be sufficient to cause mechanical damage.
Porcelain capacitors are often used at UHF (Ultra High Frequency) (300 MHz
to 3 GHz) and above because they have a low ESR.

Dissipation Factor (DF) and Loss Tangent are terms used to describe the effect
of ESR. The value of DF is given by the equation:

ESR
Loss Tangent = DF = -, where Xc is the capacitor’s reactance at
xc
some specific frequency. This is the tangent of the angle between the