Page 295 - Analog and Digital Filter Design

292 Analog and Digital Filter Design

applying a carbon film onto a ceramic rod and then cutting a spiral gap around
the device to increase the resistance. The spiral conductor is actually a lossy
inductor.

Metal film resistors have a lower noise than carbon film types and a lower tem-
perature coefficient. Component tolerances of 1% are standard, although pre-
cision devices in an E96 range of values with 0.1% tolerance and 15 ppm (parts
per million) temperature coefficient are available at a higher cost. These resis-
tors are constructed by applying a number of metal film layers, of different
metals, to a ceramic formes to achieve the correct resistance and a low tempera-
ture coefficient. A spiral gap is sometime cut around the metal film to increase
the resistance value.
All conductors have some series inductance, simply due to having a certain
length. In fact some high-frequency circuits just use a thin wire bond to form
an inductor (this will be discussed further in Chapter 12). Resistors are con-
ductors and therefore have inductance too. Some types have more inductance
than others. Even a thick-film surface-mount resistor has inductance, although
of considerably lower value than other types.

Wire-wound resistors have a significant inductance because of their construc-
tion; when a wire is wound into a coil its inductance increases in proportion to
the number of turns squared. Carbon or metal film resistors that have had a
spiral gap cut through their surface will have more inductance than a carbon
composition type. Ail these components have some inductance due to the wire
leads at either end.

Resistors also have capacitance. The two ends have a certain cross-sectional area
and are spaced a certain distance apart, separated by a ceramic dielectric. This
capacitance is small, typically 0.2 pF, but at high frequencies and in high impe-
dance circuit node this can be significant.

The Printed Circuit Board (PCB)

The circuit board on which the components are connected is important at high
frequencies and for surface-mount circuits. At high frequencies, for example,
capacitance between tracks can cause a lower resonance frequency in a tuned
circuit. Surface-mount circuits can have reliability problems due to thermal
expansion of the circuit board; components firmly attached to the tracks with
solder can be stressed if they do not have the same thermal expansion. There are
several types of board, with FR4 (fiberglass insulator) being the most common.

It is usual for an RF or high-speed digital circuit to have an earth plane on
the printed circuit board (PCB) component side. The earth plane serves two