Page 294 - Analog and Digital Filter Design

Selecting Components for Analog Filters 29

sensor circuit is being used to transmit over a twisted pair line. Power for the
sensor is DC and is applied to capacitively coupled split windings in the trans-
former, and so does not interfere with the AC signal. However, all the DC passes
through the transformer windings and the magnetic flux produced could satu-
rate the core. Transformer cores usually have no air-gap or adjustable slug and
are prone to saturate with small amounts of DC; their A, is normally far higher
than inductor core made from a similar ferrite.

Small inductors, about one centimeter long and wound on a ferrite or iron rod,
are common. These generally have a low Q, typically from 30 to 60 when meas-
ured at frequencies of about one megahertz. They are of fixed value inductance,
from one micro-henry up to one milli-henry. These inductors are useful in RF
circuits, but care must be taken to consider the self-resonant frequency.

There are other ferrite or iron based inductors. There are vertically mounting
devices that are a little smaller than the RM cores: perhaps 8mm in diameter and
up to 12mm high. These are usually wound to have a standard value (e.g., El,)
and can have values up to 100mH. Surface-mount inductors are either wound on
a ceramic or ferrite former, and usually have a low Q and a low self-resonant fre-
quency. These devices are small (size 1812, about 6mm by 3 mm by 4mm high)
often low value, perhaps up to lOOyH for the ferrite based devices. Their values
are limited to about lOOnH for those using the ceramic former. There are also
iron-cored inductors, for high value or high current applications. These are
usually restricted to power-line filters or loud speaker crossover networks.

Resistors

Resistors are used in active filter circuits, in conjunction with capacitors, to set
the frequency and the Q of each stage. Selecting the correct component value
can produce a filter with the desired frequency response at room temperature,
but unless consideration is given to temperature effects, the response at other
temperatures could be wrong. If resistors with a positive temperature coefficient
are selected, choosing capacitors with a negative coefficient may help to reduce
tuning errors.

There are several types of resistor. Wire-wound devices are rarely used, except
for power applications, and would not normally be placed in a filter circuit.
Carbon composition resistors tend to be noisy and have a poor temperature
coefficient but are good in RF circuits because of their low inductance con-
struction. Carbon film and metal film devices are most common. Surface-mount
devices are usually thick film construction.

Carbon film resistors are low-noise devices with a negative temperature coeffi-
cient. Component tolerances of 5% are standard. They are constructed by