Page 279 - Complete Wireless Design

Filter Design

278 Chapter Six

source. Most filters can easily be designed with a return loss of 10 dB or
higher. With this value of return loss, only one-tenth the power incident to
the filter’s input will not be passed on to the load, but will be reflected back
toward the original source.
Ripple—The amount of amplitude variations in the passband of a filter.
Excessive ripple causes high BER in digital systems. Measured in dB.
Ripple loss—The distinction between the maximum and the minimum
attenuation inside the filter’s passband.
Shape factor—Specifies the steepness of the skirts in a filter, and is
measured as:

BW (at 60 dB)

BW (at 3 dB)
A perfectly square shape factor would equal 1. Measured as a ratio.

Spurious responses—Since the perfect component does not exist, any filter
made of capacitors, inductors, and/or crystals can have areas within its
stopband that supply less attenuation than planned, or create ripple within
the filter’s passband. This is caused by undesired reactances from the
component’s own leads, as well as stray inductances and capacitances,
resonating at various frequencies. Crystal filters, especially those with the
AT-cut, will have these responses at roughly odd integer multiples of the
fundamental frequency, which can create unexpected consequences in the
design of the communication system if not expected. Distributed microwave
filters can also have reentrance modes, which can allow a multitude of
secondary passbands within the filter’s expected stopband. These effects
can, if required, be ameliorated by supplemental LC filtering at the output of
the crystal filter and in some of the lower-frequency distributed filters.

Stopband (reject band)—The band of frequencies that bandpass, low-pass,
high-pass, and bandstop filters attenuate to some predetermined level,
usually 60 dB (but it can be much less). The stopband is the point on the
attenuated side of the cutoff frequency. Measured in Hz.

6.1 Lumped Filters
6.1.1 Introduction
Lumped filters are any filter structures that are constructed of individual,
physical components—such as inductors, capacitors, and resistors—to form a
network that can pass some frequencies while blocking others. Lumped filters
can be designed to perform in low-pass, high-pass, bandpass, and bandstop
applications, and without any frequency reentrance (more than one passband)
problems associated with the majority of distributed filter types.

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