Page 278 - Complete Wireless Design

Filter Design

Filter Design 277

Group delay—The measurement, in nanoseconds, of the time retardation
created by a filter or circuit to any discrete signal that passes through it.
When the group delay varies across the passband significantly, as it does in
tight Chebyshev filters (and to a certain extent in Butterworth filters), this
will cause increased BER in digital radios. This variation in the group delay,
called group delay variations (GDV), is especially severe at the edges of a
filter’s passband, so any filter that has steep skirts and/or a high pole count
will have a high GDV. The problem can be lessened by widening the filter’s
passband beyond what is required by the signal; using fewer filter poles; or
choosing a Butterworth type or, if selectivity specifications permit it, a
Bessel filter response. GDV is directly equated to the differential delay
specification above.
Insertion loss—The attenuation created in a filter within the center of its
passband when the filter is terminated at its design impedance. Measured
in dB.
Insertion loss linearity—The measure, in dB, in the variations in insertion
loss with input power. Primarily a problem with crystal filters.
Passband—The band of frequencies from f to f that a filter passes
LOW HIGH
with 3 dB or less attenuation (f and f are located at points where the
LOW HIGH
insertion losses reach, typically, 3 dB). Measured in Hz.
Passband ripple—The amplitude fluctuations within a filter’s passband. A
ripple greater than 0.5 dB is usually considered unacceptable in digitally
modulated radios. Chebyshev is the dominant filter topology that contains
ripple within its passband. However, this ripple can be decreased to 0.1 dB
or less by low-ripple Cheby designs.
Phase shift—The measurement of the variation of the phase of a signal as it
moves through a filter from its input to its output.
Poles—Refers to the number of reactive components, inductors or capacitors,
in a low-pass or high-pass filter; or the number of reactive pairs in a bandpass
filter (for an all-pole filter). The filter’s order matches its poles in an all-pole
filter, and the number of poles governs the steepness of the filter’s skirts.
Q (quality factor)—The ratio of the center frequency to its bandwidth at its
3-dB-down points. The narrower the bandwidth for the same center
frequency, the higher the filter Q. Q also refers to the quality factor of the
individual components that make up the filter. This is particularly
important for inductors within the LC filter circuit, since the lower the
individual Q’s within each component, the higher the filter’s insertion loss
will be. The stopbands will also have a poorer stopband attenuation
characteristic, and the filter’s response at the band edges will be more
rounded than with high-Q components.
Return loss—The measurement, expressed in decibels, of the difference
between the signal power sent toward a filter’s input and the strength of
the RF signal power returned, or reflected, from the input back toward the

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.