Page 246 - Analog and Digital Filter Design

9
CHAPTER

PHASE -SHIFT NETWORKS

(ALL- PASS FILTERS)

An all-pass filter seems to be a contradiction in terms. A filter surely removes
some signals? Well, no. Actually, an all-pass filter modifies the phase of signals
passing through it. To be more precise, it modifies the phase in a frequency selec-
tive and predetermined way. All filters delay the signal passing through them.
The majority of frequency selective filter designs (Butterworth, Chebyshev, etc.)
produce delays that are frequency dependent, so a signal at one frequency is
delayed more than a signal at another frequency. Phase-shift networks can be
used to compensate for this, so that all signal frequencies are output from the
filter with the same delay.

Another application of phase-shift networks is in single sideband modulation,
in which phase-shifting is used to cancel out the unwanted sideband of an AM
radio transmission. This application requires a signal to be applied to two paths.
The signals at the output of the two paths are phase-shifted, one relative to the
other, by 90". This chapter gives a description of a single sideband modulator,
both in mathematical terms and with practical applications.

Phase Equalizing All-Pass Filters

Introduction to the Problem
Digital or impulsive signal processing by analog filters is becoming more
common. This is in part due to the rise in digital communication systems,
but it is also due to the need to restrict the bandwidth of impulsive signals to
meet electromagnetic interference (EMI) regulations. Most filter types (e.g.,
Burterworth, Chebyshev, and Cauer) produce unwanted phase distortion of
signals passing through them. Bessel filters have a linear phase response and
produce no in-band phase distortion. Unfortunately, Bessel filters often have
insufficient attenuation at frequencies beyond the passband, because their fre-
quency response has a gentle transition from passband to stopband. Therefore,