LINEAR AN56 - Enhanced Performance over Bessel Filters for Data Communications Quick User Guide for Linear Phase Filters

Application Note 56 January 1994

“Better than Bessel” Linear Phase Filters for Data Communications Richard Markell

INTRODUCTION Enter the LTC1264-7 Linear Phase Filter

The pace of the world of digital communications is in- The LTC1264-7 has group delay which is equal to the

creasing at a tremendous rate. Daily, the engineer is Bessel in the passband while it has rejection at the second requested to compact more data in the same channel harmonic of the cutoff frequency of –30d B versus the bandwidth with closer channel spacing. As an example, Bessel’s – 12d B. Thus, Bessel is banished, replaced by a multilevel Pulse Amplitude Modulation (PAM) systems better linear phase solution for the data transmission can be used to compress data into a bandwidth limited problem. Even the most conservative data compaction channel. The most typical PAM system is simply ones and engineer will agree that the LTC1264-7 is “Better than zeros, the binary system. By shifting from a two-level Bessel.” Enough hoopla2, let’s get into the details. system to a four-level system, we double the data band-

Linear Technology’s LTC1X64-7 family of filters incorpo-

width in a bandwidth limited channel at the expense of

rates 2 poles of phase compensation (allpass filtering)

requiring a 8d B higher signal-to-noise ratio at the re-

and 6 poles of lowpass elliptic filtering in a single 14-pin

ceiver.1 This signal-to-noise trade-off to cram more bits

package. No external resistors are required. The LTC1264-

into the same bandwidth is why filtering is becoming more

7 is the first member of the Dash 7 linear phase filter

and more critical in data transmission. This is precisely

family. The group includes the LTC1264-7, the LTC1164-7,

why the LTC data communications filters were born.

the low power (4m A) member of the family with cutoff

Filters such as the Bessel switched capacitor filter frequency to 20k Hz and the LTC1064-7, the originator of (LTC1064-3), although having excellent transient response, the family, which provides cutoff frequencies to 100k Hz. have very poor noise or adjacent channel rejection. DSP is

This application note discusses some of the requirements

a help if the designer is trying to use telephone bandwidth,

and techniques for using filters in digital communications.

but is not fast enough for efficient uses of 100k Hz of

The terms “channel bandwidth,” “eye diagrams” and

bandwidth, let alone 200k Hz, where data rates approach

“linear phase” filtering are discussed without the need for

400 to 800kbps.

the “engineering speak” which permeates many textbook explanations of the same subjects.

The eye diagram measures the “quality” of the transmis- sion channel. The eye opening provides subjective indica-

1 Kamilo Feher, “Digital Communications: Microwave Applications,”

tion of bit error rate or the “goodness of the channel.” This

Prentice-Hall Inc., Englewood Cliffs, NJ, 1981.

will be discussed in more detail later in this text.

2 Hoopla is an utterance designed to bewilder.

AN56-1