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 increasing at a tremendous rate. Daily, the engineer is
requested to compact more data in the same channel
bandwidth with closer channel spacing. As an example,
multilevel Pulse Amplitude Modulation (PAM) systems
can be used to compress data into a bandwidth limited
channel. The most typical PAM system is simply ones and
zeros, the binary system. By shifting from a two-level
system to a four-level system, we double the data bandwidth in a bandwidth limited channel at the expense of
requiring a 8dB higher signal-to-noise ratio at the receiver.1 This signal-to-noise trade-off to cram more bits
into the same bandwidth is why filtering is becoming more
and more critical in data transmission. This is precisely
why the LTC data communications filters were born. The LTC1264-7 has group delay which is equal to the
Bessel in the passband while it has rejection at the second
harmonic of the cutoff frequency of – 30dB versus the
Bessel’s – 12dB. Thus, Bessel is banished, replaced by a
better linear phase solution for the data transmission
problem. Even the most conservative data compaction
engineer will agree that the LTC1264-7 is “Better than
Bessel.” Enough hoopla2, let’s get into the details. Filters such as the Bessel switched capacitor filter
(LTC1064-3), although having excellent transient response, …