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Application Notes

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  Better than Bessel Linear Phase Filters For Data Communications &mdash AN56
  PDF, 311 Kb, File published: Jan 1, 1994
  The pace of the world of digital communications is increasing at a tremendous rate. Each day the engineer is requested to compact more data in the same channel bandwidth with closer channel spacing. This application note discusses some of the requirements and techniques for using the new LTC1064/1164 and LTC1264-7 filters which were designed specifically for digital communications. The terms "channel bandwidth," "eye diagrams" and "linear phase" filtering are discussed without the need for the "engineering speak" which permeates many textbook explanations of the same subjects.
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  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, …

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• Signal Conditioning > Filters > Lowpass Filters