Bode diagram slope contribution: In a log magnitude plot, a single pole factor (1 + s/ω) in the transfer function contributes a slope of how many decibels per octave at high frequency?

Difficulty: Easy

Correct Answer: -6 dB per octave

Explanation:


Introduction / Context:
Bode diagrams are frequency response plots showing system behavior on a logarithmic scale. The slope of the magnitude plot indicates how quickly the response falls with frequency. A single pole introduces a characteristic slope change.


Given Data / Assumptions:

  • We are analyzing a single pole contribution to a Bode magnitude plot.
  • Frequency scaling is in octaves (doubling of frequency).


Concept / Approach:

A single pole contributes –20 dB per decade beyond its corner frequency. Since one decade = 10× frequency and one octave = 2× frequency, the per-octave slope must be converted accordingly.


Step-by-Step Solution:

Slope = –20 dB per decade.1 decade ≈ 3.32 octaves (since log2(10) ≈ 3.32).Therefore slope per octave = –20 / 3.32 ≈ –6 dB per octave.


Verification / Alternative check:

Textbook Bode plot rules confirm: each pole beyond the break frequency introduces a –20 dB/decade slope or equivalently –6 dB/octave.


Why Other Options Are Wrong:

  • –20 dB/octave: too steep; this is per decade, not octave.
  • –10 dB/octave: incorrect conversion.
  • –2 dB/octave: too shallow.
  • –40 dB/octave: corresponds to two poles, not one.


Common Pitfalls:

  • Confusing decade with octave.
  • Mixing the effect of a single pole with that of multiple poles.


Final Answer:

–6 dB per octave.

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