Abstract :
Among the several graphical means being used in the frequency domain, Bode diagrams and Nichols charts are often adopted. Bode´s straight-line approximation to a log-magnitude curve is popular because of its simplicity; the correspondingly simple approximation for phase angle has a couple of versions, among which Vogt and Younis´ approach is quite elegant. However, there are no straight-line approximations on the Nichols chart, although the chart is so important in designing a control system. This note is to offer a procedure to do the approximation. The result, after routine corrections, is more accurate than that obtained by Bode´s diagram. It is, therefore, quite useful in synthesis.
