@@ -211,12 +211,13 @@ def _bandwidth(self, dbdrop=-3):
211211# check if system is SISO and dbdrop is a negative scalar
212212if not self .issiso ():
213213raise TypeError ("system should be a SISO system" )
214-
215- if not ( np .isscalar (dbdrop ))or dbdrop >= 0 :
214+
215+ if ( not np .isscalar (dbdrop ))or dbdrop >= 0 :
216216raise ValueError ("expecting dbdrop be a negative scalar in dB" )
217217
218218dcgain = self .dcgain ()
219219if np .isinf (dcgain ):
220+ # infinite dcgain, return np.nan
220221return np .nan
221222
222223# use frequency range to identify the 0-crossing (dbdrop) bracket
@@ -226,14 +227,16 @@ def _bandwidth(self, dbdrop=-3):
226227idx_dropped = np .nonzero (mag - dcgain * 10 ** (dbdrop / 20 )< 0 )[0 ]
227228
228229if idx_dropped .shape [0 ]== 0 :
229- # no frequency response is dbdrop below the dc gain.
230+ # no frequency response is dbdrop below the dc gain, return np.inf
230231return np .inf
231232else :
232- # solve for the bandwidth, use scipy.optimize.root_scalar() to solve using bisection
233+ # solve for the bandwidth, use scipy.optimize.root_scalar() to
234+ # solve using bisection
233235import scipy
234- result = scipy .optimize .root_scalar (lambda w :np .abs (self (w * 1j ))- np .abs (dcgain )* 10 ** (dbdrop / 20 ),
235- bracket = [omega [idx_dropped [0 ]- 1 ],omega [idx_dropped [0 ]]],
236- method = 'bisect' )
236+ result = scipy .optimize .root_scalar (
237+ lambda w :np .abs (self (w * 1j ))- np .abs (dcgain )* 10 ** (dbdrop / 20 ),
238+ bracket = [omega [idx_dropped [0 ]- 1 ],omega [idx_dropped [0 ]]],
239+ method = 'bisect' )
237240
238241# check solution
239242if result .converged :
@@ -552,16 +555,17 @@ def bandwidth(sys, dbdrop=-3):
552555 Returns
553556 -------
554557 bandwidth : ndarray
555- The first frequency (rad/time-unit) where the gain drops below dbdrop of the dc gain
556- of the system, or nan if the system has infinite dc gain, inf if the gain does not drop for all frequency
558+ The first frequency (rad/time-unit) where the gain drops below dbdrop
559+ of the dc gain of the system, or nan if the system has infinite dc
560+ gain, inf if the gain does not drop for all frequency
557561
558562 Raises
559563 ------
560564 TypeError
561565 if 'sys' is not an SISO LTI instance
562566 ValueError
563567 if 'dbdrop' is not a negative scalar
564-
568+
565569 Example
566570 -------
567571 >>> G = ct.tf([1], [1, 1])