From 5c37bdf106fa8138efa44a1095631301e55ee225 Mon Sep 17 00:00:00 2001
From: aWZHY0yQH81uOYvH <32046476+aWZHY0yQH81uOYvH@users.noreply.github.com>
Date: Thu, 12 Mar 2026 22:13:00 -0700
Subject: [PATCH] fix bandwidth function for DT systems

---
 control/lti.py            | 8 +++++++-
 control/tests/lti_test.py | 5 +++++
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/control/lti.py b/control/lti.py
index e4c9b2f4e..90188f556 100644
--- a/control/lti.py
+++ b/control/lti.py
@@ -211,8 +211,14 @@ def bandwidth(self, dbdrop=-3):
             # solve for the bandwidth, use scipy.optimize.root_scalar() to
             # solve using bisection
             import scipy
+
+            if self.isdtime(strict=True):
+                cvt_w = lambda w: np.exp(1j * w * self.dt)
+            else:
+                cvt_w = lambda w: 1j * w
+
             result = scipy.optimize.root_scalar(
-                lambda w: np.abs(self(w*1j)) - np.abs(dcgain)*10**(dbdrop/20),
+                lambda w: np.abs(self(cvt_w(w))) - np.abs(dcgain)*10**(dbdrop/20),
                 bracket=[omega[idx_dropped[0] - 1], omega[idx_dropped[0]]],
                 method='bisect')
 
diff --git a/control/tests/lti_test.py b/control/tests/lti_test.py
index dd95f3505..3cbb8439c 100644
--- a/control/tests/lti_test.py
+++ b/control/tests/lti_test.py
@@ -109,6 +109,11 @@ def test_bandwidth(self):
         np.testing.assert_allclose(sys1.bandwidth(), 0.099762834511098)
         np.testing.assert_allclose(bandwidth(sys1), 0.099762834511098)
 
+        # test a first-order discrete-time system, compared with matlab
+        sysd1 = tf([0.1, 0], [1, -0.9], 1)
+        np.testing.assert_allclose(sysd1.bandwidth(), 0.105207775532932)
+        np.testing.assert_allclose(bandwidth(sysd1), 0.105207775532932)
+
         # test a second-order system, compared with matlab
         wn2 = 1
         zeta2 = 0.001