滤波器的截止频率定义为频率特性下降（　　）那一点所对应的频率。

<span style、"font-family: 'Times New Roman',serif; font-size: 10.5pt; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA; mso-bidi-font-size: 12.0pt; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt;" lang="EN-US">2dB</span>

<span style、"font-family: 'Times New Roman',serif; font-size: 10.5pt; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA; mso-bidi-font-size: 12.0pt; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt;" lang="EN-US">3dB</span>

<span style、"font-family: 'Times New Roman',serif; font-size: 10.5pt; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA; mso-bidi-font-size: 12.0pt; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt;" lang="EN-US">4dB</span>

<span style、"font-family: 'Times New Roman',serif; font-size: 10.5pt; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA; mso-bidi-font-size: 12.0pt; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt;" lang="EN-US">6dB</span>