巴特沃斯LPF設計(硬體電路實現)

Handat發表於2024-08-28

高階 (2n) VSVC單位增益巴特沃斯低通濾波器設計,可分解為 n 個二階低通,透過對這多個二階低通的組合最佳化,可提高濾波器的低通特性和穩定性。

串聯的傳遞函式是各個二階濾波器傳遞函式的乘積:\({{\rm{H}}_{2n}}(s) = \prod\nolimits_{i - 1}^n {{H_2}^{(i)}(s)}\)

二階壓控電壓源低通濾波器電路圖:
image
由“虛短-虛斷”得到,傳輸函式:\(H(s) = {{\mathop V\nolimits_o } \over {\mathop V\nolimits_i }} = {{\mathop A\nolimits_F /\mathop R\nolimits_1 \mathop R\nolimits_2 \mathop C\nolimits_1 \mathop C\nolimits_2 } \over {\mathop s\nolimits^2 + s({1 \over {\mathop R\nolimits_1 \mathop C\nolimits_1 }} + {1 \over {\mathop R\nolimits_2 \mathop C\nolimits_1 }} + {{1 - \mathop A\nolimits_F } \over {\mathop R\nolimits_2 \mathop C\nolimits_2 }}) + {1 \over {\mathop R\nolimits_1 \mathop C\nolimits_1 \mathop R\nolimits_2 \mathop C\nolimits_2 }}}}\)

其中\(s = j\omega\)\(\mathop A\nolimits_F = 1 + {{\mathop R\nolimits_f } \over {\mathop R\nolimits_r }}\)

去歸一化低通濾波器的傳遞函式:\(H(s) = {{\mathop H\nolimits_0 \mathop \omega \nolimits_0^2 } \over {\mathop S\nolimits^2 + \alpha \mathop \omega \nolimits_0 S + \beta \mathop \omega \nolimits_0^2 }}\)

其中\(\beta \mathop \omega \nolimits_0^2 = {1 \over {\mathop R\nolimits_1 \mathop R\nolimits_2 \mathop C\nolimits_1 \mathop C\nolimits_2 }}\)\(\mathop H\nolimits_0 \mathop \omega \nolimits_0^2 = {{\mathop A\nolimits_F } \over {\mathop R\nolimits_1 \mathop R\nolimits_2 \mathop C\nolimits_1 \mathop C\nolimits_2 }}\)\(\alpha \mathop \omega \nolimits_0 = {1 \over {\mathop R\nolimits_1 \mathop C\nolimits_1 }} + {1 \over {\mathop R\nolimits_2 \mathop C\nolimits_1 }} + {{1 - \mathop A\nolimits_F } \over {\mathop R\nolimits_2 \mathop C\nolimits_2 }}\)

\({\omega _0}\)是截止角頻率,\(\alpha\)\(\beta\)是二項式係數,代表不同的濾波特性。

設定\(\mathop C\nolimits_2 = k\mathop C\nolimits_1\),那麼\(\mathop H\nolimits_0 = \beta \mathop A\nolimits_F\)\(\beta \mathop k\nolimits^2 \mathop \omega \nolimits_0^2 \mathop C\nolimits_1^2 \mathop R\nolimits_2^2 - \alpha k\mathop \omega \nolimits_0 \mathop C\nolimits_1 \mathop R\nolimits_2 + (1 + k - \mathop A\nolimits_F ) = 0\)(關於\({R_2}\)的二次方程),由於\({R_2}\)存在實數解,則 k 必滿足\(k \le {{\mathop \alpha \nolimits^2 } \over {4\beta }} + \mathop A\nolimits_F - 1\);

求解可得:\(\mathop R\nolimits_1 = {{\alpha \mp \sqrt {{\alpha ^2} - 4\beta (1 + k - {A_F})} } \over {2\beta (1 + \kappa - {{\rm A}_F}){\omega _0}{C_1}}}\)\(\mathop R\nolimits_2 = {{\alpha \pm \sqrt {{\alpha ^2} - 4\beta (1 + k - {A_F})} } \over {2\beta k{\omega _0}{C_1}}}\)

選定\({C_1}\),k後根據計算公式設計任意特性的VSVC低通濾波器。

歸一化的巴特沃斯多項式:
image

對於單位增益\(\mathop A\nolimits_F = 1\),二階低通,多項式係數\(\beta=1\)

那麼\(\mathop H\nolimits_0 = 1\)\(k \le 0.25{\alpha ^2}\)(k取值為\(0.25{\alpha ^2}\)時,VCVS二階單位增益低通同時具有方便、低成本和穩定的優勢)並且\(\mathop R\nolimits_1 = {{\alpha \mp \sqrt {{\alpha ^2} - 4k} } \over {2k{\omega _0}{C_1}}}\)\(\mathop R\nolimits_2 = {{\alpha \pm \sqrt {{\alpha ^2} - 4k} } \over {2k{\omega _0}{C_1}}}\)

通常情況下,為設計硬體電路方便,使得\({R_1} = {R_2}\)\({C_1}\)的選取一般根據經驗公式\({C_1} \approx {10^{ - 3 \sim - 5}}{f_0}^{ - 1}\)得出。

這樣進一步簡化為:\({C_2} = 0.25{\alpha ^2}{C_1}\)\({R_1} = {R_2} = {2 \over {\alpha {\omega _0}{C_1}}} = {1 \over {\pi \alpha {f_0}{C_1}}}\)

另外為運放正端提供迴路補償失調,取定\({R_f} \ll {R_r},{R_f}//{R_r} \approx {R_f} = {R_1} + {R_2} = {2 \over {\pi \alpha {f_0}{C_1}}}\),到此完成了低通二階巴特沃斯低通濾波器的引數配置。

對於高階LPF設計,參照多項式係數和設定的截止頻率即可完成。

例項模擬設計:以截止頻率為100khz,增益為1,設計四階巴特沃斯低通濾波器:

四階低通存在引數:\({\alpha _1} = 0.7654,{\alpha _2} = 1.8478\),f=100khz,取第一級\第二級\({C_1} = 4.7nF\)
得到:
第一級\({C_2} = 0.68nF\)\({R_1} = {R_2} = 884.8Ω\)\({R_f} = 1769.6Ω\)
第二級\({C_2} = 4.02nF\)\({R_1} = {R_2} = 366.5Ω\)\({R_f} = 733Ω\)
\({R_r}\)取定1MΩ。Multisim模擬如下:

image

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