高阶 (2n) VSVC单位增益巴特沃斯低通滤波器设计,可分解为 n 个二阶低通,通过对这多个二阶低通的组合优化,可提高滤波器的低通特性和稳定性。
串联的传递函数是各个二阶滤波器传递函数的乘积:\({{\rm{H}}_{2n}}(s) = \prod\nolimits_{i - 1}^n {{H_2}^{(i)}(s)}\);
二阶压控电压源低通滤波器电路图:
由“虚短-虚断”得到,传输函数:\(H(s) = {{\mathop V\nolimits_o } \over {\mathop V\nolimits_i }} = {{\mathop A\nolimits_F /\mathop R\nolimits_1 \mathop R\nolimits_2 \mathop C\nolimits_1 \mathop C\nolimits_2 } \over {\mathop s\nolimits^2 + s({1 \over {\mathop R\nolimits_1 \mathop C\nolimits_1 }} + {1 \over {\mathop R\nolimits_2 \mathop C\nolimits_1 }} + {{1 - \mathop A\nolimits_F } \over {\mathop R\nolimits_2 \mathop C\nolimits_2 }}) + {1 \over {\mathop R\nolimits_1 \mathop C\nolimits_1 \mathop R\nolimits_2 \mathop C\nolimits_2 }}}}\);
其中\(s = j\omega\),\(\mathop A\nolimits_F = 1 + {{\mathop R\nolimits_f } \over {\mathop R\nolimits_r }}\);
去归一化低通滤波器的传递函数:\(H(s) = {{\mathop H\nolimits_0 \mathop \omega \nolimits_0^2 } \over {\mathop S\nolimits^2 + \alpha \mathop \omega \nolimits_0 S + \beta \mathop \omega \nolimits_0^2 }}\);
其中\(\beta \mathop \omega \nolimits_0^2 = {1 \over {\mathop R\nolimits_1 \mathop R\nolimits_2 \mathop C\nolimits_1 \mathop C\nolimits_2 }}\),\(\mathop H\nolimits_0 \mathop \omega \nolimits_0^2 = {{\mathop A\nolimits_F } \over {\mathop R\nolimits_1 \mathop R\nolimits_2 \mathop C\nolimits_1 \mathop C\nolimits_2 }}\),\(\alpha \mathop \omega \nolimits_0 = {1 \over {\mathop R\nolimits_1 \mathop C\nolimits_1 }} + {1 \over {\mathop R\nolimits_2 \mathop C\nolimits_1 }} + {{1 - \mathop A\nolimits_F } \over {\mathop R\nolimits_2 \mathop C\nolimits_2 }}\);
\({\omega _0}\)是截止角频率,\(\alpha\)、\(\beta\)是二项式系数,代表不同的滤波特性。
设定\(\mathop C\nolimits_2 = k\mathop C\nolimits_1\),那么\(\mathop H\nolimits_0 = \beta \mathop A\nolimits_F\),\(\beta \mathop k\nolimits^2 \mathop \omega \nolimits_0^2 \mathop C\nolimits_1^2 \mathop R\nolimits_2^2 - \alpha k\mathop \omega \nolimits_0 \mathop C\nolimits_1 \mathop R\nolimits_2 + (1 + k - \mathop A\nolimits_F ) = 0\)(关于\({R_2}\)的二次方程),由于\({R_2}\)存在实数解,则 k 必满足\(k \le {{\mathop \alpha \nolimits^2 } \over {4\beta }} + \mathop A\nolimits_F - 1\);
求解可得:\(\mathop R\nolimits_1 = {{\alpha \mp \sqrt {{\alpha ^2} - 4\beta (1 + k - {A_F})} } \over {2\beta (1 + \kappa - {{\rm A}_F}){\omega _0}{C_1}}}\),\(\mathop R\nolimits_2 = {{\alpha \pm \sqrt {{\alpha ^2} - 4\beta (1 + k - {A_F})} } \over {2\beta k{\omega _0}{C_1}}}\)
选定\({C_1}\),k后根据计算公式设计任意特性的VSVC低通滤波器。
归一化的巴特沃斯多项式:
对于单位增益\(\mathop A\nolimits_F = 1\),二阶低通,多项式系数\(\beta=1\);
那么\(\mathop H\nolimits_0 = 1\),\(k \le 0.25{\alpha ^2}\)(k取值为\(0.25{\alpha ^2}\)时,VCVS二阶单位增益低通同时具有方便、低成本和稳定的优势)并且\(\mathop R\nolimits_1 = {{\alpha \mp \sqrt {{\alpha ^2} - 4k} } \over {2k{\omega _0}{C_1}}}\),\(\mathop R\nolimits_2 = {{\alpha \pm \sqrt {{\alpha ^2} - 4k} } \over {2k{\omega _0}{C_1}}}\)。
通常情况下,为设计硬件电路方便,使得\({R_1} = {R_2}\)。\({C_1}\)的选取一般根据经验公式\({C_1} \approx {10^{ - 3 \sim - 5}}{f_0}^{ - 1}\)得出。
这样进一步简化为:\({C_2} = 0.25{\alpha ^2}{C_1}\),\({R_1} = {R_2} = {2 \over {\alpha {\omega _0}{C_1}}} = {1 \over {\pi \alpha {f_0}{C_1}}}\)。
另外为运放正端提供回路补偿失调,取定\({R_f} \ll {R_r},{R_f}//{R_r} \approx {R_f} = {R_1} + {R_2} = {2 \over {\pi \alpha {f_0}{C_1}}}\),到此完成了低通二阶巴特沃斯低通滤波器的参数配置。
对于高阶LPF设计,参照多项式系数和设定的截止频率即可完成。
实例仿真设计:以截止频率为100khz,增益为1,设计四阶巴特沃斯低通滤波器:
四阶低通存在参数:\({\alpha _1} = 0.7654,{\alpha _2} = 1.8478\),f=100khz,取第一级\第二级\({C_1} = 4.7nF\);
得到:
第一级\({C_2} = 0.68nF\),\({R_1} = {R_2} = 884.8Ω\),\({R_f} = 1769.6Ω\);
第二级\({C_2} = 4.02nF\),\({R_1} = {R_2} = 366.5Ω\),\({R_f} = 733Ω\),
\({R_r}\)取定1MΩ。Multisim仿真如下: