新手求教这个题

wp=2*pi*Fp/Ft;
ws=2*pi*Fs/Ft;
fp=2*Ft*tan(wp/2);
fs=2*Fs*tan(wp/2);
[n11,wn11]=buttord(wp,ws,1,50,'s'); %求低通滤波器的阶数和截止频率
[b11,a11]=butter(n11,wn11,'s'); %求S域的频率响应的参数
[num11,den11]=bilinear(b11,a11,0.5); %双线性变换实现S域到Z域的变换
[h,w]=freqz(num11,den11); %根据参数求出频率响应
plot(w*8000*0.5/pi,abs(h));
legend('用butter设计');
这是错误
??? Undefined function or variable "Fp".
Error in ==> one at 1
wp=2*pi*Fp/Ft;