Who can I help me about filter design?

[longstar]

I am designing a 4th order Butterworth LPF,so I apply the cascade with two 2th sallen-key filter.We can get the parameters about the FSF and Q of each stage by looking up the Butterworth Filter Table.So I get the parameters: FSF=1.0,Q=0.5412 for 1st stage,FSF=1.0,Q=1.3065 for 2nd stage.Surely,the filter -3dB frequency is 8MHz.Then we can calculate the R and C value by the transform function of sallen-key filter and above parameters.But unfortunely,I can't get the R and C value of the 1st stage.Who can tell me why?And how to do to meet the requirement?
Thanks a lot.
longstar

 

[pixel]

You can use Filter tools from Nuhertz (free version ftp://ftp.nuhertz.com/filter/FF5_1.exe). After you select filter type i.e active Butterworth 4th order it draws a circuit with with component values.
You can also specify set of component values that you can use...

 

[melc]

There is a better filter design software suited for active filters than the free version of Nuhertz (BTW someone here ahve Nuhertz full version?):

Filter Pro from https://www.ti.com, full features and free download

thx,

 

[hippoo]

hi hi,

beside the TI filter pro, u may refer to the analog devices internet based software:

**broken link removed**

i used TI filter pro and the AD "Interactive Design Tools: OpAmps : Active Filter Synthesis" before, both of them having pro and cont...

i suggest that u use both of them, simulate both the results using spice software and select the best...

i hope this may help u.....

regards,

hippoo

 

[leelamadhav]

I think u can go through EECs 247 course frm the berkeley's webcast site