Sallen-Key LPF frequency scaling factor

Hi all,

I am trying to obtain Butterworth, Chebyshev and Bessal LPF responses with the Sallen-Key topology. I found this app-note from TI and it is very helpful. According to the document if I want a 2nd order Bessal LPF, I just use the equations on top of page 9 (first line) along with table 2 (in the same page). It is quite straight forward.

My questions are am I in the correct path and what is the effect of the frequency scaling factor (FSF)?
The FSF is 1 for Butterworth but for Bessel and Chebyshev it has an effect. Why is that?

Regards

[BTW I have asked a similar question a while back but apparently I had no idea about the subject at that time]

Frequency scaling applies for all filters. So does "impedance" scaling, where you can pick some R seed, and the capacitors are adjusted. I have a TI application (FilterPro) installed. It automates opamp active filter design. I get to choose a topology, passband ripple, number of poles, cutoff frequency, a resistor value seed. It even plots the response if resistors get replaced with nearest 5% or 1% values.

Blimey... if you want a simpler article on filter design, have a look at my website. Not sure if it helps, but it will give you a second opinion:

http://www.simonbramble.co.uk/techarticles/active_filters/active_filter_design.htm

The answer is simple:

The pole frequency fp is identical to the 3dB cutoff fc for Butterworth response only.
For all other lowpass functions (Chebysheff, Bessel,...) there is a scaling factor FSF that relates the cutoff to the pole frequency (since all filter tables and functions are based on the pole frequency).
FSF=fp/fc.

W.

Thank you all for your replies. They were very helpful !!!!!!!!!!!

gehan_s said:

Thank you all for your replies. They were very helpful !!!!!!!!!!!

Click to expand...

Here is one additional hint: Use only K=1 or K=2 (here you can use 2 identical R`s).