Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Sallen-Key LPF frequency scaling factor

Status
Not open for further replies.

gehan_s

Member
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.
 
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.
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top