Active crossover resistor/cap size

[paulhfx]

I'm designing an active analog crossover for a 4-way dipole, with mostly 4th order crossovers. I've got the basic schematics worked out and a working model laid out in LspCAD.

I understand the topologies, and the relationships between capacitors and resistors in a given circuit section. Once a particular cap size is selected the resistor value can be calculated (or vice versa).

How are the sizes of the capacitors or resistors (whichever are selected first) determined? Are there minimum/maximum size rules of thumb?

Thanks,

Paul

 

[Ed LaFontaine]

Paul,
Search for "Limitations of Sallen-Key Filters". One option is here:

http://www.elecdesign.com/Articles/ArticleID/7486/7486.html

You can choose a spectrum of combinations of R-C to arrive at a frequency. I think you will want to see "Comments about component selection" at the end of page 2. This document will help you get there.

 

[johnnyx]

Rule of Thumb

In the FilterPro program (available free from Texas Instruments website), you set a "seed" value for the resistors. I usuall set it to 10k ohms, and the program calculates from there. 10k is a good value;- lower and opamps may have problems driving the load, higher and noise and interference can be a problem.
Hope this helps.

 

[paulhfx]

Thanks Ed, that seems to be just what I was looking for.

It even includes a few comments on setting one capacitor size for different crossover frequencies and then varying resistors only, which seems to be Marchand's approach for their modular 4th order slope units.

Paul