[SOLVED] Low Pass Filter - Slope

[kjha]

Hi Everyone,

I am designing a passive low pass filter using LTspice. I can get a slope of -20 dB/Decade using one pole. Is it possible to get a slope of less than -20 dB/Decade? how do i get a slope of -10 dB/Decade or -6 dB/Decade. is there any way?? can t be done by maybe playing with the filter Q factor or something??

Thanks!
Kalpit Jha

 

[RCinFLA]

You have to put cascade filters. If you are looking for less then -20 db/decade then you have to some high pass in the high freq slope area.

Older analog based band segment equalizers use ganged staggered tuned bandpass filters.

 

[LvW]

Hi kjha,

I suppose you don't want to have a slope of less than -20dB/dec not only for one single frequency, right?
A slope of less than -20dB/dec over a certain frequency range cannot be achieved in a conventional way.

However, since several years the principle of a "fractional capacitor" is discussed in several contributions and articles.
The principle behind this idea is the following:
* Each real capacitor has an admittance Y=(jwC)exp(a) with a=0.9999... That means: An ideal capacitor with a=1 does not exist.
* Based on specific technological procedures it is possible to realize "fractional capacitors" with a<0.9999 (for example a=0.6).
Also some biological "materials" have such properties.
* The BODE diagram of these elements shows a magnitude response of -a*20dB/dec and a phase response of -a*Pi/2.

The attached document describes the principle.

 

[FvM]

You can design filters with arbitrary magnitude characteristic (arbitrary magnitude + phase has some limitation related to system causality and may be difficult or effectively impossible in analog circuits). Generally the circuit complexity tends to increase with the number of defined characteristic points.

An example for a filter with wide band 10 dB/decade slope is a pink noise filter. The analog implementation uses an RC ladder circuit. https://www.spectrum-soft.com/news/fall98/pink.shtm

You'll notice that the filter characteristic has the typical behaviour of an approximation.

 

[LvW]

An example for a filter with wide band 10 dB/decade slope is a pink noise filter. The analog implementation uses an RC ladder circuit. https://www.spectrum-soft.com/news/fall98/pink.shtm

Ohh yes, good reference.

 

[kjha]

RCinFLA , LvW and FvM Thanks a ton Guys! this was really helpful..