cut off frequency of high pass vs lowpass

[lopan]

In flipping through a book Simplified Practical Filter Design, Irving M Gotlieb, I came across a passage that confuses me. Actually more than one but lets try this.

That for a second order filter the resonance freq is 1/(2*pi*rt(LC)) this looks fine. But then he goes on to say that cut off frequency is not the same for high and low pass networks just by flipping around the reactive devices all other things constant.

What?

Next he goes on to explain further that for high pass

fc = 1/(4*pi*rt(LC))

and for lowpass

fc + 1/(pi*rt(LC))

Does this make sense to any one?

Maybe I am lost but the charcteristic equation for second order filters does not show a 4 fold difference -- in fact no difference.

Do I need to through this book away?

Any thoughts


cheers

[AndrewT]

Hi,
I don't have any reference books to hand but your 1st equation seems right if rt = sqrt.
The versions without and with 4 as a factor seem wrong and I cannot understand his idea that lopass & hipass should be calculated differently.
regards AndrewT.

[richie00boy]

Depending on filter Q, the crossover point is not always at -3dB for each filter.

The equation you give does not have a 4 times difference between the two frequencies as the 4 is on the bottom. When was the book originally published?

[lopan]

Originally posted by AndrewT
Hi,
if rt = sqrt.
I cannot understand his idea that lopass & hipass should be calculated differently.
regards AndrewT.

right sqrt is better notation
My thought exactly

Originally posted by richie00boy
Depending on filter Q, the crossover point is not always at -3dB for each filter.
The equation you give does not have a 4 times difference between the two frequencies as the 4 is on the bottom. When was the book originally published?

cheers

True the resonant point of a filter is not always at -3dB but the cutoff frequency, fc, is generally defined as the 3 dB down point. Only for first order filters does fc correspond to -3dB down.

Doesn't matter top or bottom -- multiply or divide by 4.

Book was published in 1990

cheers