highpass filter.. modify frequency?

http://www.groupdiy.twin-x.com/albums/userpics/10015/eq.pdf

gathering opinions on the best way to modify a highpass(75hz) filter.

anyone see a nifty way to modify this to either be variable(pot) or switchable. looking for around 500-600 hz to be the new frequency.

:thumb:

Simply switch the caps, or the resistors.

To raise the corner frequency, divide the caps or multiply the resistors by the factor f (Hz) / 75.

> To raise the corner frequency, divide the caps or multiply the resistors

I think you want smaller resistors for higher frequency, no?

But don't listen to me on monday morning.

You're right. Don't listen to me on a Monday morning, either. :?

I should have said:
To raise the corner frequency, divide the caps or the resistors by the factor f (Hz) / 75.

hmm I started working on it before lunch and the math didn't add up so i just assumed it was my severe hunger and went to lunch! i came back to see the next post. :thumb:

thanks guys!

One other thing: to avoid loading down the source, it would be better to reduce the caps rather than the resistors.

thanks NYD!

If you really want continuously variable it is difficult with the unity-gain topology, since variable C's are generally impractical, and the three variable R's are of different values.

If you know your output loading you can get there with the equal (variable) R and equal C Sallen-Key two-pole HP followed by a (variable) R and C, with the Q of the two-pole hiked up a bit for the desired overall response. But now all three R's can be the same, if you can tolerate a little change in the Q due to the variable effect of output loading. If that is intolerable a final voltage follower can be used. This approach will have some voltage gain which may be undesirable.

If so, there are ways: there's a maximally flat three-pole topology with equal R's and C's and overall unity gain, but it requires three (!) voltage followers. Still, it's lower parts count than the state-variable two-pole followed by a buffered single-pole (four op amps).

There's a handy online calculator for designing high and lowpass filters over at ADI...


http://www.analog.com/Wizard/filter/filterUserEntry/#top

If you are making a fixed HPF, if will do nicely.

Variable cutoff frequency is a bit more complicated to design, as bcaruso described, and the ADI calculator isn't all that helpful, but they can be done.

[quote author="Svart"]http://www.groupdiy.twin-x.com/albums/userpics/10015/eq.pdf

gathering opinions on the best way to modify a highpass(75hz) filter.

anyone see a nifty way to modify this to either be variable(pot) or switchable. looking for around 500-600 hz to be the new frequency.

:thumb:[/quote]

Do you really need something as steep as a 3rd order filter (18db/octave)?

If you discard the first cap and resistor, make both the remain resistors the same value and add 4dB of gain on the opamp, you'll get what is known as 'equal value' filter and that makes a dual gang reverse log pot for variable frequency a breeze.

What kind of ramp response are you looking for?
What's your voltage source?
Current source?
What knd of time response do you need?
Phase angle will depend on your reflection coeficient pole plot.

A continous transfer function can be determined if you decide on which topology to go with.

What kind of stop band ratio are you seeking in relation to group delay?

We need to know your band attenuation if rads per second in order to satisfy the above parameters.

May I suggest a little reading of Tremaine's Passive Networks before embarking on a seemingly simple but actuallt complicated endeavor?

In Haste,

Your Truly Departed,

Doc Holiday

Svart,

IIRC, the gain for a 3-pole Butterworth equal value Sallen-Key is 2.0, but you have to put a buffer after the first pole pair. That isn't much gain, so overload isn't likely to be a big problem. Usually you loose some amplitude when you highpass filter audio signals because most of the power is at lower frequencies. You can always pad it down by 1/2 before the filter if you need to.

In other words, if you want to use a 3-deck pot for your R's, the caps can all be the same value. The circuit goes like this

RC HPF -> op amp buffer -> 2-pole Sallen & Key HPF with gain of 2 on the op amp (two equal value resistors in the feedback divider).

Putting the gain stage second reduces the chances of overload if your signal levels are pretty hot.

If it's gotta be variable, and you want to turn a knob and keep a textbook Butterworth response, this is the only way I can think of other than the 3 -buffer approach bcarso suggested... except some gosh-awful multideck rotary switch nightmare <grin> The different R values and the loading of the first RC section by the second keeps the topology you posted from working well in a variable filter.

Don't forget to put equal value fixed resistors in series with each pot section so your filter has a maximum corner frequency stop point. If you let the pots go to zero ohms it may not be pretty. :shock:

Using 0.022 uf caps, that makes equal R's of 14.4K at 500 hz, and down to around 12K at 600 hz. if I calculated it right. So, like three 5K pot sections and three 10K fixed resistors in series with each section gives you a little wider range than 500-600 hz.

Now all you gotta do is find a 3-deck 5K pot :roll:

Hope this is what you're looking for.

:shock:

i sure opened the can on that one..


thanks guys that answered all my questions and then some!
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