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Questions about filter design
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acidblue



Joined: Jun 26, 2009
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PostPosted: Wed Dec 12, 2012 5:03 pm    Post subject: Questions about filter design Reply with quote  Mark this post and the followings unread

I an effort to learn how to design my own active filters I have a few questions.

In a low pass filter what frequencies should pass and which ones should be rejected??
Is a LP filter for CMOS different from one thats for true analog VCO's?

Which filters are best for CMOS, Sallen Key, Multiple Feedback, Twin T ect.??
Which are better for analog?? or is it all a matter of personal preference??

Whats a good Q figure for say stage 1 and stage 2 ect?
Been looking at the Fliter Design Wizard here:
http://designtools.analog.com/dt/filter/filterW.html

Any insight is greatly appreciated.
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Dougster



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PostPosted: Thu Dec 13, 2012 3:49 am    Post subject: Re: Questions about filter design Reply with quote  Mark this post and the followings unread

acidblue wrote:
I an effort to learn how to design my own active filters I have a few questions.

First, I'd like to recommend chapter 16 (page 285) of Mancini: Op Amps for Everyone

Quote:
In a low pass filter what frequencies should pass and which ones should be rejected??

Pardon me for being pedantic, but what exactly are you asking? A low pass filter will let the frequencies below a certain point (the cutoff, or "corner" frequency) to pass through while it attenuates the frequencies above that point. The choices of design and component values determine the cutoff frequency and the attenuation.

Quote:
Is a LP filter for CMOS different from one thats for true analog VCO's?

I'm not exactly sure what you are asking, but if I'm interpreting correctly, a filter is a filter regardless of what circuit you place it in.

Quote:
or is it all a matter of personal preference??

In the end, at least for audio filters, there really is only personal preference. Wink

I'd also recommend getting a copy of Lancaster's Active Filter Cookbook and if you are really serious, buy a copy of Electronotes...

Regards,
Doug

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JingleJoe



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PostPosted: Thu Dec 13, 2012 9:31 am    Post subject: Reply with quote  Mark this post and the followings unread

Main suggestion I have for you is to read up about electronics more and (this is really the biggest help I found) if possible, study electronics at college.
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Dougster



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PostPosted: Thu Dec 13, 2012 11:16 am    Post subject: Reply with quote  Mark this post and the followings unread

College is highly-overrated...

Regards,
Doug

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acidblue



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PostPosted: Thu Dec 13, 2012 1:45 pm    Post subject: Re: Questions about filter design Reply with quote  Mark this post and the followings unread

Dougster wrote:
acidblue wrote:
I an effort to learn how to design my own active filters I have a few questions.

First, I'd like to recommend chapter 16 (page 285) of Mancini: Op Amps for Everyone

Quote:
In a low pass filter what frequencies should pass and which ones should be rejected??

Pardon me for being pedantic, but what exactly are you asking? A low pass filter will let the frequencies below a certain point (the cutoff, or "corner" frequency) to pass through while it attenuates the frequencies above that point. The choices of design and component values determine the cutoff frequency and the attenuation.

Quote:
Is a LP filter for CMOS different from one thats for true analog VCO's?

I'm not exactly sure what you are asking, but if I'm interpreting correctly, a filter is a filter regardless of what circuit you place it in.

Quote:
or is it all a matter of personal preference??

In the end, at least for audio filters, there really is only personal preference. Wink

I'd also recommend getting a copy of Lancaster's Active Filter Cookbook and if you are really serious, buy a copy of Electronotes...

Regards,
Doug


What I mean is this:
Is 20khz a good cutoff frequency or maybe 10khz??
Which range(khz) is considered low?? Sorry if this sounds stupid.

For some reason I thought filters for CMOS would be different for Analog, my bad.
Thanks for reply and the advice I'll take a look thru the pdf you posted.
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JingleJoe



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PostPosted: Thu Dec 13, 2012 2:16 pm    Post subject: Reply with quote  Mark this post and the followings unread

Dougster wrote:
College is highly-overrated...

Regards,
Doug

Maybe in America it is Rolling Eyes why do you say that anyway?

Human hearing range is 20Hz to 20kHz, if you make the cutoff 20khz there will be no filtering of the audio. I recently made a VCF, low pass filter with a cutoff which can vary between about 5Hz and 500Hz, specialized for very low frequencies to get better effect on bass.

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acidblue



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PostPosted: Thu Dec 13, 2012 3:21 pm    Post subject: Reply with quote  Mark this post and the followings unread

AHA! I did not know that. Thanks

What Filter Response is good for low-pass, Butterworth. Bessel or Chebyshev?
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Dougster



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PostPosted: Thu Dec 13, 2012 4:08 pm    Post subject: Re: Questions about filter design Reply with quote  Mark this post and the followings unread

acidblue wrote:
What I mean is this:
Is 20khz a good cutoff frequency or maybe 10khz??

Well, since the typical human being can't hear above 20kHz, having a 20kHz cutoff won't do anything audible. Maybe your dog will like it...

OK, so we typically say that a human can hear from 20Hz to 20kHz. A strategic thing to remember is that twice any particular frequency is one octave up from that frequency. So, if you could hear down to 20Hz, then the octaves would be 20, 40, 80, 160, 320, 640, 1280, etc. Most people can't really hear the bottom octave, so you can probably count on 40Hz and above. By the same token, the range between 10kHz and 20kHz is only one octave, and most of us can't hear that entire range either...

Quote:
Which range(khz) is considered low??

I personally consider low to be 300Hz or less, but then I'm a bass player. For a reference, typically the A above middle C is 440Hz...

Quote:
Sorry if this sounds stupid.

We all have to start somewhere.

Quote:
What Filter Response is good for low-pass, Butterworth. Bessel or Chebyshev?

D) All of the above. Do you want smooth? Squelchy? Wet sounding? (Yep, wet. Check out Robert Rich's "Bestiary" for some very nice wet sounding filters...)

I'd like to also suggest that you check out Helmholtz On the Sensations of Tone...

Regards,
Doug

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Dougster



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PostPosted: Thu Dec 13, 2012 4:14 pm    Post subject: Reply with quote  Mark this post and the followings unread

JingleJoe wrote:
Dougster wrote:
College is highly-overrated...
Maybe in America it is Rolling Eyes why do you say that anyway?

Maybe in America? Where are *you* from? Britain?

So, just for argument's sake ("I'm here for an argument." "No, you're not!"), how many people do you know who have college degrees and they actually use anything they learned in college?

Regards,
Doug

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acidblue



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PostPosted: Thu Dec 13, 2012 4:16 pm    Post subject: Reply with quote  Mark this post and the followings unread

I'm liking these answers Very Happy
So is it Butterworth>smooth, Bessel>squelchy, and Chebyshev>wet??
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Dougster



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PostPosted: Thu Dec 13, 2012 4:18 pm    Post subject: Reply with quote  Mark this post and the followings unread

Cool

As we say, "further analysis is required". Time to get out the protoboard and find out for yourself... Wink

Regards,
Doug

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JingleJoe



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PostPosted: Fri Dec 14, 2012 12:20 pm    Post subject: Reply with quote  Mark this post and the followings unread

acidblue wrote:
I'm liking these answers Very Happy
So is it Butterworth>smooth, Bessel>squelchy, and Chebyshev>wet??

Yes Smile

And regarding college,I don't know many people in general what with being a shut in mad scientist. However, you're suggesting that the person in question will not use what they learn in college ... even if they learn about electronics, which is what they seem to be interested in and want to do. Logic has left the building Laughing

The only people who I went to college with, who aren't using what they got out of college, are the ones who failed college because they put in no effort. Most went to university to study electronics further, I was happy with my level of electro-knowledge at the end of college Smile which is why I suggest it to anyone who wants to learn about electronics.

P.S. are you aware that a person's location is just at the side there under thier picture and username?

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JovianPyx



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PostPosted: Fri Dec 14, 2012 12:42 pm    Post subject: Reply with quote  Mark this post and the followings unread

Joe, it has zero to do with the quality of education in one country or another and I'll add that remarks like that are incendiary in a multinational forum.

Rather, statistically the majority of college graduates don't end up working in their field of study. Sad, but true. The reason is that people who enter college at 18 years of age haven't had enough life experience to know what truly interests them. Often they are pushed by parents, or by counselors who tell them "there's money in this or that" or because daddy was a lawyer, so little Johnny should be a lawyer too. Couple that with the expense of college which induces people to finish what they started, even if they realize they have little or no interest in it - because it would otherwise be a waste of money. Additionally, there are fields of study offered in college that are useless unless you intend on working as a college professor, think of "classical studies" and the like. How many real jobs are there for such an education?

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Last edited by JovianPyx on Fri Dec 14, 2012 12:52 pm; edited 1 time in total
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JingleJoe



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PostPosted: Fri Dec 14, 2012 12:50 pm    Post subject: Reply with quote  Mark this post and the followings unread

JovianPyx wrote:
Joe, it has zero to do with the quality of education in one country or another and I'll add that remarks like that are incendiary in a multinational forum.

I suppose the tone of sarcasm didn't come over, so don't get your knickers in a twist, it's just the first difference I could perceive between myself and whatshisname there.

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JovianPyx



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PostPosted: Fri Dec 14, 2012 12:53 pm    Post subject: Reply with quote  Mark this post and the followings unread

What I said is fact Joe, and you tread on hurting other people's feelings like that.

If you mean it to be a joke, then add a winky or a smiley. K?

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JingleJoe



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PostPosted: Fri Dec 14, 2012 1:00 pm    Post subject: Reply with quote  Mark this post and the followings unread

I didn't decry anything you said, you can only take offence, not give it, so relax- I did put in a smiley, a rolling eyes smiley.
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JovianPyx



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PostPosted: Fri Dec 14, 2012 1:18 pm    Post subject: Reply with quote  Mark this post and the followings unread

Veering back on topic...

One major difference between CMOS filters and opamp filters is that CMOS gates like 4069UB do not have constant gain over their range of swing. In a CMOS gate, the gain is highest when the input is at (Vdd-Vss)/2. This means that the farther the input gets away from the that "center" value, the less gain it has. This causes a more or less pleasant sounding distortion with a good deal of second harmonic added.

On the other hand, an opamp has the same gain regardless of where the input is.

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Dougster



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PostPosted: Fri Dec 14, 2012 11:54 pm    Post subject: Reply with quote  Mark this post and the followings unread

JingleJoe wrote:
However, you're suggesting that the person in question will not use what they learn in college ... even if they learn about electronics, which is what they seem to be interested in and want to do. Logic has left the building Laughing

No it hasn't, it's still installed on my Mac right where it belongs.

I think you can learn plenty about electronics without wasting a penny on college. I believe you'll get more out of a copy of Electronotes than you will from a four year degree, unless of course you're taking class from Aaron Lanterman...!

Regards,
Doug

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acidblue



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PostPosted: Sat Dec 15, 2012 8:56 pm    Post subject: Reply with quote  Mark this post and the followings unread

Anybody have any experience with Linear Tech's Filter Cad??
http://www.linear.com/designtools/software/#Filter
About half way down the page.

It doesn't seem to like op-amps, just switched capacitors.
It only displays op-amps if a use a high frequency for Fs and Fc.

Designing a low pass filter with an op-amp seems impossible with that app.
I could be using it wrong though.
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Dougster



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PostPosted: Sun Dec 16, 2012 2:30 am    Post subject: Reply with quote  Mark this post and the followings unread

acidblue wrote:
Anybody have any experience with Linear Tech's Filter Cad??
http://www.linear.com/designtools/software/#Filter

Umm...

"to create filters using Linear Technology's monolithic filter ICs"

Have you looked at any of the datasheets for those filter chips?

Regards,
Doug

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abovenyquist



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PostPosted: Tue Dec 25, 2012 1:13 am    Post subject: Reply with quote  Mark this post and the followings unread

Dougster wrote:
I believe you'll get more out of a copy of Electronotes than you will from a four year degree, unless of course you're taking class from Aaron Lanterman...!


Thank you for your kind words! Smile
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Dougster



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PostPosted: Wed Dec 26, 2012 7:33 am    Post subject: Reply with quote  Mark this post and the followings unread

abovenyquist wrote:
Dougster wrote:
I believe you'll get more out of a copy of Electronotes than you will from a four year degree, unless of course you're taking class from Aaron Lanterman...!


Thank you for your kind words! Smile

And thanks for publishing your videos!

BTW, did you ever get my email about the OB-MX?

Regards,
Doug

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bubzy



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PostPosted: Thu Dec 27, 2012 9:25 am    Post subject: Reply with quote  Mark this post and the followings unread

im a bit late in this conversation, but just want to reinforce what doug and scott have said.

im pretty dumb when it comes to electronics, sure i can stuff a board and solder at 100mph, but theory in the main part, escapes me. my brain is like a fishing net with holes in, most stuff gets through, but now and then something big gets stuck in there and i remember it.

please keep asking questions here, its not a sign of weakness or of stupidity, i've learned so much from the guys here and in the forum.

and as for *WHAT* kind of filter you should build. in my opinion you should build at least one of everything Smile this also applies to lunettas, vco's vca's, sequencers, sample and holds, waveshapers................*fades off muttering synthesizer stuff*

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Dougster



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PostPosted: Thu Dec 27, 2012 2:24 pm    Post subject: Reply with quote  Mark this post and the followings unread

bubzy wrote:
and as for *WHAT* kind of filter you should build. in my opinion you should build at least one of everything Smile this also applies to lunettas, vco's vca's, sequencers, sample and holds, waveshapers................*fades off muttering synthesizer stuff*

Heehee! 'xactly!!!

Maybe I should amend my signature to be any "DIY", not just modulars...

Regards,
Doug

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