Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

The Lounge A place to talk about almost anything but politics and religion.

Low Filter Roll Off Calculation
Low Filter Roll Off Calculation
Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 24th November 2017, 10:21 AM   #21
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Quote:
Originally Posted by SyncTronX
Don't know what either F nor Fc is.
I didn't use F or Fc. 'f' is frequency. 'fc' is corner frequency.

40dB/decade (and 12dB/octave) is the ultimate slope. It will be less than this near the corner frequency.

We still have no context, and only an assurance that the filter is Butterworth from someone who might not know what Butterworth means.
  Reply With Quote
Old 24th November 2017, 02:44 PM   #22
Mark Johnson is offline Mark Johnson  United States
diyAudio Member
 
Mark Johnson's Avatar
 
Join Date: May 2011
Location: Silicon Valley
Low Filter Roll Off Calculation
Quote:
Originally Posted by DF96 View Post
40dB/decade (and 12dB/octave) is the ultimate slope. It will be less than this near the corner frequency.
Unless there is sufficient peaking in the frequency response (Q>sqrt(2)). Then the slope is greater than 40dB/decade near the corner frequency, as the magnitude falls precipitously to catch up to the asymptote. See red curves in #17.

However as someone remarked, the OP may not actually know the possible range of Q values for his filter.
  Reply With Quote
Old 25th November 2017, 10:02 AM   #23
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
He told us that it is Butterworth i.e. no peaking. I don't know how reliable this information is.
  Reply With Quote
Old 25th November 2017, 03:31 PM   #24
JMFahey is offline JMFahey  Argentina
diyAudio Member
 
JMFahey's Avatar
 
Join Date: Mar 2009
Location: Buenos Aires - Argentina
The response approximates/approaches the ideal 12dB/oct well above the cutoff frequency, and it becomes very close , say, 3 or 4 octaves above ... or below .... , BUT at the transition frequency (sorry, I am freely translating from my native Spanish, might not use the "popular" word, please try to focus on the concept Im trying to refer to) slope is not "perfect" by any means, we dont have a sharp angle breakpoint but transition is gradual.

Exactly one octave above we are still in the gradual slope area so result wont be *exactly* 12 dB down but a different (although admittedly close) value.

I guess the problem choice was not random, but to force synctronX do all the Math and justify it, instead of just answering the "trivial solution" such as offered by Abraxalito.
"Trivial solution" in this case being the Math definition of it, not the everyday language meaning of "trivial", of course.
__________________
Design/make/service musical stuff in Buenos Aires, Argentina, since 1969.
  Reply With Quote
Old 25th November 2017, 07:29 PM   #25
SyncTronX is offline SyncTronX  United States
diyAudio Member
 
SyncTronX's Avatar
 
Join Date: Oct 2008
Location: Another Project Enters the Queue Oh My!
Low Filter Roll Off Calculation
@JMFahey,

I thought I found the solution in the first post, or, got close to what I was given. The information given was limited, so I had to do it with what I know.

@Mark, I do not know the filter Q.

@DF96, that is all the information that I was given.

@PRR, Yes, I think this is also correct.

The formula in the OP was all that I could come up with originally, so the .8V could be rounded up to 1V.

Thanks for your contributions. I'll let you know what I find out.

Cheers,
__________________
-- Sync
  Reply With Quote
Old 3rd December 2017, 05:10 AM   #26
SyncTronX is offline SyncTronX  United States
diyAudio Member
 
SyncTronX's Avatar
 
Join Date: Oct 2008
Location: Another Project Enters the Queue Oh My!
Low Filter Roll Off Calculation
Post Script:

Here is the actual question:

"A typical 2-pole low pass Butterworth filter has an output voltage of 4V at 1200 Hz input. Assuming this is beyond the cutoff frequency, what is the output voltage at in input frequency of 2400 Hz."

Answer Group: a) 8V b) 2V c) 1V <<<< d) 4V

But still. I did get that correct even using the wrong formula and after thinking and Abraxilito, PRR, DF96.

JMFahey, It was the trival 40dB/Decade = 12dB/Octave.

Mark, Rayma, MarcelvdG

Thanks for your clarifications also.

Cheers,
__________________
-- Sync
  Reply With Quote
Old 4th December 2017, 10:57 AM   #27
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
So it was a simple textbook or exam question, with multiple choice answers - and all except one being obviously wrong for anyone who understand the question.
  Reply With Quote
Old 4th December 2017, 12:21 PM   #28
JMFahey is offline JMFahey  Argentina
diyAudio Member
 
JMFahey's Avatar
 
Join Date: Mar 2009
Location: Buenos Aires - Argentina
Funny, but you *should* have disclosed the full information from the beginning instead of sending us on a wild Math chase all over the place.
FWIW even "1V" is NOT the exact answer, just "the less wrong" one..

We were thinking fractions of 1% differences while the actual question was "choose between approximate answer and gross 200% , 400% and 800% errors"

Oh well.
__________________
Design/make/service musical stuff in Buenos Aires, Argentina, since 1969.
  Reply With Quote
Old 4th December 2017, 01:19 PM   #29
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Yes, don't try to learn electronics from whoever set this question as he clearly gets confused by details.
  Reply With Quote
Old 4th December 2017, 10:15 PM   #30
SyncTronX is offline SyncTronX  United States
diyAudio Member
 
SyncTronX's Avatar
 
Join Date: Oct 2008
Location: Another Project Enters the Queue Oh My!
Low Filter Roll Off Calculation
And even with the question rephrased...it still brings up uncertainties. I'm in the process of trying to recreate this Comparator question. It's one we haven't seen before, but it was on the final that I just finished.

Here is a schematic that I drew up from memory and Aol=120,000. The standard method of find the Vref doesn't work. Vref = V*R2/R1 + R2. Then we have the output voltage: And another if Vin is 10Vp, which input voltages will have positive max outputs? I'm pretty sure voltage gain in the comparator = Aol. I can't recall what all the answer groups were.

I tried simulation, but got very different answer using AC input and Function Generator and still can't figure it out.

It's a real pisser.

Cheers
__________________
-- Sync
  Reply With Quote

Reply


Low Filter Roll Off CalculationHide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Calculation of Digital Bi-quad Filter Coefficients dshyam91 Digital Line Level 2 27th July 2013 03:19 AM
Bass drivers which cleanly roll-off without a low pass filter. theophile Multi-Way 4 10th February 2011 10:50 AM
Asking for choke value or calculation for CLC filter Groundloops Power Supplies 5 15th September 2009 07:51 PM
Sime filter calculation help needed. Nordic Parts 20 15th November 2006 02:42 PM
SMPS Output filter inductor calculation. corrieb Car Audio 7 28th October 2005 03:47 PM


New To Site? Need Help?

All times are GMT. The time now is 02:50 PM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 14.29%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio
Wiki