electro-music.com   Dedicated to experimental electro-acoustic
and electronic music
 
    Front Page  |  Articles  |  Radio
 |  Media  |  Forum  |  Wiki  |  Links  |  Store
Forum with support of Syndicator RSS
 FAQFAQ   CalendarCalendar   SearchSearch   MemberlistMemberlist   UsergroupsUsergroups   LinksLinks
 RegisterRegister   ProfileProfile   Log in to check your private messagesLog in to check your private messages   Log inLog in  Chat RoomChat Room 
Live streaming at radio.electro-music.com

  host / artist show at your time

poster

Please visit the chat
 Forum index » DIY Hardware and Software » Developers' Corner
Passive Filter not attenuating
Post new topic   Reply to topic Moderators: DrJustice
Page 1 of 1 [13 Posts]
View unread posts
View new posts in the last week
Mark the topic unread :: View previous topic :: View next topic
Author Message
acidblue



Joined: Jun 26, 2009
Posts: 225
Location: The Darkside

PostPosted: Mon Aug 06, 2018 1:08 pm    Post subject: Passive Filter not attenuating Reply with quote  Mark this post and the followings unread

I have a simple saw Osc. going thru a passive low pass filter.
but the filter isn't attenuating the signal.
I put the filter after R4 on the schem.
According to my calculations the filter should be attenuating at 338 hz.
But i'm getting no attenuation.
I've hooked my Pico scope and my saw goes up to roughly 2.86khz. max.

Even going thru the filter it still maxs out at 2.86khz.

What am I doing wrong?


passive filter.png
 Description:
 Filesize:  2.56 KB
 Viewed:  22 Time(s)
This image has been reduced to fit the page. Click on it to enlarge.

passive filter.png



SimpleSawtoothVCO.jpeg
 Description:
 Filesize:  58.36 KB
 Viewed:  23 Time(s)
This image has been reduced to fit the page. Click on it to enlarge.

SimpleSawtoothVCO.jpeg


Back to top
View user's profile Send private message
Grumble



Joined: Nov 23, 2015
Posts: 827
Location: Netherlands
Audio files: 27

PostPosted: Mon Aug 06, 2018 2:57 pm    Post subject: Reply with quote  Mark this post and the followings unread

I can think of two things:
1 - one lead of the capacitor of your filter is not connected.
2 - your capacitor isn’t 47nF but 47pF

_________________
my synth
Back to top
View user's profile Send private message Visit poster's website
acidblue



Joined: Jun 26, 2009
Posts: 225
Location: The Darkside

PostPosted: Mon Aug 06, 2018 6:15 pm    Post subject: Reply with quote  Mark this post and the followings unread

Grumble wrote:
I can think of two things:
1 - one lead of the capacitor of your filter is not connected.
2 - your capacitor isn’t 47nF but 47pF


#2 is the first thing i thought of and checked it on my DMM, it's definitely a 47nF.

#1 I'll have to check later as im away from my bench.
Any more ideas?
Back to top
View user's profile Send private message
Grumble



Joined: Nov 23, 2015
Posts: 827
Location: Netherlands
Audio files: 27

PostPosted: Mon Aug 06, 2018 10:31 pm    Post subject: Reply with quote  Mark this post and the followings unread

maybe you misplaced the probe of your scope? (before the 10k instead of after)
_________________
my synth
Back to top
View user's profile Send private message Visit poster's website
PHOBoS



Joined: Jan 14, 2010
Posts: 4105
Location: Moon Base
Audio files: 607

PostPosted: Tue Aug 07, 2018 8:36 am    Post subject: Reply with quote  Mark this post and the followings unread

did you connect grounds of the VCO and filter together ?
_________________
"My perf, it's full of holes!"
http://phobos.000space.com/
SoundCloud BandCamp MixCloud Stickney Synthyards Captain Collider
Back to top
View user's profile Send private message Visit poster's website AIM Address Yahoo Messenger MSN Messenger
JovianPyx



Joined: Nov 20, 2007
Posts: 1714
Location: West Red Spot, Jupiter
Audio files: 195

PostPosted: Tue Aug 07, 2018 10:34 am    Post subject: Reply with quote  Mark this post and the followings unread

PHOBoS wrote:
did you connect grounds of the VCO and filter together ?


Good point as the way the VCO schematic is drawn it's not clear that it is powered by a dual supply, not a single 9v battery or single voltage wallwart supply.

You can see this in the layout since it shows a +9v feed, a -9v feed and a zero volt feed.

I'd prefer to see this made obvious in the schematic, but that is not the "style" used here.

The VCO needs +9v, 0v (this is ground) and -9v. That can be accomplished with two 9 volt batteries or a properly designed dual 9volt supply. And surely, if the ground is not connected at all, the passive filter will not work properly.

_________________
FPGA, dsPIC and Fatman Synth Stuff

Time flies like a banana.
Fruit flies when you're having fun.
BTW, Do these genes make my ass look fat?
corruptio optimi pessima
Back to top
View user's profile Send private message Visit poster's website
acidblue



Joined: Jun 26, 2009
Posts: 225
Location: The Darkside

PostPosted: Tue Aug 07, 2018 10:51 am    Post subject: Reply with quote  Mark this post and the followings unread

PHOBoS wrote:
did you connect grounds of the VCO and filter together ?


Yes there on the same proto board.
I tried different cap values all the way down to .22uF and it seems to attenuate the volume not the frequency-- this is the nature of passive filters, but it should be filtering the frequency as well

JovianPyx wrote:
Good point as the way the VCO schematic is drawn it's not clear that it is powered by a dual supply, not a single 9v battery or single voltage wallwart supply.


I'm powering this with a bench power supply +-12v.
I clearly have the cap coming from the 10k resistor to 0v.
Back to top
View user's profile Send private message
Grumble



Joined: Nov 23, 2015
Posts: 827
Location: Netherlands
Audio files: 27

PostPosted: Tue Aug 07, 2018 10:59 am    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
it seems to attenuate the volume not the frequency-- this is the nature of passive filters, but it should be filtering the frequency as well

No! The frequency of your signal stays the same even when you change values of your filter. The frequency is set by the saw oscillator, the filter just, well, filters the signal with a certain roll-off.
Only filters that self-oscillate change frequency when you change capacitor values.

_________________
my synth
Back to top
View user's profile Send private message Visit poster's website
JovianPyx



Joined: Nov 20, 2007
Posts: 1714
Location: West Red Spot, Jupiter
Audio files: 195

PostPosted: Tue Aug 07, 2018 11:10 am    Post subject: Reply with quote  Mark this post and the followings unread

Grumble wrote:
Quote:
it seems to attenuate the volume not the frequency-- this is the nature of passive filters, but it should be filtering the frequency as well

No! The frequency of your signal stays the same even when you change values of your filter. The frequency is set by the saw oscillator, the filter just, well, filters the signal with a certain roll-off.
Only filters that self-oscillate change frequency when you change capacitor values.


Yes, and it's probably good to think of a filter when self oscillating as an oscillator more than a filter. However, this passive filter can't oscillate ever, so it will always work like a filter.

This particular filter is a single pole passive low pass filter. It has a cutoff frequency (Fc) which in this case is about 338 Hz. When the VCO outputs 338 Hz, the filter's output will be 3 dB below the input amplitude. As the frequency of the VCO increases, the output of the filter decreases (because it is a low pass filter). Filters like this will always lower amplitude when the input signal's frequency is greater than Fc. The more it is above Fc, the more it attenuates.

As Grumble stated, a filter (not self osc) will never change the frequency of the signal.

_________________
FPGA, dsPIC and Fatman Synth Stuff

Time flies like a banana.
Fruit flies when you're having fun.
BTW, Do these genes make my ass look fat?
corruptio optimi pessima
Back to top
View user's profile Send private message Visit poster's website
PHOBoS



Joined: Jan 14, 2010
Posts: 4105
Location: Moon Base
Audio files: 607

PostPosted: Tue Aug 07, 2018 11:12 am    Post subject: Reply with quote  Mark this post and the followings unread

have you tried listening to it or only viewed it on your scope ?
_________________
"My perf, it's full of holes!"
http://phobos.000space.com/
SoundCloud BandCamp MixCloud Stickney Synthyards Captain Collider
Back to top
View user's profile Send private message Visit poster's website AIM Address Yahoo Messenger MSN Messenger
acidblue



Joined: Jun 26, 2009
Posts: 225
Location: The Darkside

PostPosted: Tue Aug 07, 2018 11:20 am    Post subject: Reply with quote  Mark this post and the followings unread

PHOBoS wrote:
have you tried listening to it or only viewed it on your scope ?


I also have it hooked to a guitar practice amp, one of the small cheap ones.
There is a volume drop as you expect from a passive filter, I hear very little change in the way of frequency, again i suppose that what I should expect from a passive filter.
Back to top
View user's profile Send private message
JovianPyx



Joined: Nov 20, 2007
Posts: 1714
Location: West Red Spot, Jupiter
Audio files: 195

PostPosted: Tue Aug 07, 2018 12:00 pm    Post subject: Reply with quote  Mark this post and the followings unread

acidblue wrote:
PHOBoS wrote:
have you tried listening to it or only viewed it on your scope ?


I also have it hooked to a guitar practice amp, one of the small cheap ones.
There is a volume drop as you expect from a passive filter, I hear very little change in the way of frequency, again i suppose that what I should expect from a passive filter.


That is precisely what you should expect from a passive filter.

And it seems that your filter is working as expected if you hear amplitude drop as you increase either the R or the C. You'll see this easily demonstrated if you replace the 10K resistor with a 1K resistor in series with a 10K variable resistor (center and one side). When you turn the knob, it will work very much like a "tone control" because you are changing Fc of the filter. Given a signal with lots of harmonics, it will more and more remove those harmonics making it sound less crisp and more sine-like as you lower Fc.

To change the frequency, other methods can be applied such as wave shaping, but in no way does a non-self-osc filter ever change the frequency of the input signal.

_________________
FPGA, dsPIC and Fatman Synth Stuff

Time flies like a banana.
Fruit flies when you're having fun.
BTW, Do these genes make my ass look fat?
corruptio optimi pessima
Back to top
View user's profile Send private message Visit poster's website
Grumble



Joined: Nov 23, 2015
Posts: 827
Location: Netherlands
Audio files: 27

PostPosted: Tue Aug 07, 2018 12:46 pm    Post subject: Reply with quote  Mark this post and the followings unread

This website: http://sim.okawa-denshi.jp/en/ is always very helpfull in designing analog circuits and in this case especially the Filter tool ( http://sim.okawa-denshi.jp/en/Fkeisan.htm )
_________________
my synth
Back to top
View user's profile Send private message Visit poster's website
Display posts from previous:   
Post new topic   Reply to topic Moderators: DrJustice
Page 1 of 1 [13 Posts]
View unread posts
View new posts in the last week
Mark the topic unread :: View previous topic :: View next topic
 Forum index » DIY Hardware and Software » Developers' Corner
Jump to:  

You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You cannot attach files in this forum
You can download files in this forum
e-m mkii

Please support our site. If you click through and buy from
our affiliate partners, we earn a small commission.


Forum with support of Syndicator RSS
Powered by phpBB © 2001, 2005 phpBB Group
Copyright © 2003 through 2009 by electro-music.com - Conditions Of Use