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Passive Filter not attenuating
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acidblue



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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?


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Grumble



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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

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acidblue



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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?
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Grumble



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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)
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PHOBoS



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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 ?
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JovianPyx



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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.

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acidblue



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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.
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Grumble



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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.

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JovianPyx



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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.

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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 ?
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acidblue



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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.
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JovianPyx



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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.

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Grumble



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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 )
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