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 Forum index » DIY Hardware and Software
Perculiar problem with simple waveshaping circuit
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JingleJoe



Joined: Nov 10, 2011
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PostPosted: Sun Aug 12, 2012 3:09 am    Post subject:  Perculiar problem with simple waveshaping circuit Reply with quote  Mark this post and the followings unread

I'm making a simple waveshaping circuit, see the attachment for a circuit diagram, it's pretty simple. It does three main things:

1 Creates PWM from a saw wave input
2 Inverts the saw wave
3 Mixes the two together

However, there is a problem. When the switch is switched to position 2, to select the inverted saw wave, the output waveform gets an odd blip at the bottom Neutral (see diagram, bottom right)
Everything is fine untill I flip the switch, I checked the output of the regular saw and inverted saw sections and they were both fine beforehand.
The blip appears on both the inverted saw and PWM comparator op amp outputs.

I have not yet tried to solve this problem however I have some ideas but wanted to get your help too! (you may know some secret I don't Wink)

I'm thinking this problem might be caused by outputs of two different op amps on the same IC being connected together (regular saw[or reg s] is coming from a different op amp [op amp B]). However I thought the 100k resistance between them would be fine, it's fine for the regular saw coming from op amp B. Damn you op amp A, stop being so picky.
I plan to increase this resistance and see what happens.
I also plan to buffer the output of the saw inverter output with the remaining op amp on the IC.
I'm hoping I don't have to use a separate IC to buffer the saw inverter or something like that Confused

I should mention too that the supply is 0 to +12V and in the diagram the saw waves marked R and I are the Regular and Inverted saw waves, respectively.


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As a mad scientist I am ruled by the dictum of science: "I could be wrong about this but lets find out"


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JingleJoe



Joined: Nov 10, 2011
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Location: Lancashire, England
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PostPosted: Sun Aug 12, 2012 4:40 am    Post subject: Reply with quote  Mark this post and the followings unread

oh wait, I think I just figured out what it might need: a resistor to ground on the inverted saw output or A3 input.

Anyways, more things for me to test later.

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JingleJoe



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PostPosted: Sun Aug 12, 2012 6:41 am    Post subject: Reply with quote  Mark this post and the followings unread

Seems I solved my own problem already Laughing but it may be a good idea to leave this thread here for posterity.

I noticed the glitch was happening at the other side of the square wave too but it was only visible when it was at a pulse width of less than 10% because otherwise theoutput was at maximum voltage so there was no room for the glitch!
Turns out it changes sides when I flip the saw-invert switch which means it was something to do with the saw wave all long.
I still don't quite know exactly what it was but the solution was to increase the resistance of the mixing potentiometer to 1 Meg.
This leads me to suspect that it was some kind of interference between the saw wave output and PWM comparator output. Still a bit of a mystery though!

P.S. Adding pull down resistors to ground on the outputs and voltage follower(A3) input did nothing.

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elmegil



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PostPosted: Sun Aug 12, 2012 7:02 am    Post subject: Reply with quote  Mark this post and the followings unread

Posterity is good Smile
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JingleJoe



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PostPosted: Sun Aug 12, 2012 10:33 am    Post subject: Reply with quote  Mark this post and the followings unread

For posterity:

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