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 Forum index » DIY Hardware and Software
Triangle Core VCO w/ thru-zero
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GlassX



Joined: Jan 27, 2008
Posts: 50
Location: Brazil

PostPosted: Sun Jan 27, 2008 9:32 pm    Post subject: Triangle Core VCO w/ thru-zero
Subject description: VCO
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Hello, friends! I've been lurking for a while, but never had the guts to post anything here until now.

I'm a musician and recently became interested in synthesizers. Being a Buchla fan, I wanted a triangle core VCO, but didn't want to deal with transcondutance amplifiers (discrete or 3080) or current mirrors, so I challenged myself to build my own design.




Ok, here it goes...

My circuit uses two operational amplifiers, one being the integrator (in a weird configuration using a JFET to change polarity of the current - and thus the direction of the triangle wave) and the schmitt trigger. The JFET+integrator topology is a novel configuration I found on some old textbooks. This is my main concern, because on EVERY oscillator I found (apart from perhaps the ARP2500) there are no resistors between the exponential current sink and the VCO capacitor.

Now on the thru-zero... It felt a bit like Doc Brown inventing the flux-capacitor, I just thought about it and it just worked when I tested...

It's based around the Electronotes circuit on Bernie's page... but instead of two op-amps and a flip flop I'm using just a single schmitt trigger.

The schmitt trigger (as everyone knows) swings the output (and thus the triangle wave direction) the moment the input reaches threshold. To arbitrarily swing the direction (as on a thru-zero VCO), we have to momentarily reduce the threshold... I use a JFET for it... and it works...



I'd like to get everyones opinion on this... because I don't really know if it's a good design or not. I'm still learning about synthesizers... I started about a month ago, so, please help me. It works perfectly on the simulator (the software even renders a WAVE file), but I don't know about the real life.

Thanks for the attention!


VCO.jpg
 Description:
Here's the SPICE (LTSPICE/SwCAD actually) screenshot. The blue spikes change the direction.
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VCO.jpg



VCO Schematic.jpg
 Description:
Sorry about the 2N2222 and LT1001... I did the schematic in a hurry and just used whatever was available =]
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VCO Schematic.jpg


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andrewF



Joined: Dec 29, 2006
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PostPosted: Mon Jan 28, 2008 5:58 am    Post subject: Reply with quote  Mark this post and the followings unread

Hello GlassX

welcome welcome welcome

Nice work - what is going on in the 'scope picture towards the right where the waveform seem to break up?
Anyway it sure looks like some thru-0 action!

Do you plan to build this cct with components?
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GlassX



Joined: Jan 27, 2008
Posts: 50
Location: Brazil

PostPosted: Mon Jan 28, 2008 10:17 am    Post subject: Reply with quote  Mark this post and the followings unread

andrewF wrote:
Nice work - what is going on in the 'scope picture towards the right where the waveform seem to break up?
Anyway it sure looks like some thru-0 action!

Do you plan to build this cct with components?


Thanks for the welcome Smile

To the right? I think the frequency is increasing, because I wanted to emphasize the time-reversal effect (I haven't designed the zero-crossing detector yet)... by the way, the pictures are heavily aliased, 'cause I don't have Photoshop here, just MSPaint... Sad

In theory the time-reversal JFET seems to work! I know that thru-zero became some of a marketing catchphrase today, but I just wanted to make it into something as easy to implement as sync...

I've breadboarded it, but I still need an oscilloscope to calibrate it... hope it works as good as on the software Smile
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Blue Hell
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PostPosted: Mon Jan 28, 2008 11:08 am    Post subject: Reply with quote  Mark this post and the followings unread

GlassX wrote:
by the way, the pictures are heavily aliased, 'cause I don't have Photoshop here, just MSPaint... Sad


No, the forum software does it, when you click on the pics they'll show OK though Very Happy

And welcome

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frijitz



Joined: May 04, 2007
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PostPosted: Mon Jan 28, 2008 1:18 pm    Post subject: Re: Triangle Core VCO w/ thru-zero
Subject description: VCO
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GlassX wrote:

My circuit uses two operational amplifiers, one being the integrator (in a weird configuration using a JFET to change polarity of the current - and thus the direction of the triangle wave) and the schmitt trigger. The JFET+integrator topology is a novel configuration I found on some old textbooks. This is my main concern, because on EVERY oscillator I found (apart from perhaps the ARP2500) there are no resistors between the exponential current sink and the VCO capacitor.

welcome
I've seen this configuration for a voltage controlled oscillator but never with current control. How does it work?

Very Happy

Ian
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GlassX



Joined: Jan 27, 2008
Posts: 50
Location: Brazil

PostPosted: Mon Jan 28, 2008 2:57 pm    Post subject: Re: Triangle Core VCO w/ thru-zero
Subject description: VCO
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frijitz wrote:
GlassX wrote:

My circuit uses two operational amplifiers, one being the integrator (in a weird configuration using a JFET to change polarity of the current - and thus the direction of the triangle wave) and the schmitt trigger. The JFET+integrator topology is a novel configuration I found on some old textbooks. This is my main concern, because on EVERY oscillator I found (apart from perhaps the ARP2500) there are no resistors between the exponential current sink and the VCO capacitor.

welcome
I've seen this configuration for a voltage controlled oscillator but never with current control. How does it work?

Very Happy

Ian


Hi Ian and Blue Hell!

Ian, this was my main doubt, actually...

I don't have much info on current-controlled oscillators apart from what I found on old patents and synthesizer circuits (your site was of tremendous help). I don't know about this yet, but I'm trying to find something in my book! It's been just a month that I decided I wanted to learn everything about analog circuits, so I'm just starting Smile

(EDIT) I've done some calculations and read some stuff here, and R3/R4 are acting as "passive current-to-voltage sources". It seens okay math-wise, but I'm still trying to find more info about the stability and accuracy of such circuit... Confused

Cheers!
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synthmonger



Joined: Nov 16, 2006
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PostPosted: Sun May 16, 2010 2:23 am    Post subject: Reply with quote  Mark this post and the followings unread

*cough* bump *cough*
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v-un-v
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PostPosted: Sun May 16, 2010 2:29 am    Post subject: Reply with quote  Mark this post and the followings unread

synthmonger wrote:
*cough* bump *cough*


Except that GlassX has disappeared (aparently, and according to some dude from NYC who flambéed me in a PM, because he was miffed that GlassX had gone awol and was nowhere to be seen Shocked ) Confused

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synthmonger



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PostPosted: Sun May 16, 2010 2:31 am    Post subject: Reply with quote  Mark this post and the followings unread

Ah really? Has anyone breadboarded this or tested it further? Seems too simple to pass up. I'd be willing to do a layout if it's okee dokee with GlassX Smile
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synthmonger



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PostPosted: Thu Jun 10, 2010 3:10 pm    Post subject: Reply with quote  Mark this post and the followings unread

breadboarded this with no thru-zero luck. It'll act as a standard sawtooth oscillator, but not triangle. Not sure how that happened exactly...Any thoughts?
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PostPosted: Thu Jun 10, 2010 4:04 pm    Post subject: Reply with quote  Mark this post and the followings unread

synthmonger wrote:
...Any thoughts?


Yes. Buy Ian's Teezer kit! Idea Wink

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synthmonger



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PostPosted: Thu Jun 10, 2010 4:07 pm    Post subject: Reply with quote  Mark this post and the followings unread

But I wanna make this one work Sad
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