How to convert this oscillator to VCO ?

Thread Starter

GTelly

Joined Jul 8, 2020
3
Hello,

This is my first post ever on this forum :) During the lockdown, I decided to relaunch one of my very old project: building a modular synthesizer. Not by copying without knowing what I'm doing but by trying to design by myself (not starting from scratch though). I read a lot of books including make synthesizers of the art of electronics focusing on the basics and oscillators.

In my learning quest, I found this nice oscillator design that seems good for the first module of my synth: the VCO. (so yes, it's a copy, but it's for learning purposes :) )

Yet, probably there's something I don't understand yet : how can I transform this oscillator to a VCO ?
I mean: modifying R5/C1 permit to change the frequency. But how can I change it by increment of volts through a control voltage (all I want for now is a linear control voltage in the audio range)?
I think to FET as a resistor (not linear) and I tried to put a current source at various point without much success... (I'm ready to be ashamed :)) )

Thank you for your help !

VCO.png
 

OBW0549

Joined Mar 2, 2015
3,454
In my learning quest, I found this nice oscillator design that seems good for the first module of my synth: the VCO. (so yes, it's a copy, but it's for learning purposes :) )
What's "nice" about it? It looks like a hot mess to me, far too complicated for what little it does.

Yet, probably there's something I don't understand yet : how can I transform this oscillator to a VCO ?
You can't. Conventional oscillators and voltage-controlled oscillators are RADICALLY different things.

But how can I change it by increment of volts through a control voltage (all I want for now is a linear control voltage in the audio range)?
Forget it. Either use the circuit above posted by @Audioguru again or use something like an XR2206 (available through Jameco, Mouser or Sparkfun).
 

ci139

Joined Jul 11, 2016
1,568
as far as i get it's a pretty cool comparator circuit . . . the R10 R11 adjusting duty ratio , but it's an unsafe option as the oscillations may stop at certain setting . . .

the VCO design depends on what features of the existing circuit you want to preserve

. . . a bad idea , but a one possibility http://tinyurl.com/y7y2ft82
 
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Papabravo

Joined Feb 24, 2006
13,939
I decided to play around with the above mentioned VCO and discovered to my surprise that the LM358 model in my LTspice library was crashing the simulator. That's a bad bad thing. I recently installed the collection of Alex @Bordodynov , and using his functional model of the LM358 things came back to normal. I include the .asc file in case anybody wants to check their own LM358 model.

VCO_AB.png

Let me know if you stumble across anything interesting.
Спасибо, Alex.
 

Attachments

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Papabravo

Joined Feb 24, 2006
13,939
In case you're bored to tears, and would like to test your Analog IQ. What component in the VCO of post #5 in this thread determines the maximum frequency that you can get from this VCO?
  1. Is it capacitor C1?
  2. Is it the LM358_AB?
  3. Is it Q1?
  4. Is is any of the resistors R1 thru R7?
  5. Would a real part behave differently than the simulation?
I think I know the answer, but I'd be interested to hear your opinion. go ahead and run the simulation if you like - all's fair in Analog Contesting.
If you'd like any of the supporting files I can post those as well, but only on request.
 
Last edited:

Zeeus

Joined Apr 17, 2019
597
Hello,

This is my first post ever on this forum :) During the lockdown, I decided to relaunch one of my very old project: building a modular synthesizer. Not by copying without knowing what I'm doing but by trying to design by myself (not starting from scratch though). I read a lot of books including make synthesizers of the art of electronics focusing on the basics and oscillators.

In my learning quest, I found this nice oscillator design that seems good for the first module of my synth: the VCO. (so yes, it's a copy, but it's for learning purposes :) )

Yet, probably there's something I don't understand yet : how can I transform this oscillator to a VCO ?
I mean: modifying R5/C1 permit to change the frequency. But how can I change it by increment of volts through a control voltage (all I want for now is a linear control voltage in the audio range)?
I think to FET as a resistor (not linear) and I tried to put a current source at various point without much success... (I'm ready to be ashamed :)) )

Thank you for your help !

View attachment 211649
Please post the .asc
Couldn't get it to work
Thanks
 

Zeeus

Joined Apr 17, 2019
597
In case you're bored to tears, and would like to test your Analog IQ. What component in the VCO of post #5 in this thread determines the maximum frequency that you can get from this VCO?
  1. Is it capacitor C1?
  2. Is it the LM358_AB?
  3. Is it Q1?
  4. Is is any of the resistors R1 thru R7?
  5. Would a real part behave differently than the simulation?
I think I know the answer, but I'd be interested to hear your opinion. go ahead and run the simulation if you like - all's fair in Analog Contesting.
If you'd like any of the supporting files I can post those as well, but only on request.

I think the op amp (and it seems circuit not working on spice with R as 100 ohms)

With the comparator like that. change in V (dv) = 4V
Vi input voltage

frequency = Vi/ (4*R*C*dv). R of first op amp

Maximum frequency? then op amp. What frequency do you want?

Anyways, thanks for circuit

And please why does vi have to be 1.5V less than Vcc ?
 
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Thread Starter

GTelly

Joined Jul 8, 2020
3
Hello ! Sure, it seems a nice design.
What's "nice" about it? It looks like a hot mess to me, far too complicated for what little it does.


You can't. Conventional oscillators and voltage-controlled oscillators are RADICALLY different things.


Forget it. Either use the circuit above posted by @Audioguru again or use something like an XR2206 (available through Jameco, Mouser or Sparkfun).
Thank you. By "nice", I meant that it seems to fill my need (triangle / square wave without any amplitude drop on the triangle). Probably, it's a mess and I trust you :)

So now I know that OSC and VCO are something really different. I was unsure of that and it's why I finally decided to ask here after many hours trying to figure out.

OK for the LM358 circuit. I will try that !
 

Alec_t

Joined Sep 17, 2013
11,414
I decided to play around with the above mentioned VCO and discovered to my surprise that the LM358 model in my LTspice library was crashing the simulator.
I found a year or two back that the LM358 model I had would crash the sim if there were two or more instances on the schematic, but the sim would usually run ok if there was only one instance. I now have 3 models in my armoury; from TI. ST and NS respectively. However, whenever I go to use one, I've forgotten which one was best :). The LM324 model has similar issues too.
 
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