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 Forum index » DIY Hardware and Software
A Different Kind of DCO
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Cynosure
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Joined: Dec 11, 2010
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Location: Toronto, Ontario - Canada
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PostPosted: Sun Nov 23, 2014 9:46 am    Post subject: A Different Kind of DCO
Subject description: CMOS PLL Synced Oscillator
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I almost posted this in the Lunetta forum, but I think that it is something that might be useful for any type of synth.

Read all the details in my blog post:

http://www.jacobwatters.com/blog/a-different-kind-of-dco/

The overall summary of the circuit is a 4046 phase locked loop that uses an external VCO. The VCO can be any type that you want.

This would pair well with an Arduino or PIC, where there is no onboard DAC. The Arduino has a Tone function that creates a square wave tone by 'bit banging' on one of its output pins.

The intended purpose was to convert square waves from Lunetta synths into sawtooth waves. The attached schematic uses a triangle oscillator, but any VCO can be used.

My next step is to convert it to a dual rail power supply.


sqr2sawtri.png
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CMOS PLL Synced Oscillator
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sqr2sawtri.png


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LFLab



Joined: Dec 17, 2009
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Location: Rosmalen, Netherlands

PostPosted: Tue Nov 25, 2014 4:12 am    Post subject: Reply with quote  Mark this post and the followings unread

Interesting!
I am trying to wrap my head around this. So what you are saying is ditch the cmos oscillator which is there now, and replace with the output of an arduino?
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Cynosure
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PostPosted: Tue Nov 25, 2014 6:13 am    Post subject: Reply with quote  Mark this post and the followings unread

LFLab wrote:
Interesting!
I am trying to wrap my head around this. So what you are saying is ditch the cmos oscillator which is there now, and replace with the output of an arduino?

Yes. Replace the 40106 oscillator with the output from an Arduino.

If you want, you can also replace the VCO section on the right side of the schematic with any VCO that you want. I tested it with a simple LM324 triangle VCO, but it should work with any VCO.

And the cool part is that there is no expo converter or tempco resistor required. The PLL keeps it all synced to the output from the Arduino.
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LFLab



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PostPosted: Tue Nov 25, 2014 6:27 am    Post subject: Reply with quote  Mark this post and the followings unread

Very interesting, what happens outside of the frequency range for C5? Too low a frequency and the output wave becomes more of a pulse shape, too high a frequency and the cap cannot be drained quickly enough?

So how does this handle frequency modulation, you'd need to handle this in the arduino right? Not in the analogue section I assume?
Really interested to hear the results of your next steps (dual supply, saw core, Arduino).

Was going to mention the Arp AXXE VCO done by Tom Gamble, it is very simple and it worked right away in my build (better so when I put in the right FET), but it really needs some work (to levels of both outputs equal).
Another option could be:
Posted Image, might have been reduced in size. Click Image to view fullscreen.

The Arp 4027 core, but can you just use the saw output as input for the PLL? Or would a squarewave output be better?
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Cynosure
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PostPosted: Tue Nov 25, 2014 2:44 pm    Post subject: Reply with quote  Mark this post and the followings unread

LFLab wrote:
Very interesting, what happens outside of the frequency range for C5?

The VCO will just not go any higher or lower than the frequency range indicated in the schematic. IT doesn't change the shape of the output. However, changing R11 will change the shape of the wave. These are things specific to the VCO I tested it with.

LFLab wrote:
So how does this handle frequency modulation, you'd need to handle this in the arduino right?

Yes, frequency modulation is most easily done in to the source square (Arduino if that is what you are using). You can probably do some stuff with the PLL to make it go out of sync and then allow it to go back in sync.

Also, increasing the value of C3 and/or decreasing the value of R4 should add portamento. However, that can also be handled in an Arduino.

LFLab wrote:
can you just use the saw output as input for the PLL? Or would a squarewave output be better?

The 4046 PLL works best with a logic level square wave. If you only have a saw, then a simple comparator will convert it into a square.

There is a simple VCO in the LM13700 datasheet. I was thinking about giving that a try when I make a dual rail version.

I am a bit busy right now, and the holidays are quickly approaching, so I don't know when I will get around to doing more testing. I will post my results here when I do get to it.
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LFLab



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PostPosted: Thu Nov 27, 2014 1:37 am    Post subject: Reply with quote  Mark this post and the followings unread

thumright
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commathe



Joined: Jul 26, 2013
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PostPosted: Sun Nov 30, 2014 7:05 pm    Post subject: Reply with quote  Mark this post and the followings unread

Nice! Definitely going to consider playing around with this
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commathe



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PostPosted: Tue Dec 02, 2014 7:29 pm    Post subject: Reply with quote  Mark this post and the followings unread

Was thinking about this circuit a little more. Have you considered replacing the 40106 that squares up the saw wave with a comparator? A schmitt trigger will produce a non 50/50 square wave when fed a saw. That might be the source of your limited frequency tracking. The phase comparator output pulses will probably be becoming too narrow at the higher frequencies to properly charge the cap producing the CV
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Cynosure
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PostPosted: Tue Dec 02, 2014 9:04 pm    Post subject: Reply with quote  Mark this post and the followings unread

commathe wrote:
Have you considered replacing the 40106 that squares up the saw wave with a comparator? A schmitt trigger will produce a non 50/50 square wave when fed a saw.

Are you referring to U3? That is a squarewave going into the schmitt trigger.

The LM324 output peaks at around 3.5V on a single rail 5V supply. I used a schmitt trigger to ensure that it got the squarewave all the way up to 5V.

It can probably be left out when using higher voltages.
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commathe



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PostPosted: Tue Dec 02, 2014 9:40 pm    Post subject: Reply with quote  Mark this post and the followings unread

Aaah. I misread the schematic. I thought you were feeding the saw into U3 to create a logic level signal from the saw. I didn't realise it was already a squared up input,
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