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 Forum index » DIY Hardware and Software » Thomas Henry designs
The VCO Maximus
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halfshavedyaks



Joined: Jul 12, 2018
Posts: 28
Location: UK

PostPosted: Sun Aug 26, 2018 2:49 pm    Post subject: Reply with quote  Mark this post and the followings unread

I replaced R19 with 56k, even though my chip says CEM3340 not AS3340

it improved it considerably the error is now less than 10 cents - still not usable but much better.

I think i need to try with a different power supply - which I don't have.
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halfshavedyaks



Joined: Jul 12, 2018
Posts: 28
Location: UK

PostPosted: Sun Aug 26, 2018 2:55 pm    Post subject: Reply with quote  Mark this post and the followings unread

can the maximus be adapted to sync to a positive trigger? would changing the direction of D1 achieve that or is a positive voltage at pin 6 of the 3340 a bad idea?
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halfshavedyaks



Joined: Jul 12, 2018
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PostPosted: Sun Sep 09, 2018 5:43 pm    Post subject: Reply with quote  Mark this post and the followings unread

I looked up the CEM3340 datasheet and it is quite clear that the maximus can't do conventional hard sync.

The datasheet does give an implementation for that but the maximus doesn't use it.
The sync on the maximus doesn't reset but rather changes the direction of the core.

no wonder it seemed wrong it's a different thing entirely.
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halfshavedyaks



Joined: Jul 12, 2018
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Location: UK

PostPosted: Wed Sep 12, 2018 1:27 pm    Post subject: Reply with quote  Mark this post and the followings unread

Proper hard sync that resets the oscillator to zero rather than just reversing the direction is essential for me so I have implemented the conventional hard sync as outlined in the CEM3340 datasheet.

Posted Image, might have been reduced in size. Click Image to view fullscreen.

It works, but it only responds to a negative pulse (as expected)

It is also finicky about the level of the pulse.

has anyone else done this?

I don't actually understand this circuit - since I don't know what's going on inside the 3340.

does anyone know how to make it respond to a positive rather than negative pulse? (other than inverting the pulse first with an attenuverter)

EDIT I used a BC557 - just what I had on hand I'm not sure how appropriate it is.
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halfshavedyaks



Joined: Jul 12, 2018
Posts: 28
Location: UK

PostPosted: Wed Sep 12, 2018 1:30 pm    Post subject: Reply with quote  Mark this post and the followings unread

I have tried some more to improve the PWM pitch shift but with no luck.

I did however notice that flexing the board by pressing on the edges creates a pitch shift. Is that normal or could it be a clue to a problem?
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halfshavedyaks



Joined: Jul 12, 2018
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PostPosted: Wed Sep 12, 2018 1:37 pm    Post subject: Reply with quote  Mark this post and the followings unread

this is a useful page - it also has schematics that are much clearer showing the hard sync circuit.

https://electricdruid.net/cem3340-vco-voltage-controlled-oscillator-designs/
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elmegil



Joined: Mar 20, 2012
Posts: 2048
Location: Chicago
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PostPosted: Wed Sep 12, 2018 3:57 pm    Post subject: Reply with quote  Mark this post and the followings unread

might be worth putting a comparator between your pulse input and this? Up to you whether you invert the incoming pulse at the same time...
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halfshavedyaks



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PostPosted: Thu Sep 13, 2018 10:02 am    Post subject: Reply with quote  Mark this post and the followings unread

elmegil wrote:
might be worth putting a comparator between your pulse input and this? Up to you whether you invert the incoming pulse at the same time...


I can see the benefit in that but I'm not really confident about how to do it. I can sort of imagine it with an opamp inverting comparator - but I'm a little beyond my expertise to actually design a suitable circuit.

also i really don't have space to add an opamp, unless I make a whole daughterboard for the mod.

would anyone be willing to draw a suitable circuit?

I had hoped there would be an easy way to change the standard circuit to respond to a positive rather than negative pulse.
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elmegil



Joined: Mar 20, 2012
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PostPosted: Thu Sep 13, 2018 11:26 am    Post subject: Reply with quote  Mark this post and the followings unread

normally to change it you'd invert it. You can do that part with a transistor inverter ... which could also then drive a stronger signal, so that *might* work.

See the response with the NPN and PNP transistors and 5V supply here:

https://electronics.stackexchange.com/questions/30238/how-to-invert-a-digital-signal

Instead of 5V use 12V.
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halfshavedyaks



Joined: Jul 12, 2018
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Location: UK

PostPosted: Fri Sep 14, 2018 10:10 am    Post subject: Reply with quote  Mark this post and the followings unread

I've done it with an opamp. I used a TL071 - maybe I will change to a LM311 later although it isn't really necessary.

It works fine, but there's a slight catch - the oscillator isn't reset to 0 or to max negative but to the output level of the comparator, which is as close to the negative rail as the TL071 can get.

Since the tri wave only does +/- 5v this means there's a big negative spike on every reset - so you have to trim the comparator output to the level you want it reset to, which I guess is -5v

using the comparator however at least means that you can control that level rather than being at the mercy of the sync signal input level.
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