# AS3340 VCO /w hard/soft sync, FM modulation

**URL:** <https://lookmumnocomputer.discourse.group/t/as3340-vco-w-hard-soft-sync-fm-modulation/249>\
**Category:** DIY STUFF\
**Created:** [February 13, 2020, 7:01am UTC](https://lookmumnocomputer.discourse.group/t/as3340-vco-w-hard-soft-sync-fm-modulation/249 "2020-02-13T07:01:03Z")\
**Posts on this page:** 1\
**Showing post:** 30

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**Author:** ![Trumac22](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/trumac22/32/3900_2.png) [@Trumac22](https://lookmumnocomputer.discourse.group/u/Trumac22)\
**Post date:** [June 16, 2020, 12:55am UTC](https://lookmumnocomputer.discourse.group/t/as3340-vco-w-hard-soft-sync-fm-modulation/249/30 "2020-06-16T00:55:40Z")

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So, I kept playing with it and managed to make some headway - I’ve now limited the pitch drift to about the the last 10% of the duty cycle. Not perfect but certainly much better!

I stumbled across this thread: [https://www.muffwiggler.com/forum/viewtopic.php?f=17&t=231130](https://www.muffwiggler.com/forum/viewtopic.php?f=17&t=231130) and saw lots people repeating the problem (especially with the AS3340, like I’m using). I managed to treat the issue by including:

- 10uf electrolytic caps on the power rails  
\*100nf ceramic cap from pin 4 to ground  
\*changed the 10k resistor from pin 4 to ground to 56k (a la the datasheet)  
\*added a 100nf cap from pin 14 to ground - don’t ask me why

I also added a 1M resistor across legs 4 and 5 (something about hysteresis, means nothing to me) and it helped, but also reduced the pulse width pot to doing its full sweep in about 10 degrees of rotation. I cba to rectify that and it didn’t seem to make a huge difference, so away with it.

Thanks for the help everyone, I just want to rack this thing and play with it now, but alas it’s 2am on a Monday

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