Lm13700 Osc - Sync?

bugbrand · Posted: Thu Jan 31, 2008 4:44 pm Post subject: Lm13700 Osc - Sync?

Hey, I'm working on a VCO along quite similar lines to Thomas Henry's LMVCO (in AS21C) and wondered how you'd go about adding a sync input???

My presumption would be to momentarily short the cap - you could do that with a 4016 gate but that'd be big and clunky (unless the other gates were used?). So, perhaps with a JFET?

Any pointers to schematics perhaps?

Thanks!

frijitz · Posted: Thu Jan 31, 2008 7:57 pm Post subject: Re: Lm13700 Osc - Sync?

bugbrand · Posted: Fri Feb 01, 2008 3:11 am Post subject:

Ah yes - info! Good point, Ian.

Ok, I'm posting up the schematic which I drew out last night - the full thing isn't tested though I've been through most of the parts on breadboard.
Its a design for a reasonably simple modulation osc - ie, I'm not too worried about perfect tracking. The Osc core is straight from the LM13700 datasheet - we then take the triangle and generate a square from it (pulse width adjustable - I didn't want to fit full PWM) and then we have a linear VC crossfade using a 2nd LM13700.

So, the point for sync would be cap C1..

Cheers!

LMShapeOscBugBrand.jpg

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(Untested) LM Shape Osc

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frijitz · Posted: Fri Feb 01, 2008 6:53 am Post subject:

bugbrand · Posted: Fri Feb 01, 2008 11:59 am Post subject:

Thanks a lot for the ideas and details Ian! That's a real help.

I think I'll certainly look first at the JFet method - that's along the lines I was thinking about, I just needed some firming up! Definitely time to bread board some more.

I'm hoping this may work out as a nice compact module - primarily aimed at LFO modulation, but of course with possibility to be a droning osc too. I envisage a bank of two or three of them with cross modulation between them all (..it'd be a lot easier if I banana systems allowed normalisation - it'll have to be switches). Anyways, if the design moves forward well I'd really love to turn this into a proper pcb and share it around..

intellijel · Joined: Sep 10, 2006 Posts: 66 Location: Vancouver, BC

Why does the JFET option require a series resistor to the gate? I have seen this in other circuits as well but I do not understand why you would need to limit the current to a fet gate.