[sdiy] 3340 FET sync

[Doug Terrebonne]

 Check the Jupiter-6. It also uses it's own method of sync.
Doug 
synthparts.com

    On Monday, January 20, 2020, 4:51:25 PM PST, rsdio at audiobanshee.com <rsdio at audiobanshee.com> wrote:  
 
 Fin,

Are there any schematics for other examples of adding sync to an integrated circuit like the 3340? Comparisons might be educational.

My first thought upon reading your original message was that you might not have direct access to the part of the 3340 circuit that resets its oscillator, so you might have imperfect results. Your ‘scope shots seem to show that, while you can reset the charge on the capacitor, you can’t actually force the internal oscillator to restart.

If this were a discrete oscillator circuit, then your FET gate signal could also force the rest of the circuit to restart. It wouldn’t have to be discrete transistors - even an op-amp based oscillator could have sync hacked on by tapping into the right control.

Note that I haven’t reviewed the equivalent circuit for the 3340 internals, so I might have missed something. It might be worth putting everything in SPICE to see what the expected behavior is, and how to change that.

Brian


On Jan 20, 2020, at 10:16 AM, Finlay Shakespeare wrote:
> Hi everyone,
> 
> b l a r p  I even made the images into teeny weeny gifs! Here you go:
> http://futuresoundsystems.co.uk/3340sync.gif
> http://futuresoundsystems.co.uk/3340syncTraces.gif
> 
> Cheers!
> Fin
> 
> On 20 Jan 2020, at 17:50, Finlay Shakespeare wrote:
>> Hi all!
>> 
>> Long time lurker here - I really haven't mailed in a long time - but wondering if anyone could shine a light on this...
>> 
>> I'm working on a 3340 circuit (using new-generation Curtis chips here) employing MOSFET sync rather than the standard hard/soft sync methods given in the datasheet. This allows for resetting of the waveform to a known(ish) point of the waveform, rather than just giving a sync-soundalike. Known-ish, however, because at certain frequency settings / ratios (between syncing and sync'd oscillator) the waveform will "overshoot" before stabilising on the next cycle.
>> 
>> I've attached both a schematic and a shot of the scope here - if anyone has any guidance for minimising (or even better, completely removing) this overshoot, that would be fantastic. On the scope, the blue trace is the pulse input to the sync circuit, and the pink trace is the waveform from the sync'd 3340's ramp output.
>> 
>> Please note that the resistor and capacitor values given in the sync network going to Q4 are not necessarily as they currently stand - I've managed to improve the situation by tweaking this network, but seemingly I can't remove the overshoot completely.
>> 
>> Cheers!
>> Finlay Shakespeare


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