[sdiy] Odp: Re: SAW core VCO flyback time

[Mattias Rickardsson]

Definitely some nice benefits there.

But how about the two alternating ramps, with two integrator
capacitors - how would you ensure that they are identical?

/mr

On 28 August 2016 at 01:20, Roman <modular at go2.pl> wrote:
> Benefits:
> - there's no fixed time period of reset pulse involved, that affects HF
> tuning
> - ramp always starts at 0V, which is not the case at higher frequencies in
> mostly used saw core design
> - integrators can use slower opamps, easier to find with super low bias
> current, while buffer after mux can be as fast as you wish, like SR=2kV/us
> because high input bias is no issue here
> - waveform is cleaner because slower reset time, that does not cause
> integrator opamps to ring/overshoot (let's assume that final buffer part is
> done right and does not ring either)
> - fastest saw reset slope in history
> - and most of all it's such a cool idea to try!
>
> Roman
>
> Dnia 27 sierpnia 2016 23:31 Tom Wiltshire <tom at electricdruid.net>
> napisał(a):
>
> Ok, I get it. Thanks Roman.
>
> I guess you could do triangles the same way, but with one integrator doing a
> down slope instead. Mux would switch back and forth between the two slopes.
> The result would be one octave down, clearly. Not sure what the benefit over
> a one integrator triangle core would be exactly.
>
> Tom
>
> On 27 Aug 2016, at 21:00, Roman <modular at go2.pl> wrote:
>
> One ramp is going up, and when reaches maximum, the MUX switches to 2nd
> ramp, which then starts exactly at this moment.
> Meanwhile 1st ramp is slowly decaying by not-so-fast switch, let's say it
> takes a few microseconds, and is held at 0V untill the 2nd ramp reaches its
> maximum, and MUX switches back again to 1st ramp.
> So reset FETs are not driven by short pulses but out of phase square signal.
> MUX switches between ramps making the reset slope fast as lightning.
> I have this core somewhere in one of my notebooks, or maybe loose paper.
> Never built it hough as it was a bit overkill.
>
> Roman
>
> Dnia 27 sierpnia 2016 19:37 Tom Wiltshire <tom at electricdruid.net>
> napisał(a):
>
> How would this work, Rick? I'm not seeing it.
>
> Does it need the two ramps to be running at half the output rate so that you
> always switch in the middle of the ramp? And don't you just swap flyback
> time for switching time as the potential glitch?
>
> Thanks,
> Tom
>
> On 27 Aug 2016, at 17:49, Rick Jansen <rick.jansen at xs4all.nl> wrote:
>
> You could do a dual integrator and alternate their outputs: no flyback time
> at all.
>
> rick
>
>
> On 27/08/2016 18:27, Mattias Rickardsson wrote:
>
> On 26 August 2016 at 06:42, David G Dixon <dixon at mail.ubc.ca> wrote:
>> I stopped building sawcore VCOs because I didn't like flyback.  That's why
>> I
>> only build tricores now, and shape them into saws.  They sound better
>> anyway, and sync in a more interesting way.
>
> I'm not a big fan of them either, as far as tracking and waveshaping
> is concerned.
>
> But regarding the sound - I made a very bad (meaning slow flyback) saw
> core, and now people shout that it sounds so great and they want it in
> more designs.
>
> Alas, the ever so unrewarding nature of good engineering! ;-)
>
> /mr
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