[sdiy] VCO unstable in Spice, stable in real world

[Harry Bissell]

Bringing the reset comparator output uhh... 'out' can be troublesome. You can easily
inject noise onto the comparator output from the outside world and screw up the VCO
reset.

I'd buffer that output if possible, a 4000 series CMOS gate would do well and the
propagation delay of 50ns (max for a 4069 on 15V supply) is unlikely to be a problem.

Of course, don't forget to add a 1K series resistor on that output :^)

H^) harry


----- Original Message -----
From: David G. Dixon <dixon at interchange.ubc.ca>
To: 'Rutger Vlek' <rutgervlek at gmail.com>, 'Synth DIY' <synth-diy at dropmix.xs4all.nl>
Sent: Fri, 04 Feb 2011 14:51:06 -0500 (EST)
Subject: Re: [sdiy] VCO unstable in Spice, stable in real world

> Thanks for all the good suggestions! Decreasing the simulation step time
> was a good idea. It didn't change a great deal, but made the saw output
> more precise since the simulation is no longer aliasing on the discharge
> pulses for the capacitor. I've also replaced the LM311's model and it
> seems to do its job much better now. I've also found out that the way I
> prefer Spice is mostly to simulate and understand small parts of a
> circuit, rather than an entire synth at once :). The VCO I'm trying to
> model is a single instance of Jurgen Haible's Living VCO. By doing a Spice
> model I'm hoping to get my electronics knowledge up to speed, such that I
> can make some modifications. One of the modifications on my list is to add
> oscillator sync.

That's the right way to use SPICE!  For sync, just add pulses to the
comparator's grounded pin.  You can even attenuate those pulses for variable
sync.  It's all I ever do, and it works a treat and sounds magnificent.

> I've been wondering by the way. If you take a VCO's rising saw output,
> invert it and shape it into a pulse that can trigger the second VCO's
> discharge path, a great deal of delay is introduced. This means that two
> synced oscillators will actually not reset at exactly the same time. Is
> this phase-difference considered useful or unwanted? Taking the comparator
> output of VCO 1 to directly trigger the second VCO's discharge patch would
> assure much better (phase-) synchronization (although switching sync on
> and off might me a little more difficult). From the VCO's I know this
> trick is never used... I wonder why?

I use the comparator output directly to trigger the JFET on the optional
inverter/follower for converting triangle to saw.  The comparator output
isn't needed internally for waveshaping in a sawcore VCO, but as you say,
taking the comparator output directly to an output jack could be very
useful.  Just remember that the output of a comparator (a proper one like
LM311) is not very low impedance (like an opamp) but is basically the
impedance of the pullup resistor (at least on the upswing).  Hence, you have
to be a little bit careful about what you send it to.  Again, SPICE is your
friend here.

> > In short, SPICE modeling is well worth the effort, but it can also be
> very
> > tricky, and many of the models don't appear to be very accurate.
> >
> 
> Yea, that's just annoying! Also, B2 Spice claims that they have integrated
> with Eagle PCB. Well... I wouldn't call importing the schematic without
> working models 'integration'. It seems they have a long way to go, but
> same holds for the Eagle designers in terms of User Interface by the way
> :). Ok, that's enough bitching. It's still great that I can play with
> different components without actually having to buy them (nor blow
> them...).

You can do what I do, and do "virtual breadboarding" while sitting in a warm
leather recliner watching Buffy the Vampire Slayer, rather than sitting in a
cold garage making an actual breadboard!  Since I started using SPICE, I
think I've actually breadboarded about two circuits.

> ps. Speaking of oscillator sync. Has anyone ever tried making an analog
> VCO that tracks relatively well and correct the remaining tuning error by
> syncing it to a digital oscillator?

Sounds effectively like PLL.  I don't think it would be all that great,
especially when it's so easy to compensate for tracking error using a diode
(or a diode-connected transistor).  I'll email you privately to show you the
"correct" way to do that using a biased inverter.  Again, SPICE is your
friend!

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-- 
Harry Bissell & Nora Abdullah 4eva