[sdiy] 2164 expo VCO tracks!

[cheater cheater]

I think it's important that you get the right JFETs!

Also, maybe think about trying the approach with two or more HF trimmers?

HF frequency is very important for FM and divide-down and the like.

Regarding the switching glitch:
it's a feature, not a problem! If you can make its 'depth' or 'amount'
controllable, then that's something very useful.

Another important thing:
how does it behave around other modules? Does it put switching noise
on the voltage bus?

D.

On Sat, Oct 17, 2009 at 11:52 PM, David G. Dixon
<dixon at interchange.ubc.ca> wrote:
> After a "somewhat" premature announcement last week concerning the
> completion of my 2164 expo VCO, I found several mistakes on my PCB, and made
> a few small design changes.  My "Rev. A" board is now perfect.  I notice,
> however, that the saw-core circuit does not behave exactly like the
> simulation, insofar as a significantly larger comparator time constant is
> required to fully reset the integrator.  I'm using an RF JFET, 2N5485, as I
> did not have access to the ones Ian Fritz suggested (2N4391 or PN4391).
> Simulations suggested that the 5485 would be ever so slightly faster, in any
> case, even though it has significantly larger on-resistance.  Simulations
> also suggested that about 560ns would be sufficient to fully reset the
> integrator with the parts I'm using, but I found that about 1.5us is
> required, and even then the comparator is only getting to within about 50mV
> of ground.  I'm also clamping the comparator to one diode drop above ground
> during reset, so I don't think that the comparator (1/4 of a lowly TL074) is
> the rate-limiting step.  That's the bad news...
>
> The good news is, the VCO is (so far as I can tell by probing around with my
> hot fingertips at 3520Hz) perfectly temperature compensated (the frequency
> didn't budge), and optimum values of the tempco voltage and CV summer gain
> are exactly what I predicted them to be (289.5mV and 54.64%, respectively).
> Here is the tracking, based on 440Hz at 3.000V CV, after high-frequency
> trimming:
>
> 1.000V   110Hz
> 2.000V   220Hz
> 3.000V   440Hz
> 4.000V   879Hz
> 5.000V  1760Hz
> 6.000V  3520Hz
>
> The tracking begins to fall apart above about 6000Hz.  Replacing one quad
> opamp with a better one and using the proper JFET would tighten it up, but
> this is better than 0.1% tracking within the "piano" frequency range.  The
> waveforms are also really nice (just the tiniest glitchy spike in the
> triangle and the sine, totally inaudible).
>
> I will be sending out all the details to my devoted fans later this evening,
> after I run a few errands.
>
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