[sdiy] Saw core JFET mystery

[jhaible at debitel.net]

I guess that's the difference between a single circuit, and a production-ready
design. The former can squeeze the best out of a device, the latter has to asume
worst case. And tolerances, and temperature variance, of junction fet parameters
are immense.

JH.

> Hey Team (especially Ian Fritz):
> 
> I soldered up my latest board revision for the 2164 Expo VCO tonight, with
> the proper JFET at last: PN4391.  After much simulation, I decided that I
> could greatly simplify the circuit around the ramp integrator and reset
> comparator, and eliminate the high-frequency trim circuit in favour of a
> Franco compensation resistor (just like everybody else).  So far so good.
> However, tonight, when I got it all hooked up, it seemed that the reset
> comparator RC time constant was too large.  I only used 10k and 22pF, or
> 220ns.  However, this seemed to be way too long.  I took the resistor off
> and rigged up a 20k trimmer instead so I could play around with R...
> 
> Long story short, using the scope I found the best resistor value to be
> about 1k.  Hence, the reset RC is only 22ns!  I'm thinking that this can't
> be right, but the scope confirms that the ramp is resetting all the way, and
> the bottom corner of the derived triangle looks very nice over the whole
> audio range, with just the tiniest inaudible reset glitch (this is the real
> indicator of resetting goodness).  I hard-wired a 1k resistor in, and it all
> works swimmingly.
> 
> So, why can this RC be so much smaller than everyone else uses?  Is the
> PN4391 that much different than the 2N4391?  They should be virtually the
> same.  My integrator cap is only 1nF, so it will drain pretty fast, but
> still...?
> 
> Now, bear in mind that I have no idea how well this beast tracks yet.  I'll
> have to look into that tomorrow, and I probably still have some optimizing
> to do on sizing the Franco resistor, but I'm expecting it to track very
> well; it certainly resets fast enough!
> 
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