[sdiy] Saw core JFET mystery

[cheater cheater]

Jürgen,
Are you saying that it might be better to use something else instead
of the jfet?
Will we have to change that part of the circuit around the reset for every unit?

David,
what happens if you change the jfet to another one of the same model?
Can you try a few copies to make sure we're not having a big problem
there? (maybe I'm just getting too worried)
What about practical tests of temp stability of the various jfets?

Thanks
D.

On Mon, Mar 1, 2010 at 10:11,  <jhaible at debitel.net> wrote:
> 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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