[sdiy] VCO reset time

[Tony Clark]

   Hi Harry,

> I'm playing with a new experimental VCO core.  I will NOT tell you what
> it is...

   Why?  Does it involve a BBD?  ;)

> (you will need a lot more to get me NOT to talk)

   What a tease...you don't even give us a price on that option!  :P

> Several questions...
> 
> 1)  What is a good ramp reset time ?   How fast have you seen ramp reset
> be ?

   A _good_ reset time would be in the neighborhood of 1uS.  This is 1/10 
of a wave cycle at 100kHz, which I would feel is quite acceptable for 
pretty much all applications.  Of course faster is better, especially if 
you plan on doing waveshaping, like I do with my VCO, then the 
reset time is extremely crucial.
 
> 2) How many octaves do you expect (in tune) without resorting to HF
> compensation

   Approximately 15 or more octaves.  I never did a complete count of 
what my own VCO is capable of, but it is staggering.  There IS a definate 
relationship with the reset time/cap size/frequency range of a VCO.  The 
smaller the cap, the smaller the current driving the VCO.  I find that 
this is a distinct _disadvantage_ in the range of the VCO.  While you can 
certainly get fast reset times usng a small cap, the low end range of the 
VCO suffers since it's harder to drive those really small currents 
without running into leakage issues.
   I opt instead to put in as LARGE a cap as possible, because you can 
run larger currents in the mirror allowing you to run the VCO into 
extremely low frequencies yet still have excellent high end response.

> (I have not tried hf comp yet...)

   I don't use it personally.  But probably someone with more golden ears 
than me would prefer it.  I suspect that really the HF compensation is 
there because of waveform distortion at higher frequencies.  With a super 
fast reset time, it shouldn't be necessary.

   Currently I'm in the process of redesigning my old VCO core (it's now 
5 years old!) and do have a better reset circuit to go in, but I don't 
really expect much of an improvement in reset time, but I should see an 
improvement in offset-to-zero.

   Hope that helps...

   Tony

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