[sdiy] More VCO fun

[ASSI]

On Tuesday 08 December 2009, Ian Fritz wrote:
> Since you ar commenting on my circuit:

This is why I was trying to take my answer off-list which I've 
obviously failed to do.  It was David's question of why *his circuit* 
would behave quite differently that I set out to answer.  So please 
accept my apologies for not qualifying all my statements properly.  
The circuit you've posted and credit to Terry should indeed work as 
you've described and I didn't mean to imply that it doesn't.

However, if the supply is changed to +-15V and/or a lower cutoff 
voltage JFET is used like the 2N5485 that David actually has in his 
circuit, the performance will deteriorate since the JFET gate is not 
actively pulled up to GND.  Increasing the "reset timing constant" 
beyond a certain point will not help in this situation since it also 
impedes on switching on the JFET fully, just as David found out.

> 4 -- This is a really old design.  You might want to look at a more
>  recent offering (posted on the e-music forum), which uses a MOSFET
>  to switch in ~ 200 nS.

I agree that a MOSFET is a better solution these days, I just wanted 
to give David a hint on where he might take the circuit he already has 
on board.  I just found your circuit on electro-music, I hope this is 
the right schematic:

http://www.electro-music.com/forum/download.php?id=15799

I notice that one of the improvements is a pullup on the comparator 
output.  Something along those lines should give the "old" circuit 
with a JFET a much better performance not unlike what you see from 
yours.


Achim.
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