[sdiy] Help needed with ASM based VCO

[Ian Fritz]

Hi Chris --

Couple of things:

>The problem was indeed the FETs in my circuit (the
>ones that switch to discharge the timing capacitor).
>I had made the stupid mistake of using standard FETs
>for this purpose; not knowing that you need high
>switching speed and very low ON resistance types.

I wonder why increasing the feedback cap didn't work, at least as far as 
getting a complete discharge.  Curious as to what FET you were using.


>I am in the UK and was kindly pointed (by Laurie
>Biddulph and Tony Allgood indirectly) to the J108 FET
>that is sold by UK company Farnell. This was perfect
>for the job, featuring only 5nS switch time and an
>immpressive <8 Ohms ON resistance, which actually
>outperforms the original specified component.

Careful there.  You don't get something for nothing.  The J108 has much 
higher leakage current -- a factor of 30 compared to the 2N4391.  (And the 
sheet I have doesn't even specify the capacitance.)  So you are trading off 
some low-end performance.  Maybe you don't care, which is fine, but the 
original FETs (2N4859 or 2N4391) were carefully chosen to give the best 
overall performance.

But an even better solution seems to be MOSFET switching, such as in the 
designs I put up a couple of monthe ago.
http://home.earthlink.net/~ijfritz/xfer.htm

   Ian