[sdiy] They aren't sawtooths, they're ramps

[Ian Fritz]

At 10:16 AM 11/5/2009, David G. Dixon wrote:
>According to the simulation, switching from the prevailing level (+/-5V) to
>ground takes about 50ns.  Then, it takes about 200ns to settle to the low
>trigger level, and about 800ns to settle to the high trigger level.
>However, the triangle has already reversed direction once the main part of
>the switching transient occurs.  Therefore, I'd say that the switching takes
>less than 100ns.

Nice!  It will be interesting to see how this does in practice.


>This is based on a discrete schmitt trigger made from two
>2N3904s, with a 47pF speedup cap (see Scott Stites's website for the VCO-1
>schematic).

This design, of course, is very similar to the old Electronotes design of 
Bernie Hutchins.  I worked for many years improving and tweaking the 
design, since it was being used at ultrasonic frequencies (the original 
DoubleDekka) and needed to track up to ~80 kHz.

As part of my work on this circuit I discovered the rather surprising fact 
that a fast opamp outperformed the discrete Schmitt trigger!  I used the 
then new LM318, which is still available, inexpensive and fast (70 V/us 
slew rate). It would be very interesting which method does better in 
simulations, since using an opamp is quite a simplification.

Ian