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

[David G. Dixon]

Well, I had some time to kill as I'm invigilating an exam in one of my
courses, so I decided to do a simulation of replacing the discrete schmitt
trigger in VCO-1 with an LM318.  It took a while to find a model for LM318
within Multisim which actually worked, but I finally found one that doesn't
blow up: National's model "LM318_4".

I'm using 20k resistors to +/-15V on the + input, and a positive feedback
resistor of 15.8k, which is giving exactly 5.000V output levels.  Sizing the
speedup capacitor across the feedback resistor is relatively important.
With a speedup capacitor of between 33 and 47p, it takes about 135ns for the
output to cross zero from +/-5V.  If the speedup capacitor is above or below
this optimal range, then the zero-crossing time climbs to about 170ns.

The discrete version takes about 200ns to switch.  It takes about 150ns to
make up its mind, but then the actual switching takes place very fast,
within about 50ns.  However, it doesn't begin switching very swiftly.

On balance, I think it will be easier to use the LM318 for designing
rock-solid threshold voltage levels, and of course it begins to switch
instantly once the threshold level is achieved by the integrator.  The
discrete switch is a little bit dicey in this regard.

I'm going to replace the 2N3904s with LM318 in my version and redo the
layout.  Thanks for the tip, Ian!



> > 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.
> 
> Yes, I've been reading about the LM318.  Hal Chamberlin recommended it for
> sample and hold circuits.  I think I might give that a go, at least in
> simulation, and get back to you.  I'm not all that impressed with the
> symmetry of the discrete ST output, either, although it doesn't really
> matter since all it does is saturate the OTA in the VCO-1.