[sdiy] Re: Leapfrog

[jhaible]

> Also, making a "sweeped sine" simulation isn't everything. It is actually
a
> rather poor method for characterizing certain aspects. A "smooth" responce
is
> not necessarilly a sign of good characteristics, but an "unsmooth"
responce is
> a good indication that it probably will not be that good.

Well yes of course, that's the general problem with Spice. You don't see any
instability with AC analysis. Typically, if you increase a filter resonance
until the real circuit breaks into self-oscillation, in Spice you see the
peaks
getting higher, and suddenly getting smaller again. So you have to start
from
the "safe" area, and increase your critical parameter in tiny steps to find
the
threshold to self oscillation by seeing the peak getting smaller again.

But this wasn't about stability - it was about this circuit being an
integrator
function. Have you seen my last mail? I've analysed the circuit for opamp
gain = infinite (i.e. V at +in = V at -in) and it's indeed a pole at zero. Any
real (finite) gain will be an approximation, of course. But good enough
above 1Hz for a TL071.

Whether the whole thing is stable for, say, 1% resistors, if the negative
resistance of the NIC slightly dominates the positive resistor(s), is a
different
question. I _expect_ it to be stable, because of the tight leapfrog feedback
loops,
and the well-damped first and last stages, but we'll only know when we
really build it. Maybe one should build a simple Biquad filter from one
inverting damped, and one non-inverting undamped integrator, to see what
happens.

JH.