Thank you all for the valuable advise on my project. I think that there are a
few points that I should clarify. I am a hobbyist at electronics, a middle
school teacher by trade. I only took 2 semesters of electronics in college, so
when someone begins to argue about theorems and laws, I'm simply lost.
My goal is to develop an ignition system for my old truck that incorporates a
PIC controlling the timing curve and coil charging.I am aware that this
project has been done several times over, but the learning experience of
building from scratch is what I'm interested in.

Clarifying a few points about the pickup and zero crossing.
The variable reluctance pickup produces a sine wave pulse every time a tooth
on  the distributor shaft rotates in and out of the pickup's magnetic field.
There are four teeth spaced 90 deg apart. I get 2 pulses per rev of the crank
and 4 per rev on the distributor. The frequency and P to P voltage of the
pulse increases proportionally with an increase in crankshaft. The pulse goes
high as the tooth approaches, falls to zero at the center and goes negative
until the tooth leaves the magnetic field. The output V at cranking rpm is
about 2v P to P and about 12V at redline. My goal is to produce an interrupt
at the PIC when a tooth aligns with the center of the pickup. It must also be
highly immune to emf noise.
 I'm considering simply running the wave through a germanium diode rectifier
and watch the trailing edge of the first pulse. When the wave falls, the slope
is nearly vertical, getting me close to the zero crossing.

What does everybody think?
Bruce