Circuits for magnetic pickups

[Sandy]

>
>I don't agree generally that Zeners are too slow, the are only a few percent
>slower than tranzorb type devices.  Zeners should be more than adequate for
>automotive use.  Make sure the voltage is high enough to prevent shunting
>your signal, though.

I tried some zeners in my circuit simulator, and I 'think' they didnt work
well at the high frequency that could be comming in from 60 tooth wheel, at
say 12,000 rpm would be in the range of 12khz. Will that be a problem? Also,
I included the inductors to keep out other high frequency noise.



                    100uh                    R1
     |-------------()()()-------*---*------/\/\/\--*----*->>> To Balanced amp +
     |                          |   |              |    |
     |                          |  ---             |    \
  [pickup]         Inductors    |  ---          [BiTVS] / Attenuate
     |                          |   |              |    \
     |                          |   |        R2    |    |
     |-------------()()()---*---|---*------/\/\/\--*----*->>> -
                                    |   |
                                   --- --- 
                                   --- --- Small Caps
                                    |   |
                                    --|--
                                     GND         


A bi-directional TVS is nice, in that it is fast and available. One problem,
is that if the voltage gets to trigger the TVS, it effectivly will be a
short across the inputs to the amp, and the system should still work, for
example if the sensor is too close to the gear. So I am going to guess that
the TVS will also need a resistor inline, so to just provide a 'clipping'
function. Does this sound plausable? As I remember the P-P voltage can be >
100 volts, but, way down to the mV range. 

As for testing the circuit, this is still a problem, but, I can easily test
for noise rejection, by just string wire to a VR sensor, and see what type
of noise shows on the output of the VR, and then finally the opamp. 

Thanks again for the help, I'll be off to the parts store to get a couple of
sensors.

Sandy