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This is an updated coil drive circuit. Since I have a preamp now, I no longer need to pick off a signal from the transmit coil to test with. So this is just to drive the coil. It is now set for 15 mA.
I added a .001 cap because I found there was processor noise on the logic DRIVE signal. That noise was getting into the drive current and showing up in the receive signal. This cap eliminates most of that noise and also softens the edges of the pulse. The air signal from the coil looks much better now.
I will have to change this again when I get a real reference diode to replace the three series diodes I am using as a reference.
Robert Hoolko