HammerHead II Waveform Question

[RexK]

I am building a PI metal detector based on the Hammerhead II (Rv A1). I am using a different controller (16F720). I have attached photos of my metal detector, I haven't implemented code for the LCD, encoder, & pushbuttons.


My coil is 10” diameter, 26 turns/26AWG, shielded with copper foil, 13 ohms, 490 uH/109pF (LCR 1000Hz). I am using a 670 ohm damping resistor (measured).


I am checking the waveshape and timing at various test points as listed on Hammarhead II drawing. I have attached screen dumps for test points TP1 thru TP4. TP1 (trigger) has a period of 1.020mSec and pulse width of 106uSec. TP2 shows a high voltage pulse generated by coil collapse (no ringing). TP3 is shown with and without preamp opamp, there appears to be a lot of ringing. Is this a problem? Any ideas to get rid of the ringing?


Thanks

PI DETECTOR_PKG.pdf

[Carl-NC]

You should not see ringing. What preamp are you using?

[RexK]

I am using a MAX412 opamp for preamp (as shown in the Hammarhead II schematic). I removed the opamp and the ringing remains see attached waveforms (TP3). See TP2, it appears to be a 'good' waveform (no ringing). Moving the oscope to TP3, the ringing appears (with or without MAX412). There appears to be only two circuits that could cause ringing. The 1K resistor/dual diode limiter between TP2 & TP3. The other is the feedback resistor/diode on 1st stage of preamp.

Rex Klopfenstein
Bowling Green, OH