Colpitts C values?

[mikebg]

I´ll try the last circuit posted by Mikebg but I have some questions:

Is the last circuit capable of operating with metal absorption principle??

In this case, the output of the 358 is the difference signal between the signal and the integral??

Is there any information about the coil??.

Samu, the TX circuits of Garrett in my postings can suppress amplitude modulation of TX caused by change of absorption (change of distance coil - ground). The output of integraror is proportional to absorption. When energy absorbtion from oscillating tank increases, the output voltage of 358 decreases in order to increase collector current in operating point. However these are fast circuits (small timeconstant). For target detection we need too large integration timeconstant in order to increase overshoot (to prolong settling time).

However the target signal is not the absorption, but the change of absorption (the derivative of output signal). For stability we need a "motion detector" or SAT (self-adjusting threshold). This is achieved by a capacitor which removes DC component.

I think the circuit will operate with original old search coil and with old capacitors.
Then it will oscillate at previous frequency.