metal detector using LC circuit in resonance

[nxt_]

dear all greetings,

i am trying to build a metal detector sensor based on LC circuit (at resonance). which leads to any metal come in the range of the magnetic field of the coil will leads to drop in the voltage so that i can sens it with amicro controller. now here is the problem the resonance frequency is 130khz (using a generator) now i need to obtain this 130 kHz square wave without the generator but using a pic18f452 micro controller from microchip . the problem i am facing here is the saturation problem (i.e Slew Rate of the used components). which must be minimum (2pi*F*vpeak) V/us.. now i need a recommendation for a transistor that capable of withstanding this frequency (130 kHz).
also is it okay for pic to produce such high frequency (130 kHz) using its internal timers?

the above are my needs. now i have a question about the lab generators (i do know that their output current is very low. now here what happens when i connect my LC circuit when i touch it with the voltmeter the wave on the scope is noticeably drop ( this is an insufficient current problem right?)

many thanks for your attention
regards.

 

[chuckey]

Your voltmeter problem, will materialize with your wanted circuit. What you need is a very Q LC circuit and putting anything directly across it will damp the Q and the added stray capacity will de-tune it. I would build a one transistor amplifier to drive the LC circuit and optimise that circuit for output voltage and high Q- use a FET transistor to buffer the voltage to read it. So now you will have your basic LC circuit but you can drive it with a generator or micro and measure the output voltage at the buffered output.
With this sort of circuit it is usual to have an earthed shield around the coil (suitablely split so not to be a short- circuit turn), so the coil is immune to external changes of capacitance (nearer or further from the ground) and just relies on magnetic induction for any de-tuning effect.
Frank