You need to define what type of metal detector you are up to building.
Generally there are 2 types, a CW/IB/VLF, and TEM/PI. BFO, Off-resonance and others are left in a history now.
A CW type relies on induction balance of the Tx and Rx coils, runs with a periodic signal, usually a sinus, and it usally processes a baseband signal in a "motion" way, e.g. a HPF. The advantage of these is ability to discriminate non-ferrous and ferrous metals. They usually react badly on salt-rich soils.
PI uses a flyback pulse after a period of a coil being "charged" with a DC voltage, so that a current is about 1A or more. The flyback reaches a few hundreds of volts and subsides via a damping resistor or some other device. What follows is an Rx sampling period, using the very same coil. These have poor discrimination, but more depth, and they are not protesting on salty soils or beaches.