Ramsey Electronics Treasure finder TF-1 Instruction Manual page 5

So this oscillation is generating a "pulsating" magnetic wave. This magnetic
wave expands and contracts around the coil. When this magnetic "wave"
comes in contact with a metal object it induces a small voltage at either end
of the metallic "target". With this induced voltage across the target object, a
small current begins to flow causing a "counter" magnetic field to be
generated by the target metal. This opposing field interacts with the search
coil, generating a counter reactance when metal is encountered. Since our
search coil is designed to work with only the magnetic field generated by the
coil, we can actually shield out the electric field interference by enclosing the
coil in a "Faraday" shielding arrangement. Seems a little crazy to enclose
most of our coil in a tube of copper, but it actually improves the performance
of the search coil!
So how do we transpose this change in reactance to a changing tone? We
use our coil in an oscillating circuit whose frequency is determined by the
inductance of the coil. This high frequency oscillation is much too high for our
senses to discern, however, so we need a way to make this changing
inductance usable. This is accomplished by "mixing" the signal with another
(local) oscillating signal. The mixer, simply put, combines the two input
T rea s ure F inde r M etal D etec tor B loc k D ia gram
S e a rch C o il S p e a k e r
M ixe r
In d u cta n ce
T u n e d
O scilla to r
T u n a b le
L o ca l
O scilla to r
frequencies and produces a sum (the inductance tuned oscillator + the
reference oscillator, and the difference (the inductance tuned oscillator - the
reference oscillator). This "mixed" (difference) frequency falls in the range of
our hearing, and the change in the pitch of the tone is proof that the coil is in
magnetic contact with some metal object.The frequency that we have
chosen for the oscillator provides for an optimal mixing frequency for the
mixer part of our circuit.
Now have a look at the schematic for the unit and follow along. The power
source for the treasure finder kit needs to be regulated to provide for drift
free operation of the two oscillator circuits that are contained in the kit. Diode
TF1 • 5