FD and TD Detectors

[Cody]

When we read a posts on the internet about metal detector one thing to notice is the use of "phase" such as phase angle and the like. The detector and electronics being discussed is FD, frequency design, and very different than a pulsed time domain machine like the Explorer. In a way it is like apples and oranges which is what Glenn has said many times. I first looked at SF, then FD, and now TD design. A detector is a detector but there is a vast difference between a BFO and VLF as an example. There are not a lot of TD detectors out there to compare that are not made by Minelab. I greatly respect the design engineers for the other types and don't knock the machines but they detector are very different when it comes to discussion how the operate.

 

[Cody]

Frequency domain design which is also referred to a Fourier Design is primarily concerned with the phase angle measured using a reference. As an example the transmitter signal can be the reference so is our zero reference for phase angle measurements. A target is place in the electromagnetic field and due to the electrical properties of the target there is a phase displacement in the signal in the receiver compared to the 0 referenced signals in the transmitter. The electronics to implement phase angle measurement are costly and difficult to work with. Consider a phase angle that is less than 1/10 of 1%. This takes some pretty good electronics and design to implement the technology.

Time Domain is based on the temporal evolution of signals. Think of a signal induced into a target. The target has L/R so the inductance changes through the resistance. This is one part of the evolution of the signal. The signal then decays which is another part of the temporal evolution of the signal. Time domain electronics is interested in measure and analyzing the distortion magnetic field in the receiver coil due to these temporal evolutions of a target. This implementation is not as costly and the electronics are easier to work with.

We then have multiple frequency and single frequency. Single frequency would be like a sing we observer on an oscilloscope for 60 cycle voltage in our homes. Multiple frequencies could use more than one sine wave for multiple frequency design. However, a square wave is comprised of multiple sign waves. By adjusting a square wave we can obtain and infinite number of sine waves.

A sine wave applied to a coil produces a continuous rise and fall of the voltage and the electromagnetic field. A pulse causes an abrupt discharge of current stored in the inductance of the coil when the applied voltage suddenly drops to zero. This induces a very strong pulse of energy into the soil.