Hardware

The initial version of this project is to be a high quality VLF metal detector that is predominantly software oriented.  This has been made possible by recent advances in sound cards.  Regardless of this there is still some electronics required to complete the metal detector although the total electronics is less than that generally used to create or record sound.

Requirements of the Interface circuit.

Some of the following requirements may be performed by the audio card.  With most of the below requirements it doesn't hurt if it is performed within both the interface circuit and the audio card.

In performing these requirements, it is very easy to introduce noise.  Achieving the following requirements while minimising noise requires careful design.

Requirements of the electronics that interface between the coils and A/D and D/A transducers include the following:

Hardware Amplification

The amplification that is available on the audio card will often allow the amount of amplification to be set by the software.

Other forms of hardware amplification can include increasing the number of windings in the coil, creating a high Q receiver circuit optimised for the frequency of interest or using an active amplifier such as Operational Amplifiers (Op Amps).

High Q Design

The definition of Q factor from wikipedia is as follows:

"The Q factor is a dimensionless parameter that compares the time constant for decay of an oscillating physical system's amplitude to its oscillation period. Equivalently, it compares the frequency at which a system oscillates to the rate at which it dissipates its energy. A higher Q indicates a lower rate of energy dissipation relative to the oscillation frequency, so the oscillations die out more slowly. For example, a pendulum suspended from a high-quality bearing, oscillating in air, would have a high Q, while a pendulum immersed in oil would have a low one. The concept originated in electronic engineering, as a measure of the 'quality' desired in a good tuned circuit or other resonator"

The principles of High Q receiver design is to design the receiving circuitry in such a way that it has a natural resonant frequency (or natural resonant frequencies) at the frequency(s) of interest.  Unfortunately, it also means that only small amounts of energy from external noise sources with the same frequency or near the same frequency also cause large changes in input.   It effectively channels external noise near the frequency(s) of interest into the input.

Over Voltage Protection

It is typical to add diodes (or zener diodes) to give some form of over voltage protection.  Circuits that contain capacitors and coils can accidentally create high voltages due to quick switch offs.  The diodes act as a sort of valve to release excess voltage from harming electronics further on.

Filtering

In particular, anti aliasing filtering is critical to any application that inputs data through an A/D converter.  The requirement is to get rid of all frequencies higher than half the sampling rate (called nyquist-shannon criteria).  Getting rid of  high frequencies is done with a low pass filter.  It is best to reduce as much as possible all frequencies higher than those being analysed.  See the following wikipedia page:
http://en.wikipedia.org/wiki/Anti-aliasing_filter

There is also a need to get rid of very low frequencies with a high pass filter.  This is mainly done to get rid of interference from 50/60 hertz power lines.  This high pass filter would be set to a cut off frequency of about 500 to 1000 Hertz so that as well as the 50/60 Hertz it also cuts out most its harmonics.

Impedance Matching

  In many circuits impedance matching is required for each interface between the system and the transmission lines that connect the circuit to other components of the system.  Impedance matching is the design of these interfaces such that they do cause reflections.

With most VLF detectors, the frequencies involved are so low and distances between the circuit and either the coils or the audio card as so small that the conductors between don't act as transmission lines.  As a consequence there isn't any real need for impedance matching.  This could change, for example, if someone implemented the computer remotely in a vehicle with a (presumably coaxial) cable connecting the electronics and coils at a distance in a field.

Electronic Components

Electronic components that are designed for an equivalent audio application are often a good choice.  For example, an op amp designed to work as an amplifier for a microphone would be a good choice for amplification of the input to the audio card as it is already designed to work as an input to audio cards.
These components are likely to meet the appropriate characteristics such as high-fidelity and low noise.
 
All components used between the Receive coil and the audio card's input have to be selected for low noise.  For Example, in order to reduce the input of noise, Metalfilm resistors are preferred over carbon resistors and FET transistors are preferred over bipolar transistors.

Similarly, amplifiers designed for amplifying the output of a sound card previous to driving a speaker could make a good choice for amplifying the output previous to the driving the transmit coil.  These would have similar high-fidelity, low distortion, power amplifier characteristics that both these applications need.

Examples

Below are example circuits that show the type of electronic circuits that are possible.

The first is an electronic circuit by Aziz who is a member of this group.  This circuit was used to create a detector that worked.  The software in this case was based on Microsoft Windows Operating system.

The circuit is unusual in that it is a completely passive circuit.  That is, no element on it requires a supply of power to run the element.  It achieves this by a being a High Q design but in doing this it makes it suitable for Transmitting and Receiving one and only one frequency.

Below is a picture of the Overlapping Double D coils that Aziz used with the above circuit.  Overlapping Double D is one of a number of coil arrangements that can be used for Induction Balance.

Below is another example of a circuit that could be used to interface an audio card of a PC to VLF Transmit and Recieve coils.  In this case it is an active circuit.  Below the circuit diagram is a diagram of how the circuit fits in.