The metal detector is one of the primary workhorses of the treasure hunter, particularly of the hobbyist THer. There are three primary types of detectors. Frequency shift designs include beat frequency oscillator (BFO), phase-locked loop (PLL), and off-resonance. Induction balance (IB) encompasses transmit-receive (TR) and very low frequency (VLF) detectors, as well as two-box detectors that are sometimes referred to as RF (radio-frequency). Finally, there are pulse induction (PI) designs.
PULSE WIDTH. The width of each individual transmitter pulse. Depending on the detector and its intended application, the pulse width can vary between 50 microseconds (uS) and 1000uS. 50 to 150uS is usually the width for ring and coin size objects while wider pulses suit detectors designed for locating larger objects. A boat towed PI designed to look for cannons would likely use 1000uS. Examples are:- Whites Surfmaster 50uS, Deepstar 100uS, Aquapulse 300uS, Superscan and Aquapulse towed 1000uS.
PULSE FREQUENCY or PULSE REPETITION RATE. The number of times the transmitter pulse is repeated every second. The frequency affects the response speed, power consumption and interference rejection, although all of these are influenced by many other factors in the circuit design. Examples are:- Surfmaster 800 pulses per second (p.p.s.) Deepstar 3000 p.p.s. Aquapulse 170 p.p.s. Superscan 66 p.p.s.
PULSE DELAY. The time that is allowed to elapse between the transmitter switching off at the end of each pulse and the start of the receiver function or receiver sample pulse. The shorter this time is, the higher the sensitivity to smaller and/or poorer conductors. On some PI detectors (Deepstar) this is called a REJECT control. The shortest pulse delay is with the control on MIN and if it is turned clockwise the pulse delay time is lengthened which enables bits of foil and pulltabs to be rejected. However, thin and lower carat rings may also be lost. For beach work on salt wet sand a minimum pulse delay of 15uS is about right so as not to get too much signal from the conductive salt water. Detectors for diving have to use longer delays as the water signal is much greater. Detectors for large objects, as well as a long TX pulse, use longer delays to minimise signals from small bits of metal.
DECAY TIME or OBJECT DECAY. The time that the small electric current induced in a metal object flows. The energy is dissipated in the electrical resistance of the object so that poorer conductors (higher resistance) have a shorter decay time. A thin ring may have a decay time of 50uS while a silver cob could be 500uS. A detector set with a pulse delay of 100uS would pick up the cob but not the ring.
SAMPLE PULSE or SAMPLE WINDOW. The period of time from the start of receiver sampling to the end of the sampling period. Usually between 15 and 50uS. In the Deepstar it is set at 20uS irrespective of the pulse delay. In the CS7 the sample pulse tracks the pulse delay i.e. it is always the same value as the delay.
DUAL SAMPLING. This is a system used by most PI
