A
Anonymous
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In conventional pulse induction (including the Fisher system), it is necessary to let the signal settle down after the flyback, before looking at the incoming signals. This impairs sensitivity to low conductivity targets-- an advantage in saltwater beach & scuba work, but a disadvantage in gold prospecting.
CCPI (continuous-current pulse induction) takes for granted that a separate receiver coil is furnished, presumably in induction balance. If the preamp is operated linearly, in most cases signals can be looked at right up to and even into the transmit phase (which corresponds to the flyback phase in a conventional PI machine), although this increases the difficulty of producing a good design. Low conductivity targets can be detected if this is a necessity, as it is in gold machines.
In order to maximize sensitivity, the receiver coil will most commonly follow the VLF/MF practice of having more turns (higher inductance) than the transmitter coil. This in turn decreases the self-resonant frequency, and blurs the time boundary between reactive and resistive signal components for the smallest targets.
Therefore, although CCPI makes it possible to detect tiny targets which would not be detectable using conventional PI techniques, in most cases it will be necessary to demodulate both resistive and reactive components of the short-delay signals, and to ground balance them using an approach similar to that used in VLF/MF machines.
DEMODULATION/GATING DURING THE TRANSMIT PHASE
Suppose (as in the example of my post of 29 Dec 01) that the transmit phase is 10 microseconds. During those 10 microseconds a relatively high voltage is applied to the transmitter coil. This causes the current to change in a linear fashion during the 10 microsecond transmit period.
The voltage induced into the receiver by a reactive target (e.g., magnetite) by the current ramp will be a rectangle having the same shape as the transmit drive voltage. In the case of a low-conductivity (pure resistive) target, the eddy current will be in phase with the voltage induced in the target, and its rising and falling edges will induce a positive and negative voltage spike in the receiver coil.
If the system is fast enough not to smear together everything that happens during the transmit pulse, the resistive signal can be either full-wave demodulated, or the preamp can be full-wave gated, by reversing the polarity of the switching mechanism in the receiver system during the middle of the transmit phase. This causes the reactive component to cancel and causes the opposite polarity resistive spikes to add together.
--Dave J.
CCPI (continuous-current pulse induction) takes for granted that a separate receiver coil is furnished, presumably in induction balance. If the preamp is operated linearly, in most cases signals can be looked at right up to and even into the transmit phase (which corresponds to the flyback phase in a conventional PI machine), although this increases the difficulty of producing a good design. Low conductivity targets can be detected if this is a necessity, as it is in gold machines.
In order to maximize sensitivity, the receiver coil will most commonly follow the VLF/MF practice of having more turns (higher inductance) than the transmitter coil. This in turn decreases the self-resonant frequency, and blurs the time boundary between reactive and resistive signal components for the smallest targets.
Therefore, although CCPI makes it possible to detect tiny targets which would not be detectable using conventional PI techniques, in most cases it will be necessary to demodulate both resistive and reactive components of the short-delay signals, and to ground balance them using an approach similar to that used in VLF/MF machines.
DEMODULATION/GATING DURING THE TRANSMIT PHASE
Suppose (as in the example of my post of 29 Dec 01) that the transmit phase is 10 microseconds. During those 10 microseconds a relatively high voltage is applied to the transmitter coil. This causes the current to change in a linear fashion during the 10 microsecond transmit period.
The voltage induced into the receiver by a reactive target (e.g., magnetite) by the current ramp will be a rectangle having the same shape as the transmit drive voltage. In the case of a low-conductivity (pure resistive) target, the eddy current will be in phase with the voltage induced in the target, and its rising and falling edges will induce a positive and negative voltage spike in the receiver coil.
If the system is fast enough not to smear together everything that happens during the transmit pulse, the resistive signal can be either full-wave demodulated, or the preamp can be full-wave gated, by reversing the polarity of the switching mechanism in the receiver system during the middle of the transmit phase. This causes the reactive component to cancel and causes the opposite polarity resistive spikes to add together.
--Dave J.