There are several ways to do CCPI. The 7k basically uses a current square wave like this:
Unlike traditional PI, there is no zero-current region for "listening" to the RX. But you don't need a zero-current region, all you need is a zero-current-
change region and that is achieved by the "constant current" peaks, hence the term CCPI. Constant current means dI/dt = 0 and therefore dB/dt = 0, and it is a changing magnetic field that creates eddy currents. So the slews create target eddies, and the peaks are where we listen.
A bonus with this kind of CCPI is that the current peaks can be held with a relatively low voltage, perhaps as low as 1V, and if energy recycling is incorporated the current slews burn very little power. Overall, this can be far more fuel-efficient than a traditional total-loss PI and can even rival VLF. Another bonus is that it is naturally bipolar so EFE is canceled. And it is capable of being run over a wide range of frequencies.
The waveform above implies the slews are exponential but in an energy-recycling design (as with the 7k) the coil current dumps onto a capacitor so the slews are actually half-cosine:
The resulting coil voltage is therefore a series of half-sine pulses:
Peak voltage can be quite high. For a given TX inductance, the less capacitance the faster the slew and the higher the voltage.
The nature of CCPI can be confusing. The current is essentially a square wave, but some VLF detectors also drive the coil with a square wave. However, with VLF detectors we are talking about a square-wave voltage drive and with CCPI it's a square-wave current drive. Likewise, there is another PI design known as "half-sine," but this refers to a half-sine current whereas CCPI has a half-sine voltage.
CCPI can also create multi-period waveforms:
My initial assumption was that the 8k would employ this, perhaps in a "fine mode" for small gold, but so far I see no evidence this is the case. And the Minelab (mechanical) engineer thought that the 8k had the same TX as the 7k, which was a single pulse rate. However, the 7k used 2 different pulse rates depending on modes: 1225Hz and 3700Hz. I would not assume the 8k is the same.
