Some Notes on Bruce Candy (Minelab) Legacy Pulse Induction/ZVT Technology Advancements 30 Years

[RONS DETECTORS MINELAB]

 

In 1995 inventor Bruce Candy released the SD-2000 (Super Detector) this pulse induction breakthrough Dual Channel technology made it possible to penetrate through the worst ground minerals that a VLF (continuous frequency) detector had no chance of penetrating through. This was capable through using pulses (pulse frequencies) to introduce a brief time off between the transmit and receive pulses to allow mineralized ground signals to mostly dissipate prior to the receive pulse coming on.

1996 Came out with the SD-2100 adding the manual tune control. 1998 Came out with the SD-2200D adding its SURFACE discrimination capabilities, tone ID and auto tune. Power to coil voltages could be increased with adding a Li-ion batteries systems that could be pushed to 7.4V safely. 1 gram nuggets could be heard if in low EMI environments using the aftermarket coils and the Coiltek pocket rocket 7.4V battery system. I have heard that a lot of millionaires owe their fortunes to the SD-2000.

Note: These Dual Channel SD-2000, 2100, & 2200 with its longer pulses (Normal Timing) was excellent at finding the larger deep nuggets in which at the time no one was complaining about because they were finding plenty of them initially.

2000 Came out with the GP-Extreme adding DVT (Transmits Two Voltage Levels) thus further refining the previous Normal timing channels of the SD series into more timing options such as Normal, Salt, and Sensitive. Sensitive timing added sensitivity to sub gram nuggets and the Salt timing adding better alkaline salt mineralization handling. 2003 GP-3000 refined threshold. 2005 GP-3500 added tracking speeds option and a GB handle button. The GP series was regulated to 6.8V. reducing the coil voltages from the 7.4V SD series.

 2006 GPX-4000 First digital version adding many audio filtering controls. The first Smooth class timing was also added for use with mono-loop coils on ground previously achievable only with a DD coil.  

2008 GPX-4500 SETA was another noticeable improvement adding better audio filtering and threshold stability while used in conjunction with all the six timing options. Sharp timing was added to replicate the higher coil voltage of the 7.4V SD for increased performance over the standard Normal timing. Enhance timing was also added as a second refined smooth class of timings for use with mono-loop coils. Note: The original 4500’s circuit boards were changed to a different board type when the lower priced GPX-4500 version was reintroduced to the U.S. market.

2010 GPX-5000 had quite a few additions but more noticeably was the ground balance was more refined or improved. A couple more timings were added (Coin & Relic / Fine Gold and a couple more were refined notably (Salt Gold / Enhance). Fine Gold was the 3rd smooth class timing added to the GPX-series and had high sensitivity in mineralized ground to the smaller gold compared to Enhance, also designed for use with mono-loop coils. Coin/Relic was a very deep timing for benign soils (quartz dumps or white sand beaches).

2012 SDC-2300 MPF (Multi-Period Fast) utilizing a faster Enhance/Fine Gold type of pulse that produced a near instantaneous pulse off prior to the receive signal coming on gave the SDC the ability to have less decaying ground signals coming through. All the previous Conventional PI detectors struggled with this un-ground balanced decay signal and is one of the reasons why the smaller gold was being largely masked in mineralized soils. Salt Mode was another timing optimized for saline conditions. It also uses a Fast and fully Automatic Ground Balance. The smaller mono-loop coils used in conjunction with the faster pulses also allowed for better EMI handling as well.   

2015 GPZ-7000 ZVT & CW (Zero Voltage Transmission & Continuous Wave) utilizing the transmit and receive signal simultaneously about all the time twice that of a PI which in comparison is transmitting about half the time and receiving for the other half of the time. ZVT has the same major advantage as PI for ground balancing in not detecting the major soil component X, but has the same advantage of CW metal detector technologies compared to PI for detecting very large nuggets because ZVT is CW (unlike PI that is not CW), and also the same advantage as PI for being relatively insensitive to saline soils compared to VLF (Candy 2015).

Note: Signals generated in soils by the transmitted magnetic field mostly consist of three different types: Reactive Soil or X component, Viscous Remanent Magnetism Component (It is usually responsible for most of the ground signals that may not be completely ground balanced, if any), and Saline or Salt Component (In PI and ZVT detectors, the receive signal processing is designed so that signals from saline eddy currents are only noticeable from more highly saline soils [Candy 2015]). 

Note: Simplistically the GPZ-7000 behaves most similarly to a GPX-5000 Normal (or also Sharp) timing when using a Mono-loop coil but with the ground noise being typically less when using the General Gold Mode than the GPX-5000 using a mono-loop. With the GPZ-7000 using the High Yield Gold Mode with the Normal Ground Type, the GPZ-7000 will have somewhat more ground noise than the GPX-5000 using a Mono-loop and Normal (Candy 2015). High Yield transmitted field switches pole direction three times more often than the General Mode making it better for medium or small nuggets. GPZ-7000 Difficult or Severe timings or GPX-5000 & SDC-2300 Fine Gold and Enhance timings and GPX-5000 Smooth timing typically produce far less ground noise than the GPZ-7000 Normal timing as the degree of VRM (Viscous Remanent Magnetism Component) soil saturation is considerably less for Difficult or Severe than Normal timings. 

2021 GPX-6000 Geosense-PI rapidly suppresses unwanted signals via three overlapping feedback systems for super-fast detection of all gold pieces. The Double‑D coil has characteristics that allow it to cancel out EMI and conductive (salt) signals. All GPX-6000 mono-loop coils are semi-spiral (Rectangular) wound. Geosense does not track the Ferrite Component of the ground. It uses other techniques to remove the X Component response that is invisible to the operator.

No KBA (knowledge Based Article) mentioned by Candy but Australian Patent No. 2020203297 talks about possibly GEO-Sense is a combining of RX Gain and Soil Cancellation into one function as stated below:

"Figure 2 depicts a single knob to control the sensitivity of a metal detector and the amount of soil cancellation from receive signals of a metal detector. When turned to one extreme (for example to the left most), the effective sensitivity of the metal detector is at the lowest; and when turned to an opposite extreme (for example to the right most), the effective sensitivity of the metal detector is at the highest. At the same time, the amount of soil cancellation is at its highest when the knob is turned to left most; and at its lowest when the knob is turned to the right most. "

Note: On the GPX-6000 Normal and Difficult timings appear to have very similar target response on nuggets but can have a strongly different sound response on more mineralized ground, especially on Hot Rocks. 

Note: The 6000 timings are like the other GPX detectors, just really skewed towards fine gold. There is also a smaller, itty-bitty pulse slapped on the end of the cycle unique to the 6. So it has long-short-short-short-tiny instead of long-short-short-short (Personal Correspondence Electronic Engineer 2025).

 

To be continued:

 

[Zephyr]

Ron, that was a really enjoyable read. Thanks for posting!

 

I have read this statement in the GPZ-7000 manual, Bruce Candys paper on zero voltage transmission, and now your write up.

”…has the same advantage of CW metal detector technologies compared to PI for detecting very large nuggets because ZVT is CW (unlike PI that is not CW)”

can you elaborate on this? 

[Mountaineer Matt]

This is very interesting Ron. I look forward for to the next post.

[RONS DETECTORS MINELAB]

Hi Zephyr,

Great question!

VLF sinewave detectors have a major disadvantage in having to ground balance out all of the soil signals simultaneously ( all three soil components X, VRM and saline).

PI metal detectors have a major advantage of not even detecting the major soil signal; the X component, but only the saline component, but this is mostly insignificant, and the VRM component. This makes the capability of PI technology to ground balance far more accurate than VLF Detectors.

A disadvantage of PI is its lesser ability to detect very small nuggets compared to VLF detectors. Another disadvantage of PI is its capability at detecting very large nuggets compared to CW metal detectors, such as ZVT. CW means Continuous Wave and includes all technologies that do no not have zero transmit periods (almost all technologies other than PI) (Candy 2015).

ZVT CW has the sensitivity close to a VLF but the ground handling of a PI, best of both technologies combined. Ron

 

[Zephyr]

Right on Ron,

I guess I should have clarified more so what I was asking, is the advantage of the CW In detecting larger nuggets over that of the  PI because it’s not limited by the pulse duration not being long enough to “excite” a large target? 

So is this saying a VLF (being CW) in ground that is non-mineralized would detect a very large object better than a PI set to a long duration like Sharp or Normal? 

[RONS DETECTORS MINELAB]

14 minutes ago, Zephyr said:

is the advantage of the CW In detecting larger nuggets over that of the  PI because it’s not limited by the pulse duration not being long enough to “excite” a large target? 

 ZVT & CW (Zero Voltage Transmission & Continuous Wave) utilizing the transmit and receive signal simultaneously about all the time twice that of a PI which in comparison is transmitting about half the time and receiving for the other half of the time. 
 

Zephyr,

I would think since the CW transmit signal is twice that of PI that it would punch pretty deep, probably like you said similar to a VLF in benign soils.

Ron

[Nikko]

On 2/5/2025 at 6:16 PM, RONS DETECTORS MINELAB said:

"Figure 2 depicts

Where's figure 2?

[Steve Herschbach]

12 hours ago, Nikko said:

Where's figure 2?

https://web.archive.org/web/20250516223922/http://www.minelab.com/__files/f/254716/KBA 24-1 Basics of the GPZ 7000 Technology Zero Voltage Transmission (ZVT).pdf

[Shojo510]

Thank you