Also called RF detectors and depth multipliers. Invented in the 1930's by Gerhard Fisher (founder of Fisher Research Labs, now part of the First Texas conglomerate), this type of detector seems to not have changed much in operating characteristics in 80+ years. From what I've found there are still three competing models in production: Fisher's current Gemini-3 model (now 30 years old) https://www.detectorprospector.com/metal-detector-database/fisher-gemini-3-r27/, Garrett's Treasure Hound Eagle Eye Depth Multiplier (quite a mouthful) which is an add-on to their aging GTI 2500 standard style metal detector https://www.detectorprospector.com/metal-detector-database/garrett-gti-2500-r44/ and White's TM808 https://www.detectorprospector.com/metal-detector-database/whites-tm-808-r38/ , a spinoff of a collaboration between White's and Discovery, another Sweet Home, OR company that appears to now be defunct.
The Fisher unit, operating at 82 kHz, seems to be the most closely aligned survivor of the early design. Garrett's 'latest' depth multiplier (easily the most expensive at $1300 retail when combined with the GTI 2500) with its 7.2 kHz operating frequency advertises its advantage of being in the VLF range, ignoring ground minerals. White's unit (operating frequency = ?) has ground cancelling controls. Unlike today's deep PI's and ZVT which can also go deep, these 2 box transmit/receive (T/R) detectors ignore small objects -- an advantage when the sought after target is large and in a trashy area.
That's a brief overview of the current commercial climate, but what I'd like to know is how much different these three units are when it comes to operating characteristics. Specifically do the differences in operating frequency and/or ground adjustments translate into better performance under certain conditions?