Coil Preamp question

[James]

I am curious why the detector companies are put the RX preamp in the coil ? If there is a advantage to using this method, what would the percentage in over all gain performance be ? Is there so much distortion in the signal going from the coil through a short piece of coax or shielded cable to really see the over all gain. I am just curious an wondered what the bases for this concept to be incorporated. Happy Holidays an best to all Jim

[6666]

piracy.

[Carl-NC]

There is potential for the cable to pick up noise, either EMI or from the parallel TX cable. With the preamp in the coil you get immediate gain that improves the SNR, plus the coil is now on a low-impedance driver.

Piracy & control of aftermarket coils is why companies are putting micros (or at least security chips) in the coil.

[Skippy]

Cable can produce its own signal when flexed, a phenomenon known as 'microphony'. If the signal coming from the amplified coil is 20 times greater than 'standard', the cable signal is effectively 20 times smaller.

Contact resistance on the coil connector can be a problem over time, especially as manufacturers seem to like using the cheapest poorest connectors, with plain tinned surfaces etc. A larger coil signal helps overcome these problems.

An amplifier allows to option of sending the signal differentially down the cable. This reduces the dependance on having a good low-impedance 0 volt ground return with minimal voltage drop. And as any electrical noise pickup is likely to affect both wires similarly, it gets cancelled out by the differential receiver circuitry. Ideally, the differential pair should be twisted together, with an overall screen. It's notable that though the Minelab Equinox uses differential transmission, the two wires are plain; not twisted, not screened.

Different size coils have different sensitivities. If there is a pre-amp inside the coil, its gain can be tailored to that coil size, without the main detector having to be re-adjusted. [ However there are other reasons why the main box should know about what coil type ( CC/DD etc ) and size is plugged in, for example adjusting the depth gauge, changing the signal filtering characteristics, dealing with problem targets ( like steel crown bottle-caps with DD coils )]

[James]

Thanks Carl, product prtection from outside sources will reduce the profit an performance of the product. In todays market place, one has to very careful with any idea or product. Best to all Jim

[Skippy]

You're welcome.

[kt315]

Is there so much distortion in the signal going from the coil through a short piece of coax or shielded cable to really see the over all gain.
----
ml does not use shielded cable. in x-terra a cable is LAN twisted pair. in GT a cable with 4 wires.

[dbanner]

Engineers like to squeeze every bit of efficiency from their designs, so if there is some little thing to be gained, even cost savings in materials, they will do so. Also, putting electronics in the search head makes it that much harder to use non proprietry coils.