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moded gp extreme

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Joeboy:
Hi Peter,

This will probably sound like a silly question, is the signal control on the 3500 is it a gain adjustment. Does your variable front gain mod provide more gain over standard and how does it effect the signal control if at all.
Cheers Joeboy.

woody:
Hi Joeboy,  Well i could go into explaining about low frequency shot noise or 1/f noise or taking advantage of a low resistance signal impedance input stage to reduce overall noise and improve the signal to noise ratio but it could be convoluted and tedious. Firstly the the coil resistance is 0.5 ohm, in a normal minelab the N channel input Fet is around 2 ohm, the secondary N or P Fet (depending on model) is around 3 ohms.  2+3 = 5 ohm, to have complete power transfer the 0.5 ohm of the coil should  see 0.5 ohm or less series resistance. Firstly I replace the input Fets with a 0.750 Ohm device and a 0.2 ohm device.  The gate capacitance is too high to effectively with the Fet Gate capacitance and does not sustain a steady gate current and thus the fluctuations in gate drive modulate the series resistors of the Fets causing noise. This can be seen in the non steady state of the integrators with shorted inputs.
So we overcome some of the inherent gate modulation and resistive channel noise by using V type Fets that actually have lower intrinsic noise. The input Operational amplifiers are switched out (due to them having fixed 12k ohm resistors internal on their input n channel transistors and going to transistor input with very low ohm base spreading resistance with a moderate Hfe to get lower noise. Then the output from dual differential input stages is then after moderate fixed gain amplification is buffered by two low noise fast opamps that are then fed to a dual channel matched tracking variable gain amplifier.  All components critical to operation are 0.1% tolerance and metal film to lower circuit noise at all critical amplification or signal shaping stages. So utilising a low noise front end you can raise the gain up or down while retaining a very low noise figure with low noise gain control. There are some other secrets in the design but everything is done to get as much low level signal and get it out of the noise to get signals that the detector would lose in its own noise and resistive losses. There is a trade off using a given amount of fixed gain as the greater the gain in the first stage dominates all noise in the system down the amplifier chain. With all detectors, noise is the thing stopping prospectors getting the gold.

Joeboy:
Hi Peter,
Thanks for your detailed explanation.
Joeboy.

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