DetectorMods

Author Topic: Major discovery applicable to all SD GP and GPX series detectors.  (Read 13552 times)

filternozzle17

  • Newbie
  • *
  • Posts: 17
    • View Profile
Re: Major discovery applicable to all SD GP and GPX series detectors.
« Reply #15 on: September 07, 2017, 07:01:43 am »
Would you clarify the difference between front end gain and rear LCD gain.  Thanks.

woody

  • Administrator
  • Sr. Member
  • *****
  • Posts: 280
    • View Profile
Re: Major discovery applicable to all SD GP and GPX series detectors.
« Reply #16 on: September 07, 2017, 08:10:45 am »
Sensitivity and low noise are dictated by where the low noise components and gain is set in the signal gain block. Firstly you get maximum depth and sensitivity the most important thing is to remove resistive loss before the first amplifier, this includes the resistive characteristics of the input mosfets. With a coil resistance of 0.5 ohm the resistance of the input fets at around 5 ohms eats up a fair amount of the signal that is available to be amplified. Other losses are the capacitance of the coil, the cable and capacitance of the input components that reduce the initial slew rate of the detector to be able to recover fast decay target signals. To recover all available signals the initial input stage should have low noise and adjustable gain to suit the ground conditions. The set gain of 33 gpx5000 or 47 gp series and gpx4500 is too much or too little depending on ground decay characteristics. The back gain takes place after multiplexing and as far as i can tell with the existing block gain there is signal loss and trying to boost it greatly increases the overall system noise and target signals can be swamped by the noise. The front end gain sets the input signal at a level that produces optimal signal levels in respect to ground noise the get an improved signal to noise ratio and get the most out of the detector. Many variables need to worked out properly and not just increase the gain, sometimes less is more and sometimes it is the opposite. I also found excess capacitance can cause an offset in the ground balance and thus needs to be greatly reduced as to get better ground balance.

filternozzle17

  • Newbie
  • *
  • Posts: 17
    • View Profile
Re: Major discovery applicable to all SD GP and GPX series detectors.
« Reply #17 on: September 11, 2017, 11:25:08 pm »
Thanks for that explanation Woody.   
I don't know if you are familiar with the old Nautilus DMC-IIb.   A 14kHz VLF detector, this has settings on it for balancing the coil to the machine.  There are two controls Resistance and Capacitance (marked R and C)  - Maybe this is why even to this day it is the deepest seeking VLF I have used.

woody

  • Administrator
  • Sr. Member
  • *****
  • Posts: 280
    • View Profile
Re: Major discovery applicable to all SD GP and GPX series detectors.
« Reply #18 on: September 12, 2017, 12:54:48 am »
Around 10 years ago I wrote an article on geotech about installing variable damping resistance into the SD2000 or any P.I detector, setting the damping to match the coil and to a degree the ground reactance can make a substantial improvement to the early time detection of small signals. It would not be too dificult to incorporate an automatic variable damping system into a P.I to critically match the coil characteristics when using different modes of timing to cover various decay profiles. It would be even better to use dual damping on different pulse lengths to optimize the decay on both the high and low voltage tx pulses to better match the the system to the coils that all differ in decay due to manufacturing variations.