MPP Rev-D ‘Falsing’ on end of swing.

[Goaty]

I will recheck those pulse widths more carefully as soon as I get some spare time as the tolerance of the caps and resistors is likely to be sus.
How close should the pulse widths be to the suggested values and to each other?

(Unfortunately going to be out of action for the next few days so it will be a while before I can get to it).
I did try several different TL062s (TL072) (Uwith no change. Also, I guess I could pull and check the two J113 fets – would it be worthwhile matching these or putting in a balance pot in front of them?

Anyway, thanks for the suggestions. Some experienced advice gives reassurance and helps to point me in the right direction.

[Qiaozhi]

It definitely appears to be an EFE problem. Since you've checked the main sample and the EFE sample are the same width, also check the EFE delay time. Otherwise it must be a problem with the sampling integrator. Did you use 1% tolerance resistors and low tolerance capacitors. If not, the two channels may be out of balance. I would have expected the ceramic magnet to be cancelled at a distance of less than 5".

[soverey]

your shielding has to be in the range of a few hundred ohms per square , lets say 400 ohms , if its of much higher resistance is may be ineffective. It may be also a case of earth field circuit that need tuning , can't give much detail as I'm not familiar with MPP circuit.

[Goaty]

My recently completed MPP seems to be working well as far as sensitivity, but ‘sounds’ with fast swings or sudden direction changes such as at the end of swings. It makes no noticeable difference if I swing the coil close to the ground or higher in the air.


I suspect that this is due to sensitivity to the Earth’s magnetic field and have read all I can find on the Geotech website and in IMTD and have also thoroughly rechecked the circuit board components, especially around the sampling integrator.


It responds to a small ceramic magnet at about 5” from the coil but only weakly to a 1/2” ferrite toroid.


The sample pulse generator section seems to be producing the correct pulses when viewed on the CRO.


I am using an 8” 3DSS with a bit of extra graphite shielding and a twisted pair feedline. I have tried another bundle wound coil with similar results.


The control board is shielded above and below in a copper clad pcb board sandwich connected to B+ (0V).


As it is, it is useable but the falsing is annoying so any suggestions on what to check and what measurements to make would be appreciated.