MPP Rev-D ‘Falsing’ on end of swing.

[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.

[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.

[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".

[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.

[qute1]

Are your coil bundles tightly bound and secured in the housing and are you checking outside away from emi (i had trouble near wifi signals and there are a lot of them where i live they propagate a lot further than you might think) just my four pennies worth so if you have checked this just ignore me.

[waltr]

Are your coil bundles tightly bound

My thought too. Also be sure nothing can move inside the coil housing and that the coil to box cable is not free to move about.
Also ensure everything inside the box is secured.

I run a PI MD without shielded coil without issues. EFE pulses seem to work as I get no falsing normally.
However, I do get a false if a bump the coil on something or catch the coil cable on stuff. It only takes a very slight movement of coil internals or cable near the coil to false.

[kt315]

sorry but MPP is simple design for beginners for getting new experience and starting the hobby.
you have to start to do new project, hammerhead, with GB tuning advantage. do not worry, you went through the first stage and are ready
overcome next one. you are on right way amigo.

[green]

I think you need to trim R24 or R25. Maybe lift ends going to R19, connect a 200 to 500 ohm pot ends to the resistors with wiper going to R19. Or connect a high resistance(500k, 1meg)pot across R24 or R25. Adjust pot for no indication when quickly lifting a ceramic magnet(fridge magnet) from the coil.

[kt315]

crossreferenced to
http://www.geotech1.com/forums/showthread.php?23898-False-beep-at-end-of-swing
http://www.geotech1.com/forums/showt...-from-EFE-beep

[Goaty]

Thanks everyone for your helpful suggestions.
I can't wait will work through them and respond when time permits in a few days time!

[Goaty]

Sorry about taking so long to reply but I have been in & out of hospital a couple of times since and am still trying to catch up.

I have not had time to make measurements on the board but have made a couple of quick replies to some of the comments -

@ soverey

your shielding has to be in the range of a few hundred ohms per square

The coil is wound as ‘self shielding’ and I don’t think the graphite layer was just an experimental ‘after thought’ - it probably does not contribute much to the overall shielding.






@ qute1

Are your coil bundles tightly bound and secured in the housing and are you checking outside away from emi

I tried a slow swing with a bump stop against my shoe and no falsing evident, so windings seem solid enough.




@ waltr

be sure nothing can move inside the coil housing and that the coil to box cable is not free to move about.

As above for windings and if I slightly unwind the coil cable from around the shaft and wriggle them there is no problem.


@ kt319

sorry but MPP is simple design for beginners for getting new experience and starting the hobby.
you have to start to do new project, hammerhead, with GB tuning advantage. do not worry, you went through the first stage and are ready
overcome next one. you are on right way amigo.

I fully agree – as a matter of fact I ordered a couple of the Mirage boards from Sven only a week or two ago. I believe these are based on the Hammerhead. This detector building hobby is a bit of an addictive drug isn’t it! And learning all the time.

Thanks for your cross references – I had found and read the first previously but not the second.

Well, enough for now,
I will report again ASAP

Thanks again for all the help

[qute1]

This detector building hobby is a bit of an addictive drug isn’t it!

I think you will find that we are all suffering from this particular affliction (i devote the majority of my free-time to the development, construction and testing of my mods and designs) drives the wife nuts there are literally MD's everywhere, running out of space to store even the regularly used ones never mind the collectors editions.

[Goaty]

@ Qiaozhi
“Since you've checked the main sample and the EFE sample are the same width, also check the EFE delay time.”


Today I spent some time re-measuring the pulse signals up to, and including, the pulse generator just to check my original settings and have listed my measurements below-


Transmit Oscillator (TP1)
Pulse rate - 1030 pps
pulse width 58 uS


Receive Pre-amp (TP3)
Damped flyback with correct response to metal near coil


Sample Pulse Generator (TPs 5,6,7,
Minimum sample delay (TP5) 22uS
Secondary sample delay (TP7) 120uS after start of Main sample pulse.
All pulse widths 58 uS


I’ve also started checking a few components in the Sampling Integrator since the Sample pulse section looks OK.


R24 2k147 ohms
R25 2K110 ohms ‘in-circuit’ measurements.


TP8 (Q3 gate) -4.72V
TP7 (Q4 gate) -4.71V


U8
Pin 3 non-inv input (main) 92.7mV
Pin 2 inv input (Sec) 91.8mV
Pin 1 output -117.8mV


Well, this is as far as I have got.
At the moment the sampling integrator circuit has me a little bewildered as to how it functions so any explanations would be appreciated.
Also, if anyone has some suggestions as to how to go about testing this section I would be appreciative as my understanding of op-amps is limited to just the basics.


And, one last thing I have noticed is that the sample pulse widths are all the same as the Tx oscillator (58uS) and yet in the Sample Timing diagram (Fig 10) it shows the Tx pulse widths being wider than the Main Sample and Sec. Sample widths – is this correct? Measurements of the circuit also indicate they are the same but but I was concerned that I may have missed something.


Once I get my head around the Sampling Integrator I will probably try a trimpot to adjust the signal feed levels to FET switches Q3 and Q4 but other than that I’m starting to run out of ideas.

[Goaty]

I have noticed is that the sample pulse widths are all the same as the Tx oscillator (58uS)

C11 (2n2) and R11 (22k) with U7 seem to set the pulse widths for the main & sec samples to 50uS.

What would be a suitable value?

Would this be enough to upset things?

[waltr]

If Main & Sec samples widths are the same then all is good. Exact width is not that important.

There are discussions in the threads on this forum about what and how the integrator works. I just don't have links.

Since the main & sec sample widths are the same then re-check that the integrator Caps and resistors MATCH. That is the RC on the Op-amp Feed back (out to inverting in match the RC on the non-inverting in to ground). A mismatch of these components can cause the same effect as mismatched main vs sec sample widths. Again Exact values are not important but values Matching is very important.