TESORO GOLDEN SABRE

[dfbowers]

thanks for response!
I guess you are right about lm308. But I will try it anyway when I receive it.
If you say that your tgsl also dont detect coin with ferrite then I guess its ok too.

TRIED that coil with thichness of 2.5mil and didnt saw differences. Inductances/dimensions was the same so I guess I need to chang those parameters to see differences.


Eduardos PCB not finished yet so cant tell my exp.

I put everything in the box today and attached actual cable (1m instead of 20cm).
All is ok but I got one problem: when i touch the boxes conductive areas I got falses. I got plastic box which have inside (and few areas of outside) sprayred with Kontakt EMI http:http://www.gemalta.com/aerosol-emi-35-200ml.html
Is this normal? touching non-conductive areas and cable, even a coil (not overlap area) is ok with no beeps...

i got few questions:
I have read about null phase and if I understand correctly - if I can ground balance, i got phase right.
But also heard that better is phase of 20 deg. So question is : 20deg leading (+) TX or lagging(-) tx?

Is it ok that my response of silver ring depend on how I have ground balanced? i discovered that I got about 30 deg turning angle on my gnd bal. pot. where I can balance out ferrite. When I gnd balance on one end of those 30deg I got no response for silver. All other metals ok. But when I gnd balance on other end of those 30deg i got silver response..
I hope you understand.

I can create false signals by touching any area of my cable that does not have a shield (like the 4 pin connector on my coil cable) but I generally do not handle that area anyway. I think that if I had a metal enclosure and it were grounded to the connector and circuit ground, I could eliminate that problem. I had problems with an external GB pot until I added a ground to that as well.

There has been a lot of discussion here in the past on null phase:
http://www.geotech1.com/forums/showt...987#post114987

If I understand this right, it provides for maximum range for the GB adjustment pot. Plus, ideal ferrite would be rejected when the Rx signal zero crossing occurs at the center of the sample pulse.

I'm trying to find a link where the 20 degree phase shift theory comes from, so maybe someone else can answer.

The GB setting is critical in that advancing the GB pot too far, you will suddenly reject thin silver. So.. you may want to set your GB pot to where a ferrite slug is just breaking up. With a PCB mounted GB pot, I start fully CCW and advance CW until ferrite is rejected. Actual ground conditions were I live allow me to set the GB pot several degrees CCW from where ferrite is rejected.

Don

[habitbraker]

thanks ! I read that thread - lots of good info. About null phase - its clear.
About my touching enclosure problem, thats how its looking:



I discovered that problem occurs only when I touch front (showed) panel. Back panel is spayed too, but no toucing problems. And in disc mode I actually can discriminate out that problem. About 60% of disc and its silent. But I disriminate useful coins as well....

[Qiaozhi]

I'm trying to find a link where the 20 degree phase shift theory comes from, so maybe someone else can answer.

The 20 degree phase shift is a requirement of the electronics in the TGSL. When the GEB pot is set to its middle position, the sample pulse is approximately 20 degrees offset from the TX signal.

[dfbowers]

thanks ! I read that thread - lots of good info. About null phase - its clear.
About my touching enclosure problem, thats how its looking:



I discovered that problem occurs only when I touch front (showed) panel. Back panel is spayed too, but no toucing problems. And in disc mode I actually can discriminate out that problem. About 60% of disc and its silent. But I disriminate useful coins as well....

Do you have the GB pot mounted on the panel?

[habitbraker]

yes - GB pot mounted on that panel.

[dfbowers]

The 20 degree phase shift is a requirement of the electronics in the TGSL. When the GEB pot is set to its middle position, the sample pulse is approximately 20 degrees offset from the TX signal.

Thanks Qiaozhi..

I'm wondering if that is just to get the phasing for DISC an GB to work correctly, and if there is some mathamatical explanation that's easy to understand.

I'm in the middle of writing a TGSL "101" manual, sort of like the "TGSL for Dummies" idea.
I would like to get that detail correct as that question comes up frequently.

When I complete it, maybe add it to the "TGSL compete details".. It may curb a lot of the repetitive questions from new builders that don't read or cannot find a lot of the important points buried in the huge thread! But.. reading all the threads give great insight on how things are arrived at!!

Of course, one of the best ways to learn something is to write about it..

[dfbowers]

yes - GB pot mounted on that panel.

Hmm... Try this. Unmount it and see if that fixes your problem. Or, ground the case of the pot with an extra wire right to ground.. I will bet that your problem goes away!

I think that adding a small bit of capacitance to the pot with your hand is just enough to cause a small phase shift in the GB circuit.

[Qiaozhi]

Thanks Qiaozhi..

I'm wondering if that is just to get the phasing for DISC an GB to work correctly, and if there is some mathamatical explanation that's easy to understand.

Although the TGSL electronics is the reason why the coil must be nulled that way, there is nothing to stop you having a different phase-shift (as long as the residual amplitude is fairly low) and you redesign the electronics to sample at the correct position. Also, if you study the output of the pre-amp for different coil overlaps, you will see that only one position satisfies all the criteria for the TGSL to operate correctly. I don't know if there is an mathematical explanation, and it may just be down to good old trial and error.

[simonbaker]

I'm still puzzling the question of null phase shift.

Here are observations I am starting with:

If you think of the RX and TX coils as an ideal transformer and the RX coil not connected to anything, then there should be two phases possible for the RX coil as you slide it across the TX coil. One is where the net magnetic flux balances one way, and one where the net flux balances the other way. The two phases should be virtually 180 degrees apart. As you slide the coils toward the null point, the RX signal should diminish in amplitude, but the phase doesn't change. It goes to zero at the null point, then suddenly shifts phase 180 deg and begins to increase again as it passes the null point.

Now connect the RX coil to a capacitor, throw in some non-ideal resistance, and you have a resonant circuit that will have a different phase, but again just two distinct phases. Are they 180 degrees apart???? Agh, need to do the math. But the phase will be determined by the LCR parameters.

With our actual TGSL, near the null point we observe that the phase shifts smoothly from one point to another. I suspect the reason is that we have some capacitive coupling from the cable or shield. The shield also magnetically couples the coils probably, and perhaps adds a signal that has intermediate phase.

Now, what is important and what can we control? Qiaohzi has explained the reason we want to choose a null phase that allows us to set the ground balance pot near its middle position to allow best practical adjustments if the ground changes. This is based on the fact that ferrite-like material modulates the amplitude of the null signal but does not change its phase.

I agree with that in theory, but I'm not sure there is much practical choice to be made with the TGSL. I think when we try to fine-tune the null signal phase by shifting the coils, we are really just varying the relative size of the capacitive component of the coupling and not changing the magnetically coupled phase. Not sure about that, but I suspect it. Anyway, my thought is that the capacitive component is not modulated by ferrite, so it really should be ignored. If you could see only the magnetically coupled null signal, it may be true that it is mainly one of two phases, like the ideal case; in which case, you are stuck with choosing one or the other by choosing one side of the null point or the other.

If you really want to change the phase of the null signal, you need to change the RX resonant frequency relative to the TX frequency. But the TGSL design pretty much fixes it in a small range if you follow the design.

In other words, my feeling is that you can't accomplish much by tweaking the RX null signal phase during nulling the coils. Picking the best side of the null is the real mission, and just enjoy whatever phase you end up with. Hopefully, the important magnetic component of the null signal will be fine and dandy.

... to be continued no doubt...

-SB

[Qiaozhi]

... in which case, you are stuck with choosing one or the other by choosing one side of the null point or the other.

The "normal" decision is to choose the side that provides the largest overlap between the coils. This will increase the size of the sensitive area.

[habitbraker]

Hmm... Try this. Unmount it and see if that fixes your problem. Or, ground the case of the pot with an extra wire right to ground.. I will bet that your problem goes away!

I think that adding a small bit of capacitance to the pot with your hand is just enough to cause a small phase shift in the GB circuit.

tried unmounting GB pot - no difference. Tried lift all panel out - no difference. And back panel is sensitive too though...
Its almoast like its detecting metal - when I dinamically move metal very close those panels, It beeps. I think I failed a litte bit with that spraying
Its hapening in all metal mode only. Disc mode is perfect quiet.
In spite of this little problem its working perfect. Although my shielded coil makes large min null (about 1.3VPP) and my null phase is far from 20deg(around 70deg), I can totally reject 20cm ferrite in close to middle GB pot, with close to max sensivity can detect silver and coins at 30cm while discriminating iron and al foil.
Next is coil enclosure and potting.

[habitbraker]

Problem solved! I was sprayed pcb standers too so that they touched PCBs GND plane on the sides(where screws are). In other words I was grounded PCB to case with more than one "wire". I guess ground loops occured.
Took off that spray from few places and it took care of case touching problem

[Dennis the Mennis]

Thanks Qiaozhi..

I'm wondering if that is just to get the phasing for DISC an GB to work correctly, and if there is some mathamatical explanation that's easy to understand.

I'm in the middle of writing a TGSL "101" manual, sort of like the "TGS(L) for Dummies" idea.
I would like to get that detail correct as that question comes up frequently.

When I complete it, maybe add it to the "TGSL compete details".. It may curb a lot of the repetitive questions from new builders that don't read or cannot find a lot of the important points buried in the huge thread! But.. reading all the threads give great insight on how things are arrived at!!

Of course, one of the best ways to learn something is to write about it..

Hi Don ,

That was my exact same idea when I started my ' TGS for Dummies '.
I've started with much enthusiasm to gather all information and would add photo's as in the 'TGScompare' document and / or with videoclips as the ones I've posted on Youtube.

Unfortunately my pcb would not co-operate is much as I would ( perhaps this is an other way to say that I just don't understand it ), especially the phase shift between Tx and Rx.
So how to write , photograph etc without a working pcb, thats why it will take some more time , the enthusiasm however is still there.

I've ran into the same problems as Habitbraker posted : Phase shift >> 20 degrees ...
I guess I've been staring myself blind on this issue and it's time to move on.

kind regards ,

Dennis the Mennis

[simonbaker]

Problem solved! I was sprayed pcb standers too so that they touched PCBs GND plane on the sides(where screws are). In other words I was grounded PCB to case with more than one "wire". I guess ground loops occured.
Took off that spray from few places and it took care of case touching problem

Good work. Just curious, what was the azimuth and right ascension of Venus at the time...?

[simonbaker]

Hi Don ,

That was my exact same idea when I started my ' TGS for Dummies '.
I've started with much enthusiasm to gather all information and would add photo's as in the 'TGScompare' document and / or with videoclips as the ones I've posted on Youtube.

Unfortunately my pcb would not co-operate is much as I would ( perhaps this is an other way to say that I just don't understand it ), especially the phase shift between Tx and Rx.
So how to write , photograph etc without a working pcb, thats why it will take some more time , the enthusiasm however is still there.

I've ran into the same problems as Habitbraker posted : Phase shift >> 20 degrees ...
I guess I've been staring myself blind on this issue and it's time to move on.

kind regards ,

Dennis the Mennis

Hi DtM! You'll get there because you have the right approach!

On the general topic:

Even though I've said my piece about the null phase, let's review what we mean.

1. When someone says 20 degrees, what signal is being measured? Rx coil leads, or output of LF353? Is the Rx coil connected to the PCB or disconnected (I assume connected).

2. What is the null signal phase referenced to? (The Tx signal I presume).

3. What does 20 deg mean, leading, or lagging? How do we measure lead or lag?

Diagrams would be useful to make sure we're meaning the same thing here.


-SB