My Homemade VLF Detector

[JamesPicard]

...Find some values of capacitance that when placed across the RX input will cause the RX coil to resonate from maybe 1.1x above your TX frequency, up to about 1.5x (just a guess). Check your discrimination as you go, discrimination is your goal and NOT overall rx amplitude. ...... Ummh, instead of your showing us TX and RX waves, it might be more useful if you showed us RX and phase detector output (square) waves (assuming you have a detector that uses phase detectors.) ...

Ok, I reread your message...so I should not focus on amplitude but the phase shift.

Also, I don't have the phase detection circuitry built yet....right now I am just gaining an understanding of what the waveform is doing when metal gets close to the coil.

Thanks porkluvr.

[ivconic]

And Garret is a holly cow because of ... what?

Shielding will not hurt your coil, but if everything else is done properly it is not doing much good. I intend to use a center tapped bifilar wound Rx coil and a true differential preamp stage. I expect it to be as effective as a good shielding, and I'll surely do some further experiments with it. Wire tap connected to signal ground or not etc.

No, i don't want to say Garrett is something special. I wanted to point on possible internal presets in coil.
Those might be limiting factor when trying to run coil on different freqencies.
If coil is suited and nulled (with internal presets) to run smooth at 7.2kHz than maybe it is not good idea to run it on more different frequency than that.
Coil is nulled at 7.2kHz. Residual voltage will dramatically change when frequency changed.
That's what i wanted to point on.
Center tapped RX and "true" differential preamp, without shielding, will be effective only in simulators and maybe in air.
On real soil, i am afraid; it will still demand shielding too.
But, i do support your enthusiasm! Give it a try! Maybe you will discover something new that we here don't know yet.
Regards!

[Skippy]

What are you trying to find the 'resonant frequency' of, the 'large coin'? If its the quarter-dollar you mentioned earlier, the 'best' freq to find that at will be around 1.5 - 2KHz, where it will give a phase lag of about 135 degrees. Just a thought, the residual coil output you are seeing is with the coil away from all metal objects, I hope? No metal workbench nearby, no radiators, fridges, steel rods in the concrete floor, etc?

[Qiaozhi]

Well I spent several hours on this. Still have not found the magical formula yet. Maybe only a little closer.

I would take a closer look at the Garrett coil before you get too involved with tweaking the values of the differential preamp, shielding, etc. At the moment you're trying to get a basic understanding of the induction balance principle, and all these other "potential" issues are just a distraction.

Ivconic is correct about the Garrett coils. Unlike Tesoro coils, which essentially consists of 2 loops (ignore whether it's DD or a concentric for the moment), Garrett coils very often contain some passive components. I don't know the details of the particular you have in your possession, but the Crossfire coils have a 470nF cap in series with the RX and a 680k resistor connected between the RX and TX. If your coil is the same, then this is probably the root of all your problems. I would suggest winding a DD test coil according the specifications given by Ivconic in the TGSL notes (don't bother about shielding at this stage), or obtaining a 5-pin Tesoro coil for the testing.

[ivconic]

I would take a closer look at the Garrett coil before you get too involved with tweaking the values of the differential preamp, shielding, etc. At the moment you're trying to get a basic understanding of the induction balance principle, and all these other "potential" issues are just a distraction.

Ivconic is correct about the Garrett coils. Unlike Tesoro coils, which essentially consists of 2 loops (ignore whether it's DD or a concentric for the moment), Garrett coils very often contain some passive components. I don't know the details of the particular you have in your possession, but the Crossfire coils have a 470nF cap in series with the RX and a 680k resistor connected between the RX and TX. If your coil is the same, then this is probably the root of all your problems. I would suggest winding a DD test coil according the specifications given by Ivconic in the TGSL notes (don't bother about shielding at this stage), or obtaining a 5-pin Tesoro coil for the testing.

Exactly!

[Skippy]

Have you seen this thread? The ACE coil looks conventional to me, the passive parts are just a nulling circuit. The L values are given, so you should be able to load the RX with 120nF (I calculated this for 9KHz tuning).and try it out.
http://www.geotech1.com/forums/showthread.php?t=17565
Another thing re. the residual output signal, did you connect up the screen/shield wire correctly? If this isn't done right, the null could be upset due to capacitive coupling between TX and RX coils.

[ivconic]

Have you seen this thread? The ACE coil looks conventional to me, the passive parts are just a nulling circuit. The L values are given, so you should be able to load the RX with 120nF (I calculated this for 9KHz tuning).and try it out.
http://www.geotech1.com/forums/showthread.php?t=17565

Yes i saw that. But those are fixed values inside, meaning NULL is already "fixed" for 7.2kHz.
I guess it will allow slight variations in frequency, but not more than that.
Simple check:
measure residual voltage right now at 9kHz,
than return "outer" values to suit 7.2kHz and than check residual voltage again.

[Skippy]

I was assuming that the TX should be driven at 6.5KHz. I feel the urge to dismantle an ACE I have at the moment, to see what the RX front-end is....

[ivconic]

I was assuming that the TX should be driven at 6.5KHz. I feel the urge to dismantle an ACE I have at the moment, to see what the RX front-end is....

Even better if you do that!
You can measure residual voltage at original setup than and compare!

[ivconic]

I just tried to find exact specs, seems it is 6.5kHz and not 7.2khz???
At some sites there is 7.2 and on some is 6.5kHz!!??!?

If you dismantle your Garrett, please do measure frequency first!
Everything else what we mentioned so far is valid.

Look here:
http://www.findmall.com/read.php?32,626370

and than here:
http://www.metaldetector.com/learn/f...garrett-ace250

!?!?!?!?!??!

[WM6]

, Garrett coils very often contain some passive components.

Some of last one probably not.
Example: AT PRO and AT GOLD use the same (equal) coils.
Working frequency of AT PRO is 15kHz and of AT GOLD 18kHz.
Seems adapting problem is solved in control box and not in coil.

[ivconic]

Some of last one probably not.
Example: AT PRO and AT GOLD use the same (equal) coils.
Working frequency of AT PRO is 15kHz and of AT GOLD 18kHz.
Seems adapting problem is solved in control box and not in coil.

More i try to investigate this thing on Garrett's working frequency - more i start to suspect in all what's written in those internet articles!!!

That's why is best he to dismantle his Garrett and measure exact frequency!

[JamesPicard]

Since this KelloCo Excelerator EQ2 coil is made for the ACE250, I think that thread http://www.geotech1.com/forums/showthread.php?t=17565 is probably correct. But I think even if there is passive components already built inside the coil...I would think the purpose is only to null the coil.

I'm not sure, maybe it will have some good information for us all if you look at your Garrett, but even some Garrett detectors will vary in design so it might be misleading.....?

Today, I'm going to the hardware lab and I will measure the resistance and inductance of this KellyCo coil with an accurate instrument. Will let you guys know what I find out.

[ivconic]

Since this KelloCo Excelerator EQ2 coil is made for the ACE250, I think that thread http://www.geotech1.com/forums/showthread.php?t=17565 is probably correct. But I think even if there is passive components already built inside the coil...I would think the purpose is only to null the coil.

I'm not sure, maybe it will have some good information for us all if you look at your Garrett, but even some Garrett detectors will vary in design so it might be misleading.....?

Today, I'm going to the hardware lab and I will measure the resistance and inductance of this KellyCo coil with an accurate instrument. Will let you guys know what I find out.

Good decision!
Along with resistances and inductances; would be very beneficial (for you on the first place) if you do measure exact TX frequency, and most of all: residual voltage at RX coil leads or at first opamp output... or both even better.
That's how you will have all important references that concerns you in your future work with that coil.

[Davor]

Ah! I got it. In case coupling between those two coils is 1 it would be true broadband balanced contraption, and in reality it is well balanced only within some frequency span. Actually it makes coil nulling easy. Even I could do it I guess the trimmer is not accessible. Otherwise you could fine null your coil wherever you want.