Nulling Tesoro Coils For Multifrequencies

[Silver Dollar]

I've decided to stick with Tesoro mainly for my growing metal detector collection.
I have a couple of coils I am just about to null but wonder how to make them
work with 10khz 12khz and 14.7khz like the originals.

I could null at the low, the mid or the high frequency and with or without a preamp.
I've seen big differences with and without the preamp and I'm not sure which way is
correct there too.

Anyone have experience with this? Or maybe you took a tour of Tesoro and saw the
magic frequency they used? (Like maybe tune at 20khz and it will work at everything lower)..

[bernte_one]

i think it is done by tuning the caps on the tx and rx stage to a specific ratio
frequency of rx and tx 1:1,2

i am far away from tesoro in moment just restart with a new golden sabre

[Carl-NC]

10-15kHz is a pretty small range and should be fairly easy to null. Low inductance coils are much easier to broadband-null, high inductance coils often need additional components to broadband-null them.

[Silver Dollar]

I have a factory Tesoro coil and Bandito. I made a Bandito and several TGSL's.
The factory coil works on the Bandito at 10 khz and on the TGSL at 14.7khz.

To get my new coil to work on both of those what frequency is best to use for
nulling is my question. I can null it at any one of them and it will most likely work
on the others but maybe using the highest frequency or lowest will make it better
on the other ones?

I'm using the standard 6.0mh TX and 6.5mh RX...

[Carl-NC]

Null at the highest frequency.

[Silver Dollar]

OK. Thanks for the Info!

[Silver Dollar]

Well I decided to build the TFT oscillator (Triple Frequency Transmitter) to test this out;


I used the 440 foam and coils (Note: They were in my lap during testing);


It was very hard to tell the appropriate null point at higher frequencies. I connected the
TFT to the TX and Buck coils and attached the second channel scope probe directly to the
RX leads and obtained the following waveforms;

10 khz (7mv p/p RX - Assume Nulled)


12.7 khz (~20 mv???) - How to tell where the RX wave is?


14.8 khz Again how to tell how big the RX is?


Maybe the scope probe is causing some ringing?

[Silver Dollar]

I also wonder which side I should be on?
This is the side with more windings on the buck coil;


This is minimun null;


This is less buck windings;

[Silver Dollar]

Well now I'm really confused. I remembered my second scope probe broke
so the scope thinks it is a 1:1 when it is a 10:1 so all my readings are off by 10x!

Also I decided to use a tuning cap to stop the ringing in the RX wave but what value
to use? I tried .015uf and .033uf with the following results;













Channel 1 is the RX and the voltages show should be correct but remember these
are straight out of the coil with just a cap across it. I thought we were aiming for 4mv...

[Silver Dollar]

So now I tried tuning with a 015uf and nulling at 10k;


Then checking at 15k;


The null voltage doubled! So I tried changing the cap to 033;


Then renulling at 15k;


Then dropped the freq to 10k;


There is less change this way but I'm still concerned the null is never very low amplitude.
40mv x 44 gain of input = 1.7V...

[Silver Dollar]

Well I found a thread here that said to load the coil with 10k and .1uf so I tried that.

I also found that if you have the correct phase when you wave a piece of ferrite at
the coil the amplitude will increase but if the phase is wrong the signal will shift before
the amplitude increases. So I'm going with that for now. I still don't know why my null
is not very deep...

[Silver Dollar]

Well I got to thinking and realized I have a know good working coil in the original
brown Tesoro that came with my Bandito so I decided to see how it performs
using the 3 different frequencies.

I used the 10K .1UF load but also tied the grounds together as they are in the
brown coil. I found that the Tesoro coil sits at low amplitude (2.25mv - 1.2mv
RMS AC) on the "other" side of the zero crossing. That is to say if you bring a
ferrite rod down to the coil you see the RX (in phase with TX) shrink and then
grow in amplitude with 180 degree shift passing through zero if you will.

TX - 10.4 khz 17.4V P/P
RX - 2.25 mv RMS - Clipped 46 mv RMS - Clipped is with the ferrite fully centered in the open hole in the coil. Max Amplitude if you will.


TX - 12.7 khz 19.1V P/P
RX - 1.7 mv RMS - Clipped 42 mv RMS

TX - 14.9 khz 20.2V P/P
RX - 1.2 mv RMS - Clipped 32.4 mv RMS

With the Fisher 440 11" coil I could not get all 3 to the "other" side and a low amplitude. Also the clipped amplitude was around 7mv
though the coil didn't allow the ferrite to be centered as no center hole...

I'm not sure if that's good enough? I'll have to set up the Bandito electronics and do some airtests. My Belden cable came in today
so I can wire these coils up now!

[Silver Dollar]

Well I found if you null to the highest frequency you can get 14 mv p/p at 10 khz
and 4 mv at 12.5 khz. I've seen these numbers quoted for the residual volts on some
factory Tesoro's so it seems they are in the same boat.

I still do not know which side I should be on. I was thinking I should be where a ferrite
brought into the center of the coil causes the RX voltage to go through zero then gain
amplitude but if I wave a gold ring at the coil I see increasing amplitude with no phase
shift.

Some claim that the residual RX voltage doesn't matter as long as it doesn't saturate the
preamp. The factory Tesoro does shift through zero but what frequency was it tuned too?
And is it the best tuning for a coil at a fixed frequency?

[Davor]

Lately I learned about 2 frequency true benefits and I'll be watching your advances.

[Silver Dollar]

I have tuned and closed up 2 coils, the 11" and 6". I noticed something funny,
one seems to like 10 khz and the other likes 14.8 khz. I tried to tune them similarly
so don't know why. My indicator is the null signal response to ferrite. I'm not even
sure that tells you anything.

I tried to set them up with 12.5 khz minimally on the "other" side of zero. It ended up
being ~1.5mv rms for both coils. This makes the 10 khz be 4 - 5 mv rms on the "other"
side and 14.8 khz to be 3 mv on this side. But waving a frerrite stick at them shows
a difference in the amplitude reached at each frequency. The 6" gains the highest amplitude
at 10 khz and the 11" gains at 14.8 khz. I will have to attach them to a working detector
and do more testing to see if they work OK or not...