Tesoro Cibola

[Jerry]

With additional infos that i got recently; i made corrections in Cibola schematic and finally made it.
There are many differences between "original" one and the one i made.

Yep I agree, Ivconic gave credit where it was due and also stated that he had made some changes. I see nothing wrong with that.

Ivconic, just keep doing things the way you do. I greatly value your posts on Geotech.

Jerry

[bernte_one]

many thanks ivconic

had all made today by tonertransfer method , had some freetime





regards bernd

[Altair]

Hi Ivconic, stay with us forever!

[bernte_one]

hello

i´m short before finishing the board
an there is something unclear
right beside the 2907 and some mm over the 150R resistor are three holes palnned on this board
but i dont see any function for this holes on shematic
is htis a place for a potentimeter or something else with three connections?

or is this the switch for all metal and disc mode
i think it is, right?

and what is the best combination of npn and pnp transistors
for the npn marked on board i used bc547c
may bc557 for pnp?

and where to connect idx vdi (x and y position)
regards

[nakky]

Sorry I am wrong,to fast,did"t check.



Grt Nakky

[Jabberwock]

hello

i´m short before finishing the board
an there is something unclear
right beside the 2907 and some mm over the 150R resistor are three holes palnned on this board
but i dont see any function for this holes on shematic
is htis a place for a potentimeter or something else with three connections?

or is this the switch for all metal and disc mode
i think it is, right?

Hi, when looking at the schematic, it appears to be the frequency shift switch.
The one with B,W,R,G,Y labelled pins.

I'm in the process of repairing a Cibola, and this schematic is helping me a lot to check where things are going because previous owner attempted to repair with too hot soldering iron and many tracks are gone..

Btw, I'll soon try to build a DD coil for the cibola. If someone is interested in gauge used and number of turns, I can provide some info (untested for now, but should match original values)

Regards

[golfnut]

Lost here!

On the preamp the Rx coil/cap is in series. So approaching resonance this coil/cap will be a short to ground - this would loose your Rx signal ?


How does that work??

S

[Davor]

No, you'd lose only the inductive reactance, which is good. This is the series resonant circuit and you can observe it as a voltage source in series with a coil and a capacitor, so the best energy transfer and the least noise happens at resonance. This resonance is very blunt as the Q factor is rather low, below 0.5, but phase at resonance is 0° and you do not need detuning to make it behave.
In my opinion this is far superior setup than the parallel resonance one, provided you have a real low noise op amp there. At resonance the op amp sees the coil resistance as the source impedance, and that is your ultimate noise source. In case your Rx coil has 50ohm resistance, you could benefit from opamp with 1nV/sqrt(Hz) noise.

Trouble with parallel tanks at off resonance is that such setup provides AM->PM conversion due to the off resonance amplitude and phase slope response, resulting in fuzzy discrimination.

[golfnut]

No, you'd lose only the inductive reactance, which is good. This is the series resonant circuit and you can observe it as a voltage source in series with a coil and a capacitor, so the best energy transfer and the least noise happens at resonance. This resonance is very blunt as the Q factor is rather low, below 0.5, but phase at resonance is 0° and you do not need detuning to make it behave.
In my opinion this is far superior setup than the parallel resonance one, provided you have a real low noise op amp there. At resonance the op amp sees the coil resistance as the source impedance, and that is your ultimate noise source. In case your Rx coil has 50ohm resistance, you could benefit from opamp with 1nV/sqrt(Hz) noise.

Trouble with parallel tanks at off resonance is that such setup provides AM->PM conversion due to the off resonance amplitude and phase slope response, resulting in fuzzy discrimination.

Still a bit lost to be fair.

Parallel tank is high Z and will give largest voltage out at resonance.
Series tank will give low Z at resonance and no output volts.

Dont tell me that these basics have changed and nobody told me!

S

[Davor]

This reasoning is perfect in case you apply signal from outside to the tank, parallel has high impedance, and series shunts it to the ground. But here the source of signal is a coil, and you can picture it as being in series with the coil. That makes detector coil somewhat counter-intuitive.
In case of a series tank, a voltage source is in series with a series resonance, as if it is not there at all.

But there is a very bad situation happening with parallel tank that is usually overseen - it works as a L configuration low pass filter, providing some level of impedance transformation near resonance, but also having nasty phase transition, hence a nasty group delay spike near resonance. Trouble is that VLF metal detectors use delay-induced phase response of various targets, and varying delay at frontend is just bad because it gets superimposed over target responses. The biggest trouble with this setup is that the tank Q factor changes with changing ground proximity, hence phase shift and group delay change with it, making discrimination fuzzy. There is no such change with series tank.

Just to illustrate the parallel tank trouble, see the attached picture, make sure you grasp the low pass nature of parallel tank behaviour...

[solv]

Thank's Davor , allmost clear.

[golfnut]

Surely Im right on the impedance bit. wtf.

AWR is on my side right..

[Davor]

You may use smith chart if you want, but you must know what you are after. Considering your op amp has 4nV/sqrt(Hz), you'd draw a circle at 1k impedance, and see where coil impedance crosses it.

When you look at nowadays coils of recent rigs, you'll find that coil R is ~30 or so ohm, which is near to 60ohm equivalent of 1nV/sqrt(Hz)
You are free to use some op amp at 20nV/sqrt(Hz) input noise for which the above Smith chart example would be a perfect match, but I would highly recommend against it.

Using a coil with larger inductance is the same as transforming up the voltage, and impedance, of a smaller inductance coil. As there are no resonance nasties related to such matching, Id sure go for it. At least if I'm in for stability, sharpness of discrimination, flexibility in coil replacement, noise figure... series tank is just better.

[golfnut]

When you look at nowadays coils of recent rigs, you'll find that coil R is ~30 or so ohm, which is near to 60ohm equivalent of 1nV/sqrt(Hz)
? maybe ?

You are free to use some op amp at 20nV/sqrt(Hz) input noise for which the above Smith chart example would be a perfect match, but I would highly recommend against it.
? oh ??


Using a coil with larger inductance is the same as transforming up the voltage, and impedance

Yes, More turns will give more output volts for a given field incident on the coil, plus more turns gives more L and R.


As there are no resonance nasties related to such matching, Id sure go for it. At least if I'm in for stability, sharpness of discrimination, flexibility in coil replacement, noise figure... series tank is just better.

? urm, oh ?

S

[Davor]

...
? oh ??
...
? urm, oh ?

George often reminds us to use posh English at this forum