Surf PI 1.2 increase sensitivity ??

[Tony60]

Hi All I have successfully built the surf pi 1.2 the first MD so far. I am very satisfied but I would like to ask you can help me to increase sensitivity. Impulse is 12,5v and about 60 uS lenght of time(analogic oscilloscope). Now , in the air, see a coin of 30mm to 17 cm; I would like to bring it at least 20 cm... I enclose a photo of the PCB and coil.
Thanks in advance and greetings.
Tony60 (For OM also ik2czq).

[Floki]

Tony60, everything depends on metal of which the coin is made.

[Tim]

Tony the ohms count sounds a bit high-should be about 2 (?). The wire does look reasonably thick, so should be lower ohms. My suggestion is try some different wire.

[Tony60]

Hi Floki and Tim, thanks for suggestions.
All the best.
Tony60.

[Jose]

Tony.
To improve sensitivity, try increasing the pulse width TX to 100 uS or more.
It will also increase the current count.
The coil can be improved, if it is done with more insulated cable.
There are many examples in the forum.
Regards

[waltr]

Increasing the PPS rate may increase sensitivity - Read Carl's explanation of how the PI Integrator works.

Increasing coil current may or may not help!
I found that decreasing peak coil current with a series R (10-15 Ohm) actually increased sensitivity. Reason is the series R decreases the coils TC and allows the coil current to reach a peak and level off before turning off. Theory has it that this gives a cleaner Tau decay from a target which improves sensitivity. The other reason lower peak coil current can help is noise and stability of the down stream gain stages, etc.

Of Course, Noise through out the signal chain will also decrease sensitivity. Ensure and circuits have low noise to allow lower threshold.

Very small targets and high conductors (pure silver) have very short decay Tau requiring very early sampling.
Earlier sampling is a function of Coil TC and pre-amp recovery from over Voltage.
The forum here has lots and lots of discussion about these topics.

[ionut_mtb]

Hi All I have successfully built the surf pi 1.2 the first MD so far. I am very satisfied but I would like to ask you can help me to increase sensitivity. Impulse is 12,5v and about 60 uS lenght of time(analogic oscilloscope). Now , in the air, see a coin of 30mm to 17 cm; I would like to bring it at least 20 cm... I enclose a photo of the PCB and coil.
Thanks in advance and greetings.
Tony60 (For OM also ik2czq).[ATTACH]42655[/ATTACH][ATTACH]42654[/ATTACH]

hello,
Did you adjust the dumping resistor for your coil? How does it looks output of ne5534 at pin 6 on the oscilloscope? You can make the coil 230-300uh. And after you can increase the tx pulse width from 60Us to 100us how jose said.

[daverave]

i used 30 swg enamelled copper wire with some of my surf pi projects with an inductance of around 400 uH and i got very good results on low conductive items.

[Jose]

Yes, I agree with you Waltr.
I suggest increasing the pulse width, because it is the first and basic thing that can be done to increase sensitivity, but as you mention, the issue is complex.
If it is a question of improving the response to objects of low driving, one of the modifications that gave me good results was to decrease the amplitude of the pulse applied to the gate of the mosfet.
This was also suggested several times in this forum.
I enclose the scheme that I am currently using in my Surf.
Jose

[waltr]

Another good suggestion. lowering the MOSFET gate resistor increases the Current to the gate during switching which can increase the MOSFET switching speed.
The one I think would make the difference in the resistor turning the MOSFET Get OFF. Fast switch off is important in a PI MD, fast switch on is not.

Critical Damping of the coil is also very important as ionut_mtb stated. Adjust the dumping R while looking at the Pre-amp output on a scope.

[Tim]

Process of elimination Tony. Start with the simple things first, like a different coil, before you get yourself totally befuddled and all the adjustments way out of whack. Not to mention your board all snotted up.

[Tony60]

Hi to All, very very thanks for many suggesstions!!!
All the best.

Tony60

[MartyJ1963]

Hi Tony60,,,,,Which PCB and parts list did you use to make your build? Is there any chance you could post it on the forum please?,,,I've tried other pcb's that have been posted and I've not managed to make a working one yet,,,,you did a nice neat job of yours. Thanks Marty

[Tony60]

Hi MartyJ, thank you for the compliments. The PCB was designed by me using CIRCAD of the holophase using a format of its own; I will try to send it anyway. For the rest as soon as possible I'll pass the list and the outline.
Best regards.
Tony60.

[MartyJ1963]

Hi again Tony60,,,thanks for the reply. You are a gentleman. I have a lot of folders full of different information on many Detector circuits,,so if there is anything you are looking for just ask and If I have it I will gladly post it to you. Thanks again. Marty