NEW !!! Full differential PI front end.

[moodz]

Earth Field Solution and other benefits

One of the problems with sensitive PI detectors is earth field. If you wave a coil through a static field ( eg the magnetic field of the earth ) a slight voltage is induced that is proportional to the rate of change of flux through the coil. eg at type of generator. This leads to false target detection. However if the PI is driven by an alternating Tx pulse the receiver will see two things ...

1. Targets will respond with an alternating field as the eddy currents alternate in track with the stimulating tx pulse.

2. The earth field will produce a steady ( like DC ) response depending on the motion of the search head.

The only thing that would fool this would be if you sweep the search head at the same frequency as the alternating Tx pulse. ( if you drink lots of coffee you might be able to reach a 1000 Hz sweep speed )

The other benefits ? .... have you heard of chopper stabilised amps ? ... read up .... if you want to measure really small voltage changes like in a PI this definitely is better.

Coupled with the differential coil this system changes the PI landscape forever .... bisymmetric differential drive system schematic is published below ....

... wait till you see the ground balance system I have devised ... more soon.

Moodz

[Aziz]

Hi Paul,

that is a bit expensive, isn’t it? Let’s wait several years .. the competitors are not sleeping.

We can use a trick to increase the flyback voltage further: transformer principles.

Following configurations are possible:
a)Single mono coil (as main stimulator) and a differential balanced auxilariy coil (as second high voltage stimulator + receiver coil). The coils are 1:1 coupled, which form a mono coil at the end. The auxilary coil is galvanic decoupled from the main coil (has huge benefits).

b)like a) but a differential balanced main mono coil + differential balanced auxilary coil.

As the main and auxilary coils were 1:1 coupled, the total inductance of the trasmitter coil increases. We have mutual inductances and both coils form a high inductance coil (making the coil speed slower). So the main coil could be kept at low inductance/low resistance, whereas the auxilary coil could be higher. The magnetic energy will be transfered from the main coil to auxilary coil during the transmit on time.
I thought something in the range of 20-50µH for main coil and 100-200µH for auxilary coil. The auxilary coil would not avalanche (not fet there, no fet driver inbalance there, ...). It also would not have any laekage currents. If the coils are dimensioned correct, the auxilary coil would do the main target stimulation.

With these principles, one can push the flyback voltage up to 2-3 kVpp with low voltage fets (< 600V). The higher the flyback voltage, the more capacitive effects of the coil are becoming important. A capacitor at high voltage can store more energy of course, which limits the flyback voltage at the coil.

I have started some simulation models but not finished yet. I also want to do some high voltage experiments using these principles. My ordered high voltage probe isn’t arrived yet... but the SSM2019!!!

Aziz

[moodz]

Hi moodz,

instead of the IRFP460 you could probably use the NMOS SPP20N60S5 fet. It is a 600 V/ 20A / 0.19 Ohm rated fet, which allow a bit more target kicking. I got two samples today (5,47 EUR here), which I consider to try out this fet. I will spice this fet and will see.

BTW, Infineon has some very interesting mosfets like the IPW90R120C3 (900V, 36A, 0.12 Ohm!). That would give the ultimate flyback kick (1.8 kV pp). I couldn't find a source for this fet yet but all the spice models are already available for download.

Aziz

Hi Aziz ... there are 23 of those IPW90R120C3 in the UK and they are $A70 each not including postage..

I have been looking at MOSFETS with very low input / output capacitance ... the capacitance causes increased decay times and lower peak voltage.

I am going back to the 22 turn coil ... the circuit does not need the 10 K balancing resistors ... 3us is too fast anyway ... lose half of sensitivity and cannot stimulate larger targets.

moodz

[Aziz]

Interesting MOSFET's

Hi moodz,

instead of the IRFP460 you could probably use the NMOS SPP20N60S5 fet. It is a 600 V/ 20A / 0.19 Ohm rated fet, which allow a bit more target kicking. I got two samples today (5,47 EUR here), which I consider to try out this fet. I will spice this fet and will see.

BTW, Infineon has some very interesting mosfets like the IPW90R120C3 (900V, 36A, 0.12 Ohm!). That would give the ultimate flyback kick (1.8 kV pp). I couldn't find a source for this fet yet but all the spice models are already available for download.

Aziz

[moodz]

Hi Moodz,
I used the 12mm dia. aluminium as an example of how strong it could be, I was thinking more along the lines of a square coil with parallel 5-6mm dia. rods where at the corners they crossed over from inside to outside to keep the lengths the same,that would stop the inside being shorter than the outside, or if an insulator was put between the two rods they could then be twisted.
I have" no idea" as to whether or not this would work for you, I'm only mentioning it purely for a very light simple coil that might benifit you and your endeavours, on that note i will leave you to it.(best of Aussie luck to ya!!)
Regards Ron

Hi Ron ... it is an excellent idea ... I will be looking out for some in suitable rod in the bargain bins ... I have still more work to do on this front end though .. amps .. sampling and microprocessor .. your idea is on my todo list ... if someone does not post it first you will see it here .. one day ha ha.

cheers from moodz.

[moodz]

Hi Paul,



also quite interesting finding. More interesting is the 4 matched transistor pair:

http://www.thatcorp.com/300-series_Matched_Transistor_Array_ICs.html

This allows a discrete solution well under 1nV/sqrtHz.

I couldn't find a source for any THAT-chips here. I could get the MAT02 transistor pairs. Anyway, the INA163 and SSM2019 are already ordered yesterday.

Aziz

OK ... I probably wont get anything till I see your results

[authere]

Coil

Hi Moodz,
I used the 12mm dia. aluminium as an example of how strong it could be, I was thinking more along the lines of a square coil with parallel 5-6mm dia. rods where at the corners they crossed over from inside to outside to keep the lengths the same,that would stop the inside being shorter than the outside, or if an insulator was put between the two rods they could then be twisted.
I have" no idea" as to whether or not this would work for you, I'm only mentioning it purely for a very light simple coil that might benifit you and your endeavours, on that note i will leave you to it.(best of Aussie luck to ya!!)
Regards Ron

[Aziz]

Hi Paul,

Hi Aziz ... have you seen this diff amp ... it is called THAT1510 .... 7 Mhz bandwidth at gain = 100 and 1 nv root hz at 60 db gain.
Seems to be a bit faster than the SSM2019.
$A6.44 here in oz.

moodz.

also quite interesting finding. More interesting is the 4 matched transistor pair:

http://www.thatcorp.com/300-series_Matched_Transistor_Array_ICs.html

This allows a discrete solution well under 1nV/sqrtHz.

I couldn't find a source for any THAT-chips here. I could get the MAT02 transistor pairs. Anyway, the INA163 and SSM2019 are already ordered yesterday.

Aziz

[moodz]

Hi moodz,
Your coil, you said that you only need 8 turnsx2=16turns could you design your coil with one turn/bi-folar(solid rods-Alum.),this would make for a superlight coil especially in large coils and the fact you don't need shielding,on that point you wouldn't even need a casing for the windings as aluminium can be tougher to bend than steel. I have some 12mm round that cannot be bent easily by hand as an example.
Just thoughts,Regards Ron

Hi Ron ... 12 mm round would have a very low inductance and fairly high capacitance unless the loop was very big .... I have not tried it ... I have a two meter dia copper loop made from water pipe but it is not bifilar. As far as I know the jury is still out regarding if the target responds to the flyback or the tx pulse or both. If the current was high enough in your aluminium loop it might work as the resistance should be very low.

regards .. moodz

[authere]

Bi-folar coil

Hi moodz,
Your coil, you said that you only need 8 turnsx2=16turns could you design your coil with one turn/bi-folar(solid rods-Alum.),this would make for a superlight coil especially in large coils and the fact you don't need shielding,on that point you wouldn't even need a casing for the windings as aluminium can be tougher to bend than steel. I have some 12mm round that cannot be bent easily by hand as an example.
Just thoughts,Regards Ron

[moodz]

Hi Aziz ... have you seen this diff amp ... it is called THAT1510 .... 7 Mhz bandwidth at gain = 100 and 1 nv root hz at 60 db gain.
Seems to be a bit faster than the SSM2019.
$A6.44 here in oz.

moodz.

[moodz]

Hi Aziz ... thanks ... I have mainly been using the old 40 because it demonstrates that the differential method that you originally devised works better even with old amps and also because I dont feel guilty if I blow one up ... which I actually have not yet with this circuit despite the p/p voltages reaching over 1000 volts .

A further note on the schematic .... V+ for the mosfet drive was set to 6 volts but the amp supply was 12 volts otherwise the mosfet went into avalanche and the balance point shifts unpredicatably ... maybe someone could spice this ... dont know.

Regards,

paul.

[Aziz]

Hi Paul,

Well done mate.
When will you get rid of your CA3140? This mature chip is quite noisy.

Aziz

[moodz]

doh ... forgot to crop schematic after resizing ... sorry Carl.

[moodz]

Thanks Pork ... I do try to avoid complexity if possible .... anyway here is the v6 schematic ... this is the actual circuit that produced the results above ... I have even added some bias for Carl .

NOTE : if the MOSFET goes into avalanche you will NOT get balance ... you will get something ... but not the nice pix previously posted ... adjust V+ to maximise flyback peak voltage to below MOSFET avalanche or get a higher rated MOSFET .... and where are the clamping diodes ? ... you tell me
... remember this aint no spice simulation.
There are only 8 turns in the coil ... whaaaat you say ... remember it is bifilar so it is the equivalent of a 16 turn coil. Save your money ... use plain 0.8mm dia magnet wire .. twist with electric drill to 1 turn per inch ... I think the turns provide air spacing anyway ... sure you can use more turns ... 22 turns gives me 8 us sampling .. which is also good .. this needs to be experimented with ... hey you ( the reader ) could do it and let us know !

Regards,

moodz