NEW !!! Full differential PI front end.

[moodz]

Hi moodz,

but you need the dampening resistor to eliminate the self resonant frequency of the coil. Consider, if there is a resonant system, it is very susceptible to ground effects (capacitive effects, relative permeability, ...).

Dampening is a fundemantal benefit of a PI system.

Aziz

You are right Aziz ... however an oscillating system is one that does not have constraints ... the damping resistor is one kind of constraint however it is passive ... by applying an active forcing function of known limits the oscillation may be removed in the same manner ... this is done in amplifiers for example with compensation which is not a damping resistor. The feedback in an amplifier is a type of active forcing function that modifies the input condition.
I will need to investigate further ... however my aim with this circuit is to produce a very fast reversal magnetic field in the ground / target.
So far the results are good ... the down side is the extreme voltages produced at the MOSFET ... even 5 volts of drive produces over 700 volts ... this is probably due to the 'Q' of the 'filter' circuit I am using to produce the coil forcing function.
Regards,

moodz.

[Aziz]

Hi moodz,

but you need the dampening resistor to eliminate the self resonant frequency of the coil. Consider, if there is a resonant system, it is very susceptible to ground effects (capacitive effects, relative permeability, ...).

Dampening is a fundemantal benefit of a PI system.

Aziz

[moodz]

Near ideal magnetic pulse transitions from the TX coil can be achieved if a forcing function is applied rather than a simple square wave pulse.
In the plot below you can see the current flow reversal 3.5 amps to -3.5 amps ( and hence the mag field ) that reverses in about 0.7 uS ... which is quite good ... however what is notable about this result is that the flyback decay is constrained to the current reversal time .. ie there is no significant flyback voltage outside the coil current decay time .. thus the flyback has decayed in 0.7us also.
A indication that the flyback is constrained to the decay time is the peak voltage ... nearly 5 kv in this case .. because the flyback energy has to be constrained to the coil current decay time ... ie the shorter the time the higher the voltage.

The red trace is the coil current ( coil = 220 uH 0.3 ohms 100pf )
The green trace is the fly back voltage at the MOSFET end of the coil.

The tx pulse is still 100 uS ... the forcing function is generated by a 'filter'
.... there are no resistive components in the 'filter' ... so this timing was achieved with no damping resistor at all !!

Note that this is a result from an LTspice simulation of the circuit as my CRO cannot take this kind of abuse from the actual circuit.

moodz

[moodz]

hi all ... o/p from full diff sim ....
green trace is +ve fly back into RX fet switch
blue trace is -v fly back into RX fet switch
red trace is +ve o/p from RX fet switch
light blue is -ve o/p from RX fet switch

note small < 0.5 us discontinuity due at RX fet switch on time.

the oscillations at the end are where the damping resistor FET switch turns off.
enjoy.
moodz.

[moodz]

hi all .... fully differential bipolar drive spice sim.
features front end FET RX switching and Switched Damping resistor.
works even nicer on the breadboard.
The part types are representative ... the breadboard uses different physical semiconductors.

[Aziz]

Hi moodz,

thanks for the hint. It seems to be expired and obviously in the public domain now .

BTW, I will regulate the coil supply voltage to minimize further noise and instability. That will give allways fixed transmit pulse energy emission.

Aziz

[moodz]

Hi Aziz ..

.... of course forgot the coupling k ... I am used to lumped inductances.

Have a look at this patent ... bipolar pulses in 1979 ... some of the ideas in this patent seem to have been 'lost' by current designs ... and even possibly re-patented under obscure descriptions and circuits ... ( not mentioning any names )

corbyn_patent.pdf


moodz.

[Aziz]

Hi moodz,

Hi Aziz .... I will be very interested to see the effect of the shielding on your coils ... I have not used any shielding yet ..... what resistance are you getting ?? ... and your total inductance is 4 times each side ??

moodz.

My multimeter isn't accurate enough to measure this exactly. The wire I am using is a 0.5 mm core copper diameter. With insulation, it has 1 mm diameter. The calculated resistances should be 1.8 and 3.6 Ohm (on both ends).

I am allways referencing to the total coil inductance seen on both ends of the coil. If I have a center-tapped balanced coil, then each half has only one forth of the total inductance. Inductance is direct proportional to the square of number of windings.
If both coil halves are 1:1 coupled (and they are, coupling k=1):
Ltotal = Lh1 + Lh2 + 2*k*sqrt(Lh1*Lh2), where Lh1 = Lh2 = Lh
Ltotal = Lh + Lh + 2*sqrt(Lh*Lh) = 4*Lh
Lh = Ltotal / 4 (q.e.d.)


The coils are not tested yet. Just working on a single supply version to get rid of the P-MOSFET using direct DC coupled coil signals and bipolar transmit pulses. The bipolar transmit pulses have huge inherent advantages . I also want to make same grounding for laptop and PI board to drive both devices with same power supply.

Regards,
Aziz

[moodz]

Hi Aziz .... I will be very interested to see the effect of the shielding on your coils ... I have not used any shielding yet ..... what resistance are you getting ?? ... and your total inductance is 4 times each side ??

moodz.

[Aziz]

Hi moodz,

I am also preparing two center-tapped coils. In this case, they will be shielded with AL mesh stripes (like the scotch 24 shielding tape).

The bigger one has 2x19 turns, 33 cm (13 inch) diameter, approx. 1.06 mH (266 µH each half), made from twisted pair cable.
The smaller coil has 2x15 turns, 20.5 cm (8 inch) diameter, approx. 400 µH (100 µH each half), made from two loosely coupled parallel cables.

The number of turns came from only the wire length, I could find at home. So they are somehow arbitrary and not intentionally choosen.

I want to see, how shielded coils perform on SSM2019 and INA163.
Aziz

[moodz]

Great news Aziz .... I am in the process of making a bigger coil ... 18 inch.

moodz..

[Aziz]

Hi moodz,

Hi Aziz .... looks good .... have you tried the SSM 2019 yet .......

moodz

yes, I have compared both SSM2019 and INA163 together at gain of 1000. Nearly same good results.

The differential coil signal has to be filtered further to get rid of the high frequency noise. So the INA163 gives the possibility to modifiy the core amplifiers (both buffers and differential stage). SSM2019 gives not the option for this.

I will wire a new circuit for the INA163 soon and will see more.

Aziz

[Mechanic]

Looks good, no, fantastic! Might have to order some parts.
Cheers

[moodz]

Hi Aziz .... looks good .... have you tried the SSM 2019 yet .......

moodz

[Aziz]

Hi all,

why we should use differential PI front ends?

See the difference:

http://www.geotech1.com/forums/showpost.php?p=93751&postcount=678

Paul, go ahead. That's the way to go.

Regards,
Aziz