MOODZ BI-MODAL PI - VLF TRANSMIT CIRCUIT

[green]

Is the tilt polarity correct for the green trace post #22? Why less tilt for the blue trace?

[moodz]

Moodz, Sample during the transition (S1), then after the Tx current is constant (S2). S1 and S2 will have opposite polarities with metal targets. A differential integrator will sum these and output a usable signal. S1 will need to be adjusted in time to sample during the ferrite(GB) pivot. It's a lot more sensitive than normal subtractive pi sampling (S1 -EF)

This is the method I have been using for my THS pi's

I will try your method first though Altra .... noting also the other comments from G.

[moodz]

...This circuit is not a regular PI circuit ... its a resonant system that ping pongs the coil energy at each current transition. The energy builds up in the circuit like a flywheel.
You can see this effect from the start up from power on. The Q of the resonant circuit on my test bed is about 100. ( there is no damping resistor )

The presence of a reactive ground will affect this Q and thus the energy stored in the coil each cycle.

This is where the current sensing comes in useful ... I am thinking I will use it to ground balance.


Below is a pic of the current waveform in the TX coil ... with (blue) and without(green) reactive ground.

The presence of ground causes a decrease in the peak coil current.

The RX signal shown below also shows the expected result for ground during the coil transition.

[Altra]

Curious what you consider S1 and S2. Moodz scope picture post #14 has three times (0 to 1.5us) (1.5 to 4us) and 4+us.

In post 2, the sim scope shot1 "Rx waveform in PI mode", I assume he is using an induction balance coil. Sample S1 would be during the transition or voltage spike. Assuming a well nulled coil. Sample S2 would be to the right of the spike like a normal delayed pi sample. Post 14# looks like super early target responses which will include reactive ground signal.

[green]

Moodz, Sample during the transition (S1), then after the Tx current is constant (S2). S1 and S2 will have opposite polarities with metal targets. A differential integrator will sum these and output a usable signal. S1 will need to be adjusted in time to sample during the ferrite(GB) pivot. It's a lot more sensitive than normal subtractive pi sampling (S1 -EF)

This is the method I have been using for my THS pi's

Curious what you consider S1 and S2. Moodz scope picture post #14 has three times (0 to 1.5us) (1.5 to 4us) and 4+us.

[Altra]

Hi Green ...

I am still working out the math on it ...

but I am getting the strongest target signals during the current transitions (eg 2 microseconds ) ... not 7 microseconds later.
So I will be using samplfication ( sample then amplify ).

The response is prob due to fairly large targets just unbalancing the DD.
I can get much smaller delays ... theres a large capacitance across the TX coil ... BUT if you knock down the decay ... peak flyback goes up and you have to drop the peak TX currents.
WIth 2 amp swing the peak voltage at the SIC mosfets is 1100 volts ( rated at 1200 volts ).

At 1 amp peak to peak ... we could manage 1.6 microseconds.

No free lunch.


moodz

Moodz, Sample during the transition (S1), then after the Tx current is constant (S2). S1 and S2 will have opposite polarities with metal targets. A differential integrator will sum these and output a usable signal. S1 will need to be adjusted in time to sample during the ferrite(GB) pivot. It's a lot more sensitive than normal subtractive pi sampling (S1 -EF)

This is the method I have been using for my THS pi's

[green]

Hi Green ...

I am still working out the math on it ...

but I am getting the strongest target signals during the current transitions (eg 2 microseconds ) ... not 7 microseconds later.
So I will be using samplfication ( sample then amplify ).

The response is prob due to fairly large targets just unbalancing the DD.
I can get much smaller delays ... theres a large capacitance across the TX coil ... BUT if you knock down the decay ... peak flyback goes up and you have to drop the peak TX currents.
WIth 2 amp swing the peak voltage at the SIC mosfets is 1100 volts ( rated at 1200 volts ).

At 1 amp peak to peak ... we could manage 1.6 microseconds.

No free lunch.


moodz

A normal PI looks at target decay after the target charge signal, first 4us. Don't know how ground balance could be done looking at signal during target charge. Wondering what is causing the no target decay signal after 4us+. Maybe coil wire? AWG24 has a TC near .8us about the no target decay signal.

[moodz]

Hi Green ...

I am still working out the math on it ...

but I am getting the strongest target signals during the current transitions (eg 2 microseconds ) ... not 7 microseconds later.
So I will be using samplfication ( sample then amplify ).

The response is prob due to fairly large targets just unbalancing the DD.
I can get much smaller delays ... theres a large capacitance across the TX coil ... BUT if you knock down the decay ... peak flyback goes up and you have to drop the peak TX currents.
WIth 2 amp swing the peak voltage at the SIC mosfets is 1100 volts ( rated at 1200 volts ).

At 1 amp peak to peak ... we could manage 1.6 microseconds.

No free lunch.


moodz

[green]

Forgot to include my scope pictures post 14. Amplifier out gain about 450. 5mm square coke can side looks similar to no target at amplifier out but easy to see at integrator out.

[green]

Hi moodz
Trying to understand your scope pictures. What I'm seeing, maybe all wrong. No target trace similar to target traces. Would be at least 7us before signal could be looked at if coil signal was amplified. Was thinking one of the advantages of CC Tx is early sampling.
Including some scope traces with normal Tx, can start looking at signal about 4us after coil off. No target not the same as bigger targets.

[moodz]

This looks as versatile as you can get. Thanks for posting moodz, this goes in my bucket list to try.

Thanks Eclipse ...

I have rewound a more efficient transformer and now get 2 amps peak to peak in PI mode for 500ma @ 5 volt supply = 2.4 watts ( the original transformer was 1 amp at 5 volts = 5 watts for the same PI pulse )
Can still run the circuit at lower voltage (eg 1 amp peak to peak in PI ) for 1.5 watts consumption.

Also here is the FIRST evidence of discrim on the new circuit ... This is straight off the RX coil of the Minelab DD .. no amps / filters etc.

Blue = No target
White = rusty iron sledge hammer head
Green = 10 x 10 cm aluminium foil

Targets were 10cm from coil.
There is clear pivot of the ferrous vs non ferrous visible in the waveform. The pivot is at the graticle center line.

PS ... I dispensed with tilt control.. you dont need it if the losses in the transformer are minimised. The result below has no tilt control.

moodz

[moodz]

Hello brother it looks good. Will you share pcb files for check and what distance do you think distance with minelab coin ?

Thanks .. I will post some board info when I get the final tweaks done. Cant really tell what distance on a coin ... as this is only the TX .... no RX circuit.
Will let you know the RX test results shortly.

moodz

[eclipse]

This looks as versatile as you can get. Thanks for posting moodz, this goes in my bucket list to try.

[wtrk]

Hello brother it looks good. Will you share pcb files for check and what distance do you think distance with minelab coin ?

[Altra]

Thanks for the detailed reply!