field test unit no 001 "model T"

[moodz]

Now with added ground sampling

During the TX pulse valuable information about the ground is thrown away in the previous design version ... also it caused balance point instability.

Below is the revised schematic with supply rails referenced to the signal ground ( +12 ) and the addition of a sample channel for the ground signal. This value will be sampled during the TX pulse and fed to a ground balance GB channel circuit ( not shown yet ). Happily the TX pulse drives the ground sample channel.

The supply rail has been lifted from 7.4 to 12 volts


moodz.

[mickstv]

Hi Paul, Thats some interesting changes to the design. One question have you changed the code and is it now sending pulses starting +5 and pulsing low ?




Mick

[ashkelon]

Where is the pulses timing diagram ? i understand nothing about this circuit

[moodz]

Where is the pulses timing diagram ? i understand nothing about this circuit

Ashkelon ... these are development threads ... not a recipe for making a detector ... if you read all you will see the timing here

http://www.geotech1.com/forums/showp...1&postcount=35

moodz

[moodz]

Hi Paul, Thats some interesting changes to the design. One question have you changed the code and is it now sending pulses starting +5 and pulsing low ?




Mick

Mick ... this post answers your question also ....

There are two versions of the firmware "H" and "L" the H is non inverted TX and the L is inverted ... you could substitute TC4422 non inverting or some similiar type ... I only put the 4421 because I have it in stock ( but I am going to order some 4422 ).
To invert a sample pulse you set S1 off before S1 on time instead of S1 on before S1 off.

http://www.geotech1.com/forums/showp...1&postcount=35

PS the ground balance sample needs more work and is only experimental at this stage

moodz

[moodz]

Thanks Midas ... the diode in the descrim balance control disconnects the damping resistance during receive .. otherwise the coils would be unbalanced during receive and you are correct about the mosfet body diode and output capacitance effects. I tried the floating drive technique but could not stabilise the balance point ... it kept drifting off.

There is another problem ... the method of producing the +VE supply rail via D5 and C4 is too clever by half.... the problem is that a large target or ground effect will substantially effect the magnetic coupling between L1 and L2 this causes a relatively large change in the voltage stored on C4. Because C4 stores a charge that is somewhat proportional to the coupling between L1 and L2 there is a "memory" effect a large target induces a voltage which remains for some time after the target has been removed from the coil....
The effect of this is to unbalance the balance point ... especially at high gain.
.. I will have to use a more conventional synchronised bias generator to generate the +VE.


However all is not lost ... the charge on C4 via D5 is actually a representation of the ground effect ... and could be used for ground balance .... C4 will have to be alot smaller .... so stay tuned .... I have a plan for this piece of information.

The 4066 sampler starts sampling at about 6 - 7 microseconds after TX switch off ... the sampler turns off at about 22 microseconds ... this gives a good descrim point.

I am commencing another rebuild of the board with the alternative +VE power supply.

moodz.

Midas ... actually I did have a circuit for full damping control across both coils ... it is published here and from memory it did work but the flyback time is increased.

http://www.geotech1.com/forums/showp...59&postcount=8

moodz

[ashkelon]

i slowly understand what you did , i tested with a small circuit.

[Midas]

Bi-directional Resistance Control

Midas ... actually I did have a circuit for full damping control across both coils ... it is published here and from memory it did work but the flyback time is increased.

http://www.geotech1.com/forums/showp...59&postcount=8

moodz

No.. that's really not what I was trying to describe. In fact that circuit would suffer even worse since its main positive pulse on driven side has a low resistance path through the body diode (and the two 330ohm resistors) to the un-driven side of the coil. The single sided damping scheme without the extra blocking diode wouldn't have suffered as badly since it was only reflected negative pulse being over-damped.

Now you've got the blocking diode in place its a different story. The difficulty may be with this circuit is that you've only got one oscillation to get all your damping done because if it goes negative you don't have any damping at all. You may be able to do just that of course but is that level of damping required to do that really optimum?

I suspect that only being able to control the resistance in one direction may put you at a disadvantage. If that does prove to be the case then I was thinking something along these sort of lines may work better. You can juice up the gate capacitor during quiet time when you not transmitting and the coil has settled so you can rely upon the mosfet sources being at the supply voltage.

[Midas]

Midas ... actually I did have a circuit for full damping control across both coils ... it is published here and from memory it did work but the flyback time is increased.

http://www.geotech1.com/forums/showp...59&postcount=8

moodz

No.. that's really not what I was trying to describe. In fact that circuit would suffer even worse since its main positive pulse on driven side has a low resistance path through the body diode (and the two 330ohm resistors) to the un-driven side of the coil. The single sided damping scheme without the extra blocking diode wouldn't have suffered as badly since it was only reflected negative pulse being over-damped.

Now you've got the blocking diode in place its a different story. The difficulty may be with this circuit is that you've only got one oscillation to get all your damping done because if it goes negative you don't have any damping at all. You may be able to do just that of course but is that level of damping required to do that really optimum?

I suspect that only being able to control the resistance in one direction may put you at a disadvantage. If that does prove to be the case then I was thinking something along these sort of lines may work better. You can juice up the gate capacitor during quiet time when you not transmitting and the coil has settled so you can rely upon the mosfet sources being at the supply voltage.

It may even be worth revisiting the damping across both coils using this circuit (If it works at all of course, no guarantees there, its untested, and I'm not an engineer...). I seem to recall you saying you were getting good sensitivity with a 1k resistor in this arrangement. So with the body diodes neutralized.. who knows..

[moodz]

i slowly understand what you did , i tested with a small circuit.

Very good .... post your results so others can see also.

thanks

moodz.

[moodz]

It may even be worth revisiting the damping across both coils using this circuit (If it works at all of course, no guarantees there, its untested, and I'm not an engineer...). I seem to recall you saying you were getting good sensitivity with a 1k resistor in this arrangement. So with the body diodes neutralized.. who knows..

It could work .. I will probably play more with the damping once the rest of the detector is put together. At the moment the existing damping circuit can balance the detector even at extremely high gains ( 1000 - 2000 x ) on the diff amp.... however it drifts but that is for other reasons ... not the damping.

moodz.

[Midas]

Drift musings

It could work .. I will probably play more with the damping once the rest of the detector is put together. At the moment the existing damping circuit can balance the detector even at extremely high gains ( 1000 - 2000 x ) on the diff amp.... however it drifts but that is for other reasons ... not the damping.

moodz.

Yeah I doubt my circuit will be any better in terms of drift in any case.
But are you sure its not the mosfet damping causing the drift ? What are your other contenders? Because I would expect the gate voltage to resistance relationship of a mosfet to be somewhat temperature dependent (shame there's no direct information on this in the datasheet) so without any feedback or temperature compensation I would think it would be impossible to eliminate drift completely. Easy test for you anyway, try freeze spraying your damping mosfet. That will prove that it is a source of drift, though I guess not necessarily that's its the dominant source under normal conditions.

[moodz]

Yeah I doubt my circuit will be any better in terms of drift in any case.
But are you sure its not the mosfet damping causing the drift ? What are your other contenders? Because I would expect the gate voltage to resistance relationship of a mosfet to be somewhat temperature dependent (shame there's no direct information on this in the datasheet) so without any feedback or temperature compensation I would think it would be impossible to eliminate drift completely. Easy test for you anyway, try freeze spraying your damping mosfet. That will prove that it is a source of drift, though I guess not necessarily that's its the dominant source under normal conditions.

Most of the drift comes is contributed by C4 and D5 .... the charge on C4 is not deterministic due to ground and large target effects ....that is why I changed to a separate bias generator and floating ground in the latest iteration of the design.

moodz.

[Midas]

Most of the drift comes is contributed by C4 and D5 .... the charge on C4 is not deterministic due to ground and large target effects ....that is why I changed to a separate bias generator and floating ground in the latest iteration of the design.

moodz.

Yeah right that's not really drift though, its motoring in the wrong direction
But anyway, on the bench, without moving the coil at all, are you happy with the stability? I guess a bit of slow drift isn't necessarily a problem anyway. Although it may force the design into the motion required detector category (or is it there already?).

[mickstv]

Hi Moodz, on IC4 4066n are you supplying voltage from the +/- 5volt regs or is it getting supply from the VPP 12 volts ?



Mick