Triangular Wave Technology

[Dave J.]

Dave, it'll please you to know that your Impulse TX circuit inspired me to do a 6-month long (spare time, of course) investigation into ramp TX methods. The result is a 3-frequency ramp TX that looks nothing like your circuit but works a whole lot like it anyways. The goal is a next-gen version of BBS/FBS (I call it VBS as an internal joke, 129 frequencies!) that actually has excellent low-conductor sensitivity (also unlike BBS/FBS). I just got the PCBs and it is being built by my tech right now, though I don't know when I'll get to play with it as I have real product-projects that the boss insists I finish.

BTW, the reactive time difference between transmit and flyback is due to the flyback snubber diodes, they create a step-up voltage effect during flyback and increase the slew rate. Add more & more series diodes to see it approach a vertical, and watch your recycling efficiency drop to zilch.

- Carl

I know you're sandbagging us, it's really 131 frequencies, not a mere 129. Wishing you luck with it, Carl...... plenty, but not too much!

--Dave J.

[mickstv]

No, but I think I saw the circuit posted in the schematics forum.

Dave are you talking about the fisher impulse circuit or something else ?

[Dave J.]

yes.

[mickstv]

Thanks.

[Ferric Toes]

Phew! Let's ask a few relevant questions and see what comes up.

PRO FLYBACK. Are you saying that if you pulsed a coil with, say, just a 2uS 500V pulse, then that is a good as a long low voltage (100uS or more) conventional pulse that generates a 2uS 500V flyback anyway?

Hypothetical situation. We have a ferrite magnet made of a material that is also a high explosive. Put the magnet close to a metal target that is fixed to an immovable object. Explode the magnet. Do we generate eddy currents in the metal target the same as if we had used a switched current carrying coil to set up a magnetic field? Is there a high voltage spike when the magnet is exploded, and if so, where do we see it?

Eric.

[Mechanic]

Hi Eric,

Hmmmm, well firstly it would be a bit hard to just pulse a coil with a 2uS 500v pulse. It is all about dI/dT. When a conventional transmitter turns on, it generates eddy currents in the target whilst expanding and intensifying the magnetic field. The the transmitter is turned off, the magnetic field needs to go back into the coil and depending on what the flyback voltage is limited to it will go back fast, very fast. Meanwhile, the rapidly changing magnetic field is inducing a voltage in the target, much stronger than the initial eddy currents that were generated during the tx on period.

With the magnet, as you put the magnet near the metallic object, there were eddy currents generated. When the magnetic field stopped changing, these eddy currents decayed away, probably to nothing(this depends how long the magnetic field just sat there). Then when the magnet exploded, the magnetic field collapsed very rapidly, causing eddy currents to flow in the target, causing a voltage spike(how high I don't know) in the target.

I would expect to see better target response results by turning the transmitter on allowing it to reach its maximum current not too quickly, say 3A, then holding that max current for enough time for the turn on eddy currents to completely decay and then switching the transmitter off allowing that 3A back into the detector in 4 or 5uS.

Cheers Mick

[deemon]

Hypothetical situation. We have a ferrite magnet made of a material that is also a high explosive. Put the magnet close to a metal target that is fixed to an immovable object. Explode the magnet. Do we generate eddy currents in the metal target the same as if we had used a switched current carrying coil to set up a magnetic field? Is there a high voltage spike when the magnet is exploded, and if so, where do we see it?

Eric.

By the way , we even don't need to explode the magnet . It's enough to reverse it - quickly turn it to opposite position , and this action will produce the same effect , but with double magnitude .

[Ferric Toes]

By the way , we even don't need to explode the magnet . It's enough to reverse it - quickly turn it to opposite position , and this action will produce the same effect , but with double magnitude .

Yes, there are a few different ways of doing this hypothetical experiment. I chose the exploding magnet as a quick way of shutting off the field, even so I doubt if the magnet would be reduced to dust in 2uS. Quickly turning it in a fast enough time might prove even more difficult.

Eric.

[Aziz]

I would be interested in the Impulse schematics (the TX section). Anyone have a link?
Aziz

[mickstv]

It's at the link below Aziz.


http://www.geotech1.com/forums/showt...fisher+impulse

[Aziz]

It's at the link below Aziz.


http://www.geotech1.com/forums/showt...fisher+impulse

Thanks Mick.
I'm really glad to see, that it is a totally different TX than I'm working on it at the moment.
Cheers,
Aziz

[Sean_Goddard]

Also the same method used in Minelab BBS/FBS. Flyback is intentionally suppressed so you are left with a more CW-type system. You can either demodulate in a traditional VLF-style CW demod (isn't that what CZ does Dave?) or in a time-domain-style demod like Minelab.

BTW, here is the Minelab TX voltage and current waveforms:

[ATTACH]22806[/ATTACH]

OK, I'm missing a few brain cells here. Can someone PLEASE explain the difference between CW demod (I presume Synchronous demod) and TD demod (preferably citing practical schematics) - THOSE I understand ;-)

[Sergey_P]

Hypothetical situation. We have a ferrite magnet made of a material that is also a high explosive.

No dФ/dt -> no E, no eddy currents.

dt -> 0 => E -> delta function

[Carl-NC]

Hmmmm, well firstly it would be a bit hard to just pulse a coil with a 2uS 500v pulse.

Maybe not, I'm currently able to pulse ~120v @ 3us. Right now I'm limited by my boost regulator and available high-voltage capacitors, both of which are not difficult to solve if I had an overwhelming urge to solve them.

[Carl-NC]

OK, I'm missing a few brain cells here. Can someone PLEASE explain the difference between CW demod (I presume Synchronous demod) and TD demod (preferably citing practical schematics) - THOSE I understand ;-)

There isn't much of a difference. Traditional CW demod looks at the entire waveform (as in the case of full-wave demod) or half of the waveform (half-wave demod). TD demod looks at a smaller snippet, and often combines snippets from various portions of the waveform. I found the Tayloe mixer interesting as it's a little of both.