minimum sample delay

[KingJL]

I can't justify purchasing 5000', otherwise this would be GONE!!!
https://www.ebay.com/itm/5000-FEET-S...gAAOSwnHZYj5by

What the H#!!, bought it anyway. If anyone needs some, let me know!

[6666]

What the H#!!, bought it anyway. If anyone needs some, let me know!

Maybe , would like to see a picture of the wire when you receive it thanks

[Mdtoday]

TX current waveform (triangular, half sine, trapezoidal, etc.).



As food for thought about the above discussion, I give you a link to an article to tickle your brain cells.
http://iieta.org/sites/default/files..._A/54.2_02.pdf

Now that is is a great little read... thanks for sharing.

Cheers

Mdtoday

[Mdtoday]

Have a read of this article. It changed my thinking on using expensive Litz wire.

https://engineering.dartmouth.edu/in...s/stranded.pdf

Eric.

Very good read with practical applications, thanks for sharing Eric.

Cheers

Mdtoday

[green]

TX current waveform (triangular, half sine, trapezoidal, etc.).

What you are doing is commendable and worthy of continued efforts. However, I would also think of your frame of reference... "minimum sample delay". In reference to what? In reference to removal of TX input pulse, actual MOSFET current shut-off (can be in excess of 2 usec after removal of gate signal), after coil current achieves some threshold level, etc. What is the objective? To achieve a fast coil or to detect a specific minimum TC target goal? To achieve the later does not necessitate the former... it is one means to achieve the result, but is not the only one. In fact I know of one detector that has excellent performance against a 3-5 usec TC target and samples at 50 usec after TX turn-off with a sample width of 50 usec... it really does not care about "how fast" the coil is (it has a TON of capacitance in the TX). What really needs to be thought about during observations of target responses is what is actually happening with target eddy currents within the context of the exciting TX current waveform (like Carl keeps saying "coil current waveform, not voltage"). Then you can implement a suitable solution to obtain the desired result. But that is probably suitable for a separate thread and discussion.

As food for thought about the above discussion, I give you a link to an article to tickle your brain cells.
http://iieta.org/sites/default/files..._A/54.2_02.pdf

would like to chart AWG28 solid that I have been using to make my coils to see if it charts a TC just over .3usec projected on the chart. At 5TC's, 1.5usec AWG28 would lose 99% of it's peak signal .http://www.geotech1.com/forums/attac...4&d=1526683069
Interesting about detecting a 3usec TC target with a 50usec delay. I'll have to try and understand how it does it.

[KingJL]

Interesting about detecting a 3usec TC target with a 50usec delay. I'll have to try and understand how it does it.

The reason is the TX/coil current waveform... The detector is the Fisher Impulse which I believe was designed by Dave Johnson (Dave J). That is the problem I have with referencing sample time relative to TX-off. In my opinion, sampling needs to be referenced to the trailing boundary of the coil current waveform envelope. If you reference the sample timing to the trailing boundary of the coil current waveform enveope, the sample time for the Impulse becomes ~15 usec.

[6666]

Interesting about detecting a 3usec TC target with a 50usec delay. I'll have to try and understand how it does it.

I'm curious to.

[KingJL]

The reason is the TX/coil current waveform... The detector is the Fisher Impulse which I believe was designed by Dave Johnson (Dave J). That is the problem I have with referencing sample time relative to TX-off. In my opinion, sampling needs to be referenced to the trailing boundary of the coil current waveform envelope. If you reference the sample timing to the trailing boundary of the coil current waveform enveope, the sample time for the Impulse becomes ~15 usec.

A view of the Impulse coil current in relation to TX pulse.



The impulse is a bipolar TX. By referencing the sample period to the trailing edge of the coil current envelope, the sample delay is < 15 usec (closer to 5).

[6666]

Thanks Kingjl, is it this circuit ? always been curious about it, DaveJ has said a few times its worth investigating.

[KingJL]

Thanks Kingjl, is it this circuit ? always been curious about it, DaveJ has said a few times its worth investigating.

That is the circuit! Note in the bottom right in the info block "FISHER - IMPULSE".

[KingJL]

The focus on the impulse is probably dragging Green's thread a little off track. The reason I introduced it was to emphasize two points. (1) "minimum sample delay" to what reference, and (2) a fast coil circuit is not the only way to achieve detection of small resistive targets... there are other methods. I think the discussion of small fast tau target detection and the focus on the religeous need for ultra fast coils actually deserves it's own thread. There are a lot of moving parts to the physics that need to viewed and understood as an integral and not in isolation. There is no "holy grail"!!

[rickb]

Have a read of this article. It changed my thinking on using expensive Litz wire.

https://engineering.dartmouth.edu/in...s/stranded.pdf

If the bottom line is cost, and you're making thousands... Given the time required to make a quality coil, it makes no sense to use ersatz litz wire. If the product is a cheap switching supply made in china, then oxidized stranded copper may be good enough.

[green]

The focus on the impulse is probably dragging Green's thread a little off track. The reason I introduced it was to emphasize two points. (1) "minimum sample delay" to what reference, and (2) a fast coil circuit is not the only way to achieve detection of small resistive targets... there are other methods. I think the discussion of small fast tau target detection and the focus on the religeous need for ultra fast coils actually deserves it's own thread. There are a lot of moving parts to the physics that need to viewed and understood as an integral and not in isolation. There is no "holy grail"!!

If the objective is to measure the TC of AWG28 solid wire and it can be done without a fast coil circuit no problem. I've always referenced to coil off command thinking that made the most sense, maybe not. If there is a better way, I'm all for it.

[KingJL]

... I've always referenced to coil off command thinking that made the most sense, maybe not. If there is a better way, I'm all for it.

I have wrestled with the refernce point for well on to 2 years now. For it to have any meaning, "sample time" must have the same reference from circuit to circuit. Personally, I have come to using the point to where coil current has decreased to 1 mA, for my personal comparisons. That way, it takes into account the actual removal of excitation of the target and takes away some of the mystery of why one particular detector appears so sensitive to fast tau targets and another is not.

[green]

I have wrestled with the refernce point for well on to 2 years now. For it to have any meaning, "sample time" must have the same reference from circuit to circuit. Personally, I have come to using the point to where coil current has decreased to 1 mA, for my personal comparisons. That way, it takes into account the actual removal of excitation of the target and takes away some of the mystery of why one particular detector appears so sensitive to fast tau targets and another is not.

How do you determine when coil current has reduced to 1mA?