Pulse Devil - The inside scoop

[pml]

Both Eric and Dave's base frequencies are really low. Not sure what harmonics they have since that could vary between designs. I believe they both said they checked emissions and it seems fine.

My design on the other hand is the complete opposite as its pulse width will be adjustable as low as 500 nanosec and the base frequency will be adjustable to as low as 1.5 usec. I use a single turn 3' hollow Al tube, which has very low inductance, which means di/dt is extremely high. The signal is very square, which all equates to a lot of noise. See attacked image for computer spectrum generation. Top of image is signal. Middle of image is a blow up of spectrum, 4X. Bottom of image is full spectrum. In the 4X the red line = 100 KHz, yellow = 200 KHz, green = 300 KHz, purple = 400 KHz. As you can see, 100 KHz is low even though it's the base frequency. Peak is at ~ 900 KHz.

Paul

[Unregistered]

JC1

Hi All,

Hey nobody is talking about the current that is going through the coil. At milliamps, may be hard to violate. At amps will be another story. In fact I am probably in violation of the Law everytime I turn on a homebuilt unit.

On the bench as I vary the freq. or pulse length I can screw up the picture on my T.V. (I don't have cable).

Also find places to mess up A.M. radio, 560kHz to 1.05 MegHz or something like that.

So keep the xmit power low enough and may be ok.

How low?

Well now thats how compliance labs make their money.

Paul, Bluntness is very dull. And I wouldn't want to try and mske that compliant, unless you are running very low power.

JC

[Unregistered]

JC1

I forgot to add, I am an unintentional radiator, so since I don't mean to radiate I am probably ok.

JC

[pml]

Paul, Bluntness is very dull.

Unfortunately JC1, quite the contrary. I wish you were correct, but I'm sure you were just being humorous. Truth and positive attitude is exciting! Life is what you make of it.

JC1, my Tx coil is built to go over 20 Amps. Of course it's only 1 turn.

Paul

[Unregistered]

JC1

Hi Paul,

20 Amps !!! Honk!!! You just failed.

FCCCP is headed your way!

JC

[Unregistered]

JC1

Hi Paul,

One turn, I like that! In the end the best metal detector should be made with one turn.

So you are on the right track, but compliance could be a problem. Also on your frequencies, the ground can be pretty absorbtive (is this a word?) of high freq. vs the air.

Hows your depth? By the way when I speak of PI, I am talking about the "standard" stuff out there.

Wonder if the Minelabs are compliant (U.S.) that is.

JC


JC

[pml]

Hello JC1,

Hows your depth?

I was laughed at last time I mentioned depth, so I'll just wait till the final design is fully built and photos. My last design provided sufficient data to predict final depth of a 1" diameter thick lead. Let's just say it's a whole lot deeper than I would ever want to dig.

So you are on the right track, but compliance could be a problem.

About every ms I do a very long base sample. That change from MHz to KHz every ms is probably the main reason I am clobbering AM radio. The long pulse is to detect really big stuff and for manual guesstimate of possible electrically conductive iron.

My goal is to get into medical scanning or anything that can help society in some small way. I had a thought yesterday about security scanners for places like airports. Also those military mine scanners. I read somewhere in the FCC document that medical equipment is except from such rules. Not sure about military stuff such as mine scanners. I've seen a lot of security scanners are FCC approved. So I guess security scanners are not except.

Paul

[Ferric Toes]

Hey nobody is talking about the current that is going through the coil. At milliamps, may be hard to violate. At amps will be another story. In fact I am probably in violation of the Law everytime I turn on a homebuilt unit.

On the bench as I vary the freq. or pulse length I can screw up the picture on my T.V. (I don't have cable).

Also find places to mess up A.M. radio, 560kHz to 1.05 MegHz or something like that.

So keep the xmit power low enough and may be ok.JC

Hi JC,

Amps is OK, and I read somewhere in the FCC regs. that homebuilt stuff is not breaking the law, unless you are interfering with local services.

If you screw up your TV picture, then something else is wrong. I had a PI design that failed an emissions test, because I had a long track on the pcb that carried a clock pulse with a fast edge. The track was acting as an antenna. In PI designs, you want to use the slowest logic that you can get away with, to minimise harmonics from that area.

The 3kHz whistle from the transmitter of my GS5, with 11in coil, is barely audible at 500kHz on an AM radio, 4m away. On 200kHz, long wave, I can just pick it up at 8m, but as soon as I tune into a station, it is inaudible. This is with the radio aligned for maximum pickup.

Eric.

[Unregistered]

JC1

Hi Paul,

Medical equipment opens another pandora's box which I'm not going to get into too deep and don't know all the rules.

Do know they are not going to let anyone fire up a device which wipes out other medical equipment in the hospital.

Cat scanners and the like do produce strong fields and are exempted, once again why? because they want to use them.

Fast track to no problems, but rest assured the RF environment in the hospitals are well understood. and some guys make a living off just this aspect.

Finding coins swallowed by kids is a strong market and one I was approached to design by a technical investor. I don't know what all he found out, but gave up the effort having to do with all the stuff medical equipment has to meet.

Finding metallic objects after surgury is another market if you want some ideas. Body scans before you leave to find out if any wrenchs were left in.

Ah the medical stuff is so gory.

JC

[pml]

One thing I noticed is that you can have a strong AM signal, but if it's static then I can't hear any disturbance. We all know that's due to the way AM works-- by amplitude modulations. I wonder if the FCC cares about modulation or not.

As with ic's, I have good luck with very close metal shielding, like a small faraday cage. Just don't use iron or any magnetic materials. I guess if you get to close to the ic then it can mess up your circuit, but less than a cm should be fine for a lot of circuits.

Paul

[Unregistered]

JC1

Hi Eric,

Yes, as an experimenter (see its all a matter of sematics{this is a joke}) you can play around and not get busted. But thanks for the reminder, I was just having a little fun.

Seriusly, though the investor found out why there are not metal detectors in hospitals, he felt every nurse should have one, miss diagnois due to swallowing coins. Such a humanitarian he was, of course he had also "run the numbers".

screwing up the T.V., you could be right, this is from the breadboard, the logic is 4000 series or equiv, nice and slow, what there is of it. but may very well not be off the coil, about 2 amps or bit more.

In the end, still if the device is not too out of compliance or is out at places where there is not too much nowadays it would interfere with, probably can get an ok doky.

JC

[pml]

JC,

You made some really good points. I never thought about interfering with other medical equipment. I not familiar with the body scanners hospitals use to detect swallowed coins and such. Sounds great. Is there still a good need for improvement or does the industry already have a good handle on this?

What really interest me is MRI and CAT scans and such. Like you said, if it's something very very important machine such as CATs then they can make special cases. Perhaps by creating a special room just for the CAT scanner. I can how they would not want to set aside a special room just for a machine that's not vitally important.

One thing I love about MRI is working with NMR (Nuclear Magnetic Resonance) and possibly ESR (Electron Spin Resonance). I have a lot of family members with critical illnesses. Any headway in detailed scanning would help a lot of people.

Paul

[Unregistered]

JC1

Hi Paul,

Yes, lots to think about at the hospitals. One of the biggest problems is liability I think. I never got all the info on medical approval so real weak on this one, but if some doctor cuts on some kid cause your detector went off, this can be a problem. So have to back up with Xrays and need to put that in the manual, speaking of the manual, you better write a real one and teach classes on the use of this thing, cause if some nurse is waving it over some kid "while he is laying on a metal table" and she doesn't realize that the table is a problem, and the thing is going off like crazy,,,, well you get the idea.

Anyway he gave up, and it was the detector design or the emission requirements that were the problem.

yes, the people who sell cat scan equipment and the like, set it up for you and tell you where you can build the addition to your hospital for it and how much metal that will be in walls. So the equipment itself can radiate like crazy.
I''m guessing.

JC

[Unregistered]

JC1

Crap!

and it was the detector design or the emission requirements that were the problem.

should be

and it was NOT the detector design or the emission requirements that were the problem.

[Unregistered]

JC1

Hey Paul,

on your pulses, I've seen that done for measuring the hysterises on iron and such, but the first pulse will induce eddy currents one way, the second pulse will induce them the other way, somewhat cancelling the first pulse and then you sample what is left? (not much? in non ferrous) little more in iron?

I don't get it.

JC