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Offline moonshadowTopic starter
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« on: May 15, 2009, 02:21:46 pm »
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Back to metal detection after a 30 year lapse. I bought a Garrett Ace 250 to see what modern detectors perform like. Lot's of nice graphics and annoying noises that tell you very little about the ground or the target. So I decided that since I cannot find any land to search I would sit down and design the perfect metal detector .... for me !. Basically the idea is to cut out all of the unwanted variables in a design to see what is practically possible. The aim is to produce three detector designs ...

VLF phase angle 'detector on a stick'.
A phase angle twin box design for deep detection of caches.
A coin pin-pointer.

The first two require an identical transmitter circuit, so I have decided to tackle that first. The first thought  was that the transmitter had to be xtal controlled, which normally returns a square wave output. To give us perfect transmission through both coils in the search head requires pure sine waves and so far I have been looking at the perfect conversion of xtal controlled square to sine conversion. If any tech types are reading this I have got the harmonics down 130 db's  with three components. If that does not impress you ... it sure as hell impressed me !. Will I succeed .... what goes on in a designers mind .... all is revealed at .....

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Offline jgedde
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« Reply #1 on: May 22, 2009, 06:04:50 pm »
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While I haven't used the techniques I'm about the descrive to make a metal detector, here's what I have done many times in order to generate a low distortion sinusoidal drive for resolver coil excitation:

1) Generate a square wave at the desired frequency.  Pass it through a pre-filter to eliminate anything higher than 10F.  Now, use a high-Q (or many pole filter) to remove all but the first harmonic.  This technique works, but can be tricky.
2) Use a micro or an FPGA to generate PWM at a carrier frequency 20+ times the desired modulation frequency.  Again use a roughing fikter to remove anything above 10F.  Now Use a 4 or more pole LPF to recover the modulation.  This works EXTREMELY well and the technique has been used in numerous spaceflight and military designs I have done.

-130 db rejection of harmonics?  My feeling is that it's gilding the lily a bit but good work nonetheless  Great.  You might also consider using a sharp notch filter on the receive side to eliminate any IM.

In this day and age, I would use a DSP to process the receive signal.  With an FIR filter it's possible to get near brick wall filter response and linear phase response to boot.  For non-spaceflight designs, I use DSP heavily with success in motion control application.

john

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Offline moonshadowTopic starter
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« Reply #2 on: June 01, 2009, 01:47:37 am »
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My only concern was to make sure there were no glitches that might pass through the zero crossing points when squaring up the received signal. Yesterday I had got to the point where the birds nest on the development board was driving me nuts and I decided to stop there and make up a PCB. It will not be the finished version but only for development. I have added several spare invertors to the circuit with some inks for 180 degree phase changes to make sure the RX and TX signals overlap. It was rather comical because I still had a blank space in one corner of the PCB and then realised I had forgotten to include a phase shift circuit for 'tuning' purposes and corrected that by including a 555 one shot, that will be used with a ten turn pot.

A little bit of resonance in the search coils can iron out any residual problems. I think I had better draw a circuit diagram of what is on the board, before I forget. Old age and brain damage can be a wonderful thing !.

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Offline Paul A
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« Reply #3 on: June 01, 2009, 05:13:30 am »
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Hi Moonshadow,
Wherabouts in the U.K. are you?
I'm north herts (Hitchin).....
 Great

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Offline moonshadowTopic starter
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« Reply #4 on: June 04, 2009, 02:41:00 am »
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Andover in Hants.

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« Reply #5 on: June 04, 2009, 07:04:20 am »
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hi . i was going to make my detecter just big enough to fit in my smaller back pack . but still quick to put back in to one .and go  Undecided  Smiley Smiley Smiley

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« Reply #6 on: June 26, 2009, 09:56:05 pm »
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try something using radiation

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« Reply #7 on: July 06, 2009, 04:09:42 am »
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moonshadow;

I think there is an easier way, you are making something simple, really complex and throwing a PIC into the mix is definately complicating matters

First, I dont think a sine wave matters and you still need to square it (and use one-shots to equalize pulse length) for a phase detector, whether you employ an XOR or a D flip-flop to do the job. The PIC is not a sine wave user, either. Try using an LM567  to filter out noise, regenerate the signal and maintain your phase variations. (you can get a square wave from pin 5 or modified sine from pin 6 but dont load either pin with less than a 10k ohm load.) Using it this way, use 0.1 ufd on each of pins 1 and 2.

By the way, I just finished reading a PDF on CMOS crystal oscillators, they can be designed to produce a sine wave, even an LC CMOS is good. When you are using the Tx frequency for the reference frequency, super stability is not an issue.... let it drift! The source for reference and search (phase change) frequencies are the same.

Ive been considering phase change detection, for some time, (my get up and go got up and went) just never got to it but I have pursued  the idea from a different angle (no pun intended.)

I downloaded your site and will read it and make suggestions, if I see some .... points, otherwise I will just let you build your complex infernalmachine, as you wish. Ha.

I presume you will be adding to your site, as things develope?  I will look, now and then.
Brian AKA goldigger

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