PI Preamp?

[Anonymous]

First off, let me say that I don't know squat about detector circuits, but I have a question.
Would it be possible to build a preamp into the coil to boost the receive signal before it has a chance to pickup interference? Or is this not an issue with PI detectors?
I got the idea from my Toshiba VCR which has a preamp in the head.
Just a thought
Tom

 

[Anonymous]

I think some Minelab models have a preamp in the coil. I would suspect most interference is from the coil itself, very little from the cable.
However, if the preamp is in the coil you eliminate cable capacitance and you might be able to better dampen the coil and get better sensitivity. You might need two more wires to the coil for power. I think you will need to put the switching transistor in the coil, too, so another wire to clock the transistor. But the long clock wire will slow down the pulse edges, so maybe put the whole pulse generator in the coil. Now we have to get the pulse back up the sample switch. Ouch, my head is starting to hurt.
Oh, I just remembered, placing parts in the coil is generally not good, esp on a PI detector, because they will get detected. So in the end, I would not do it.
Sorry for the rambling, typing as I think...
- Carl

 

[Anonymous]

Fisher puts it all pretty darn close to the coil on their Pulse 12 boat towed detector. Its not the electronics that keeps me from building one that way, its the mechanics of it. I wouldn't know how to keep water out of it? Any ideas?
Thanks,
Charles

 

[Anonymous]

Couldn`t you just seal the electronic parts in a epoxy or gel cube. The wires could be coming out of the cube in a thicker waterproof cable housing so the preamp could be mounted on or in the coil or even a little ways up on the shaft.

 

[Anonymous]

I think Charles is looking more for a boat-towed fish, is that right Charles? I couldn't remember what the Pulse-12 looked like so I popped over to the web page. Basically, with a coil that big you might not be so sensitive to detecting the circuitry.
Charles, back on the Side Scan forum a couple of guys had a fantastic fish built up, but they could not get the circuitry worked out. Maybe you could steal some ideas from them. Another thing is to get some sources for waterproof connectors. Maybe Eric can help with this. I can also ask Tesoro, I like the Sandshark coil connector.
- Carl

 

[Anonymous]

Yes Carl, I'm interested in making a boat tow. After reading your post and thinking about it, I'm not sure I want to go the route of putting electronics at the other end of the cable. Thinking about it, probably the only way I'd be able to do it to my satisfaction would be to make a module that would basically be the complete metal detector and just feed power down to it and a serial signal to adjust parameters such as pulse delay and threshold and a serial transmission back from the fish to receive "HIT" information. Eric seemed to indicate, quite awhile back, that it was more trouble than it is worth as opposed to just running a long coax to the coil. I think I'll take his word for it <IMG SRC="/forums/images/smile.gif" BORDER=0 ALT="">
I missed the SSS forum while the site was down and so I actually went through it last week and re-read all the old posts and looked at the pictures. There is a lot of good info there, too bad the group didn't complete a working unit. I suspect that it'll happen someday.
Take care guys,
Charles

 

[Anonymous]

Hi Tom, Carl, Charles and JCP,
There are some situations where having the RX amplifier and TX switching circuit close to the coil are beneficial. I have done this on industrial detectors where one wants as high a sensitivity as possible to small metal objects. To do this, you need a short pulse delay and a fast TX switch off speed. The main benefit of this arrangement is that you can do away with the cable and its associated self capacitance. This raises the self resonant frequency of the coil circuit and when properly damped, results in a faster switch off of the coil current. There is no benefit to the noise performance as electromagnetic noise is all picked up on the coil, as Carl says. Also, at these high sensitivities, the coil will respond to many of the metal parts in any electronics in close proximity. Particular things to watch are the cans of electrolytic capacitors and the leads of some diodes which are tin plated steel. Provided the signals from these metal parts are not too great and that there is no relative movement between them and the coil, auto-zeroing circuits can back these signals off.
With boat towed detectors, there is a different set of problems. Usually the TX pulses and sampling pulse delays are much longer, as you are looking for large targets in conductive sea water. The small electronic parts down near the coil are not then a problem and you can gain a benefit in noise performance. The reason is this; if you have all the electronics in the boat and 50 or 100ft of coax cable down to the coil, you can get microphony as the cable stretches and flexes in the coarse of towing. This is a similar problem that you get with marine proton magnetometers and I once designed a magnetometer fish for ELSEC that incorporated the polarising circuit and preamp in the fish to overcome this problem. Both with a PI or a magnetometer, cable noise can also be cured by using a screened twin cable and using a differential input preamp circuit. The cable noise then appears as a common mode signal and cancels out.
Eric.

 

[Anonymous]

No one has mentioned the extra weight at the end of the stick after you put the electronics with waterproofing, etc down there! Or are you just out for exercise?