PCB DD coil? Reg?

[Anonymous]

Hi,
I think one of the Whites PI's uses a flat PCB coil? Any thoughts on how this technique would go on a DD? I expect the mutual inductance to be very low if the Tx and Rx coils were seperate, making orientation much easier.
For the GQ the coil resistance is approximately 6 Ohms, would having a trace length on the PCB equivalent to this bring the inductance anywhere near 370uH? Or would there need to be more turns or even a multi-layered PCB to increase L?
Any help much appreciated.
Cheers Kev. <img src="/metal/html/help.gif" border=0 width=23 height=15 alt=":help">

 

[Anonymous]

Hi Kev,
I am sorry, but I have never tried to build a coil by using a pcb trace, so I really can't answer your question. I guess, one would just have to try it to see.
The technique of using a pcb doesn't fit well with my shielding techniques. I am very pleased with the shielding of each of the windings. In the past, I tried different shielding methods and I felt there was an increase in the noise level. So, I have just stayed with the technique that seems to work the best for me.
As for the inductance and resistance relationship, I would not want to speculate. There are too many variables involved.
I will say that I frequently use the website below to calculate the inductance of a coil. One might be able to get an idea of what to expect by plugging in numbers, once several factors are known such as the width of the trace, spacing between traces, etc.
As for my coils, I will calculate the inductance of a round coil having the proper diameter and then form the coil to the shape needed to make the DD coil. The altering of the shape doesn't seem to make much difference in the inductance providing the shape distortion isn't really dramatic. I have made DD coils in a round configuration and ellipticals using this method and all have worked.
By using the website listed, I can quickly determine just what I need. BTW, if you try the site, once you plug in the appropriate number of turns, and the diameter of the coil, you might experiment a little with the coil length. I have found that about 0.22 in works quite well for all the coils I build. Also, once numbers have been entered, you will have two answers or options as to the inductance. I pick the .1% accuracy and have found it to be quite accurate.
I will caution that it really helps to have a scope to observe waveforms to assure there is proper damping. Otherwise, it will be almost impossible to assure the coil will work properly, especially at the real short delays.
Again, sorry I can't provide the info you requested.
Reg

 

[Anonymous]

Thanks Reg for your feedback,
I tried the coil calculator applet out and got quite an unexpected result. The number of turns required for the given 'L' was much lower for the 0.1% accuracy figure than one would require to achieve the 6 Ohms DC resistance.
I realise though that the formula used will not be tailored for a flat axial coil.
I may just have to bite the bullet, invest in some board and experiment. I've almost finished plotting a design, although it's not as compact as I would like as the thickness is only 35 microns. I may yet need to tin plate of electroplate the traces to build them up to achieve more compaction.
My daughter has a book of gold leaf, I hope a few sheets of this will provide some measure of sheilding, but then again, I don't know.
Cheers Kev.

 

[Anonymous]

Hi Kev,
I would concentrate more on obtaining the proper inductance than worry about maintaining the same resistance also. Eric's PI's are quite forgiving in this respect. My coil resistance values do not match Eric's since I use a slightly different size wire. Eric mentioned what wire size he uses on the GQ coils in a post on October 15, 2003. He mentioned the specifics in metric units, but it figures out to be about 30 awg.
In the case of a pcb coil, the coil length would be interesting since it would be minimal. As such, the 0.22" that I use wouldn't be applicable. It works fine for wound coils, but most likely wouldn't be even close to accurate on your application. Sorry, I didn't think of that when I mentioned what I used.
In my case, the coil length is basically the same as the thickness of the windings bunched together. In my application, the .22" works even though the windings are not quite that thick. However, the coil calculations are for a single layer coil also, so I guess it all works out.
I can see plusses and negative aspects to a pcb DD coil. There certainly shouldn't be much movement between the windings, but determining the overlap and adjusting the null could be a challenge.
If you do try to build one, let me know how it works out.
As for shielding, I am not sure how the gold leaf would work out. A long time ago, I saw a very interesting technique used for Faraday shielding, on a AH Pro, off resonance design. The bottom of the pc board was etched with fine lines very close together in sort of a partial grid fashion. Actually, the coil windings were etched on the top side of the same pc board, so it made for a very thin coil. Unfortunately, I don't remember exactly how the shielding looked. I don't know if this would work for your application, but it is something to think about.
In your case, I suspect that you would make the xmit on one side and the receive on the other. If the pc board idea were to be used for shielding, then a second pc board would be needed. This might add more weight. If it were me and going to use the coil where there was brush, I would shield the top also.
Reg

 

[Anonymous]

It was Tesoro that used/uses printed coils for their PI's.
First used on the original Tesoro Piranha. That one was of a White's Surfmaster PI design.
Mr. Bill

 

[Anonymous]

Thanks again Reg,
So as long as I obtain the correct 'L' I could tune the coil's time constant by adjusting the value of a series resistor, right?
I've calculated that my traces are 0.015 sq mm, so I will definitely need to build up the thickness to approach Eric's 0.049 mm sq conductor.
That's interesting you mention a meshed PCB for a shield. We have what we call the 'RF Room' here at work, it's used for testing designs for EM emissions. It is basically a room completely covered with grounded wire mesh, it's non compliant of course, but gives us a ruff idea of any possible issues. I imagine that it would be fine for intermediate frequencies, say 50 cycles up to near microwave. Static discharges I believe cover a very wideband, and would likely still reach the coil. I will keep that in mind though Reg. I'm using a CAD package to plot the coil, so I may be able to apply a crosshatch ground plane and then run an EMC check to see how effective it is.
Cheers Kev.

 

[Anonymous]

It is my understanding that the biggest problem with the Tesoro PI is the PCB coil design, if it was replaced would probably make a better performing detector. It has something to do with the eath and capacitance with the coil that limits performance, a White's engineer also commented on its use, something to think about if you are going to spend alot of time constructing, unless you atre just curious, but maybe your design might be different, but notice also no other manufacturer uses it, including Eric!

 

[Anonymous]

Hi Don,
I have designed and used pcb coils, but only in industrial PI detectors, so far. There is no reason why they could not be used effectively in a treasure detector and have at least as good a performance as a conventional wire wound coil on certain types of PI. PI

 

[Anonymous]

Hi Kev,
Be careful about using a crosshatch ground plane. A joined crosshatch will contain many small loops which will sustain eddy currents. See my reply to Don Jones above.
The Goldquest transmitter circuit has 39 ohms in series with the coil, which is 6 ohms. You could of course have a 12 ohm coil and 33 ohms in series, or a 23 ohm coil and 22 ohms in series. It doesn't matter where the resistance is. This means that you can have very thin tracks to pack the turns in.
Eric.

 

[Anonymous]

Excellent,
This is encouraging. As Reg notes, my main problem with the DD PCB will be getting the overlap of the Tx and Rx boards correct, but if it can be done once, it will then be easily repeatable, and possibly replicated complete on a single PCB?
I see that the screen mesh covers the whole PCB area and not just the trace area. Is there a reason for this? I thought that minimal accumulated metal within the shield was best.
Thanks
Kev.

 

[Anonymous]

Thanks Eric,
Those points will no doubt save me some materials and time. <img src="/metal/html/smile.gif" border=0 width=15 height=15 alt="">
I expect that since the traces are very flattened they can be brought much closer together before attaining a capacitance figure of wire with a similar cross sectional area?
I wonder what the field around flat conductors looks like, an ellipse?
Cheers Kev.

 

[Anonymous]

I defer to the man who knows best, my info was second hand thru forums, I have also owned a Tesoro Sandshark and they do seem limited on depth, which may be attrituted to other factors, the coil has always been critized for performance, undoubtly the package is not conceided right. I think Tesoro should have had you design coil! LOL

 

[Anonymous]

The biggest problem with the Sandshark, is that the sample delay is set out at about 22us. I replaced the crystal in mine, and got the delay down to 15-16us, and picked up a good 2-inches on gold rings.
I also wound a quick-and-dirty coil to compare with the PCB coil, and got roughly the same results. I suspect that, once the sample delay is reduced, there is still room for improvement with the coil, either by using a wire coil, or a better PCB design. I do know that their coil PCB is single-sided, so making it double-sided should help.
- Carl

 

[Anonymous]

Hi Carl and all,
I don't know what the field from a single sided pcb coil looks like, but to get the number of turns required, the reducing diameter of the inner windings must be significant both for receive and transmit. At a guess, it is probably equivalent to a close wound coil of the mean diameter of the inner and outer turns. As you say, a double sided pcb is better.
Regarding a DD pcb coil, you could probably work out the overlap with a good electromagnetics software package, but it would be prudent to make provision for a wire bucking coil for final tweaking. I think what I would do initially is to make two identical but separate D windings and bolt them together to make a DD, using small stainless steel screws. If the holes in one board were slotted, this would give provision for adjustment before finally tightening.
Eric.

 

[Anonymous]

Hi Eric,
I'm not that familiar with bucking coils, I have seen an excellent run down on co-planar construction that Dave Emery did. I assume that essentially it would be the same for DD's. That is, a Bucking coil would be in series with the Tx coil and lie over the RX coil, but it's turns would rotate in the opposite direction to the TX and Rx coils?
It's primary purpose being to equalise the energy that the Rx coil receives from the Tx pulse, therefore it would only require sufficient turns to nullify mutual inductance between the Tx and Rx coils.
Dave also says that a damping resistor connects across both the TX and Bucking coils. Is this the same damping resistor used on the mono coil with adjustment to compensate for the addition of the Bucking coil?
I've decided to experiment with wire on my DD before committing it to FR4. It seems to get more involved the more I learn about it. <img src="/metal/html/confused.gif" border=0 width=15 height=22 alt=":?">
Thanks
Kev.

 

[Anonymous]

Hi Kev,
A bucking coil on a pcb DD would likely be just a turn or two, about the diameter of a quarter. This would be manouvered on the pcb until an exact null was obtained, then glued down. With this arrangement, one damping resistor across both would be OK, as the bucking coil inductance would be very minor compared to the main coil.
Eric.