IB Rx coil at Tx off

[green]

Posted this scope picture in another thread. https://www.geotech1.com/forums/atta...6&d=1590432292 Wondering what is causing and how to minimize the oscillation. The coil(Rx two 8inch round connected figure 8, Tx surrounds Rx). Wanting to make a bigger one and would like to minimize the signal I'm seeing. Coil not connected to preamp in picture. Coil looks IB but 5mV difference in the two signals would saturate my preamp.

[Carl-NC]

Did you individually dampen each RX coil?

[green]

Did you individually dampen each RX coil?

Not with the Rx coil connected to the resistors only. Can't now because can't get to center tap. Will bring center tap out when I make the next coil. Tried connecting 1k across each figure8 coil when connected to the amplifier. Looking at amplifier out, signal as good with center open(2k across Rx). Didn't try with just the resistors(no amplifier). Was wondering if the two coil ends being the same phase meant anything. I do have another coil not potted I can play with. Suggestions for things to try? Does looking at coil just connected to resistors(no amplifier)with scope make sense?

[Carl-NC]

I really need a drawing to fully understand how things are connected. But a Figure-8 RX needs to have each coil individually dampened because they tend to talk to each other. Here is how I do it in a walk-thru design:



Sounds similar to what you describe except for the damping R's. The Rin's become the feedforward element in an inverting opamp.

[green]

I really need a drawing to fully understand how things are connected. But a Figure-8 RX needs to have each coil individually dampened because they tend to talk to each other. Here is how I do it in a walk-thru design:

[ATTACH]50367[/ATTACH]

Sounds similar to what you describe except for the damping R's. The Rin's become the feedforward element in an inverting opamp.

Drawing for coil same as yours. My scope picture, your Rd is my 1k. Scope ground leads connected to your ground. Scope leads connected to other ends of Rd in your picture. My first picture ground lead connected to Tx circuit ground, second picture scope floating from Tx. Playing with another coil same as above but not shielded. Will post some pictures when I learn something.

[green]

Some scope pictures. First picture repeat of above test with same type coil not potted or shielded. Each figure8 Rx coil does need to be damped at the coils. Carl, thanks for the reminder. Learned at dual field thread but forgot. Amplitude less than what I got with shielded coil. Maybe shielding, Tx Rx spacing or something else I'm missing. Time to try the larger coil.

Coil leads, twisted pair about 30inches.

[Carl-NC]

Amplitude less than what I got with shielded coil.

Amplitude of what? A target signal, or the no-target residual?

[green]

Larger coil? Goal to detect a US quarter at 24inches on the bench. Would like to keep battery power needed to a minimum. Tx rate effects battery power so make Tx rate 500Hz unipolar to keep it simple. Coil diameter___? Inductance___? Wire size___? Peak current(can it be done with 4A or less)___? I'm thinking the figure8 coil I play with so I need to answer the questions for both Rx and Tx. Any suggestions for the figure8 or a mono appreciated. Coil is for bench testing so doesn't need to be practical for detecting.

[green]

Amplitude of what? A target signal, or the no-target residual?

No target turn off. Peak amplitude for picture(shielded coil)reply #1 is more than 10 times not shielded coil reply #6(maybe shield, Rx Tx spacing not the same or something else). Damping at the coil solved the oscillation problem so I stopped testing the not shielded coil. Rx connected to scope to see turn off, no amplifier so hard to compare effect with a target. Thanks again for suggesting damping at the coil.

[Carl-NC]

First, your no-target amplitude is a function of coil matching, parasitic matching, and coil alignment. Less amplitude is better. Apparently your potted coil was not quite as good as the unpotted and, yes, shielding may play a role in that.

Second, the Figure-8 is a good choice for indoor bench testing but not a good choice for raw depth. The farther away a target is, the more the + and - signals blend and cancel each other. For raw depth, I would stick with a mono or maybe a DD. The DD has the advantage of earlier sampling (that helps) and you can increase the RX turns for more RX signal.

[green]

First, your no-target amplitude is a function of coil matching, parasitic matching, and coil alignment. Less amplitude is better. Apparently your potted coil was not quite as good as the unpotted and, yes, shielding may play a role in that.

Second, the Figure-8 is a good choice for indoor bench testing but not a good choice for raw depth. The farther away a target is, the more the + and - signals blend and cancel each other. For raw depth, I would stick with a mono or maybe a DD. The DD has the advantage of earlier sampling (that helps) and you can increase the RX turns for more RX signal.

Trying to understand effect of turns. Signal from target is proportional to ampere turns of Tx coil. Double peak amps or turns and target signal doubles. Rx signal proportional to turns, double turns double signal. Wondering if noise doesn't double also, S/N at Rx stays the same. Increasing Tx turns only helps if circuit noise is greater than coil pickup noise?

[green]

Larger coil? Goal to detect a US quarter at 24inches on the bench. Would like to keep battery power needed to a minimum. Tx rate effects battery power so make Tx rate 500Hz unipolar to keep it simple. Coil diameter___? Inductance___? Wire size_AWG19 Peak current(can it be done with 4A or less)_4A? I'm thinking the figure8 coil I play with so I need to answer the questions for both Rx and Tx. Any suggestions for the figure8 or a mono appreciated. Coil is for bench testing so doesn't need to be practical for detecting.


Filled in some of the blanks. Have a lot of AWG19 magnet wire so would like to use it for Tx coil. Try for 4A peak current with 12V battery for supply. Still thinking figure8 Rx because of less noise. Should try DD also to see if better detection depth. Need to select coil size, inductance and Tx on time to get 4A peak with AWG19 wire for Tx coil. Anything else to consider for Tx coil? Have a lot of AWG28 magnet wire for Rx coil.

[Altra]

Hi Green, one suggestion is to change your pulse rate from 500hz to a multible of 60hz. Such as 480hz, 540hz or 600hz then add a fine tune to your oscillator so that it can be adjusted +/-10hz. This can really help with noise when testing around ac currents.

You will probably need a coil diameter over 15 inches? Large coils tend to lose sensitivity to coin size objects as the diameter increases. Maybe the gold prospectors could tell you if it is possible? They all seem to be using large coils these days.

[Carl-NC]

Trying to understand effect of turns. Signal from target is proportional to ampere turns of Tx coil. Double peak amps or turns and target signal doubles. Rx signal proportional to turns, double turns double signal. Wondering if noise doesn't double also, S/N at Rx stays the same. Increasing Tx turns only helps if circuit noise is greater than coil pickup noise?

Generally speaking, for a fixed power supply voltage fewer turns on the TX produces more ampere-turns. Ideally you would want a 1-turn coil but then your power supply current would be stratospherically high. So you compromise to something you can live with.

On the RX side the opposite is true: more turns produces a higher induced voltage. Ideally you would want a gazillion turns but then you couldn't lift the coil, and the parasitic C would kill you. And, yes, more turns will also induce more EMI noise and generate more thermal noise. So you compromise to something you can live with.

If using a mono coil, then the two coils must compromise to a single winding, and that seems to have settled out to be around 300uH. If using 2 coils (whether IB or not) you can individually optimize them. I would look at 100-300uH on the TX and 500-1000uH on the RX. IB is preferably if you want seriously early sampling.

Goal to detect a US quarter at 24inches on the bench.

A quarter at 24" would be world class... I don't know of a detector that can do that. I have a cache of 220 silver quarters, the best I could ever do was 23". That was 12+ years ago and I'm sure I could get it beyond 24" now, but not a single quarter. The new Fisher Impulse/AQ is getting nickels at 20+ inches in wet salt sand, a little less in the air, and a quarter at even less since quarters tend to be more difficult.

[6666]

The new Fisher Impulse/AQ is getting nickels at 20+ inches in wet salt sand

Is that with the 12" coil ?