Dual coil for mpp

[Carl-NC]

Also, if somebody can state what is the value of TDI external Rd, I think this would help put more light on subject.

680 ohms

[dbanner]

Thanks Carl.
Setting asside other factors such as pulse width and frequency, an external main Rd of 680 ohms and assuming 300uH, seems to be within a normal range value. The 4.7K seems way high for a calculated inner coil Rd based on JUST THE PARAMETERS OF THE INNER COIL, even moreso when you consider that it is "in-circuit"
So maybe most of the damping is taken care of by the main external Rd, and the internal Rd takes care of any remaining contribution of the small coil to the overall ringing of the entire setup. This tendency of the small coil to ring may be enhanced by the reflective effects of the coupling with the larger coil, and could not be totally eliminated by the external Rd alone, since it is isolated by the larger coil.

[green]

Used the small and large coil to make a dual field coil. inductance and SRF measurements a little different than picture. Maybe measurement difference or I super glued the wire to the backing after measuring the first time. Wires were loosening from tape.
coil__________uH________SRF MHz
small________115________6.67____calculated Rd=2410, small and large coils measured one at a time, not near each other
large________115________4.5
series________271_______4.16_____connected in series, outer small to inner large
series________271_______1.25 MHz circuit resonance with added graphite shield and 30 inches twisted pair lead. connected to Tx circuit, MUR460 diode to inner small, outer large connected to +supply. external Rd not connected. 2410R across small coil to damp small coil.

adjusted external R for critical damping looking at coil decay on scope, Rd=1390R, calculates 1064R, didn't see any ringing, scope trace decay looked good. Didn't try internal damping both coils, looked good with just the small coil damped.

Tested the completed dual field coil, shielded with 30inch lead with a MPP today.

Placed a loop on the dual coil. Reduced Rd across small coil until oscillation went away. Scope picture of MPP amplifier out. Repeated with calculated small coil Rd(6.67*115*pi=2410). Don't see a difference, Maybe calculated or a little higher is good.
Removed loop when recording amplifier out, acted as target. Lowering circuit Rd lowers overshoot but still flattens out around 10us.

[bbsailor]

Tested the completed dual field coil, shielded with 30inch lead with a MPP today.

Placed a loop on the dual coil. Reduced Rd across small coil until oscillation went away. Scope picture of MPP amplifier out. Repeated with calculated small coil Rd(6.67*115*pi=2410). Don't see a difference, Maybe calculated or a little higher is good.
Removed loop when recording amplifier out, acted as target. Lowering circuit Rd lowers overshoot but still flattens out around 10us.

Green and all geotech1 members

Here are some general rules of thumb that affect the damping resistor (Rd) value. Generally, higher capacitance and higher power in coils requires lower Rd values. Here are the sources of the total TX circuit capacitance and power that affect the Rd value.
1. Lower resistance coils with a higher current draw typically produce a higher flyback peak voltages and require a lower Rd value.
2. Higher inductance coils compared to lower inductance coils require lower Rd values.
3. Higher TX circuit capacitance which includes:
a. Coil turn to turn capacitance.
b. Coil to shield capacitance.
c. MOSFET COSS or output capacitance, higher is Rd lower.
4. Cable between coil and metal detector circuit length and the cable capacitance.
5. First RX amplifier stage and the amount of time it needs to come out of saturation. The fastest design uses two stages of lower gain than one stage of higher gain.
6. As delays get lower, the coil wire holding eddy currents may begin to look like a target at lower delays. Use thinner wire, stranded wire or Litz wire to offset this effect.

As you can see, making a fast coil requires balancing many factors to get the least delay time between the TX pulse turn off and the RX turn on. Ensure that the electronic circuit resistor values in the delay circuit will allow a faster delay than the stock values. Carl's Hammerhead PI circuit and article shows which circuit value need to change for a faster delay time modification.

Fully stimulating a small low TC target requires that the TX pulse turn off time constant, not discussed much, requires a TX discharge TC to be 5 tomes faster than the target TC. This is calculated by the coil inductance divided by the Rd value. Smaller, low TC targets require a higher Rd value to make the discharge slope more vertical and better able to fully stimulate smaller low TC targets.

Creating a good and accurate mental model of how all these pieces fit together will help Geotech1 members alter coil and circuit designs and have a good understanding about how they all fit together to optimize performance for a specific range of target sizes and TCs.

Bottom line: detecting smaller, low TC targets generally requires a higher coil Rd value. The Self Resonant Frequency (SRF) measured at the end of the coil wire coax will indicate the total coil capacitance, but not the circuit capacitance. Higher SRF means lower capacitance in the total coil design.

I hope this helps.

Joseph J. Rogowski

[dbanner]

Tested the completed dual field coil, shielded with 30inch lead with a MPP today.

Placed a loop on the dual coil. Reduced Rd across small coil until oscillation went away. Scope picture of MPP amplifier out. Repeated with calculated small coil Rd(6.67*115*pi=2410). Don't see a difference, Maybe calculated or a little higher is good.
Removed loop when recording amplifier out, acted as target. Lowering circuit Rd lowers overshoot but still flattens out around 10us.

[dbanner]

Next to consider what effects increasing the PRF would have on damping a dual coil, right out to 3000 pps

[green]

I'm not clear about the steps you are taking for damping the dual coils.
For example, my feeling is the external Rd value should be the primary damping variable, the actual inner coil Rd value in practice would always be much higher than a calculated value derived from the open coil SRF parameters of the inner coil.
I guess what I'm saying is that the external Rd does damp the ringing energy attributed to both coils(both coils are after all, in series)

If you lower the value of the external Rd, this will allow a significant increase of the internal Rd, which is more preferable, because you want most of the energy passing through the inner coil instead of bypassing via the inner Rd.

If not, you may see perfect waveform with no ringing, but the sensitivity may be poor, especially for the inner coil.

First this what I did, maybe not the best way. Completed dual field coil connected to MPP

Can't see the small coil oscillation at amplifier out so used a loop(ground lead connected to probe)on the coil to look for small coil oscillation. Used a pot to adjust small coil Rd, reduced resistance until small coil oscillation stopped. about 3500R. Don't remember what circuit Rd was, maybe open with just Rin connected. Then adjusted circuit Rd with a pot(loop removed). Repeated with small coil Rd=calculated 2400R. Amplifier out looks the same with small coil Rd=2400 or 3500, don't know if making it higher would make the signal better. I'll try again with a higher small coil Rd and adjust circuit Rd to see if small coil oscillation stops and verify what I had for circuit Rd when looking for the small coil oscillation to stop.

[godigit1]

Hi Green,
Ok now I know exactly whats wrong!
What you just did did not damp the coil for the mpp it only damped your test circut. How are you laying a loop over the coil to damp it
That Mpp Is componenet for component the same test board I have right down to the upgraded op amps.


You need to hitch it up and damp the coil on its own merets on the mpp. Properly damp per forum procedure monitoring op amp and fet out or raw coil fly back.
That will the damping value on the small coil.
The 12 inch dual field from whites damps on my mpp at exactly 681 ohms now verified as the value on the tdi as well.
These is no way I can accept those damping #s
Im Sorry.
If your willing to test with forum procedure I will test your coil. other wise were not even close enough to the ball park to see it.

On the SRF math I will explain my math a bit.

For inductance all you have to do is ring the bell on the coil and you cant go wrong.
6.9 mhz =3.9 pf calcs to 136 uh
3.3 mhz =17.23pf calcs to 135 uh
1.5 mhz =83.39pf calcs 135uh.

using Pi*l*srf

6.9 =2926 ohms
3.3 = 1399 ohms
1.5 = 636 ohms. closest to my actual.

There are two different formulas on the whites patent depending on where you look on google patents its rd=l*PI*c on free patents its square root of Ldevided by pi *c

The formulas work out in invers to each other as one is using capacitance the other SRF.
With the freepatents whites patent I get.
6.9 Mhz 127 ohms
3.3 mhz 267 ohms
1.5 mhz 587 ohms.
The 587 ohms is closest to actual damping.

As far as me bailing earlier it just got too crazy and the belittling comments. and please a 4 k ohm resistor in the dual field and no one questioning it but me. Anyone whos damped Pis should question that. I would suggest you recheck a couple more screens with different color resolution the multiplier is black to me making the resistor 417 ohms whites knows what they are doing.

This is mind blowing we are not even close enough to be in the same ball park.
Im sorry this is just too hard to get things straight I know my on mpp damping values are right.
.
My efforts here to help with the damping of the dual field are usless as no one believes me or my oscope.

When you do a final damp on both coils you can see the effect of the inner resistor in two ways. If the inner coil is underdamped you will see ringing when target testing.
If the coil is overdamped its harder to tell, you can only see it in coil speed by trying multiple innercoil resistor values but if the inner coil is over damped it slows down the coil.
Take it or leave it !!!!!
Im sorry for seeming abrupt this just doesnt make sense to me.

[dbanner]

godigit1, The resistor in the whites coil is clearly a 5 band resistor, The colors I see are Violet, yellow, black, and next comes the multiplier, which is definitely NOT black, it is most likely brown, making the resistor 4.7K. It couldn't be red, that would multiply to 470K. A totally unrealistic value for damping the inner coil or any PI coil for that matter.
The last band is the resistor tolerance, which is irrelevant.

[dbanner]

First this what I did, maybe not the best way. Completed dual field coil connected to MPP

Can't see the small coil oscillation at amplifier out so used a loop(ground lead connected to probe)on the coil to look for small coil oscillation. Used a pot to adjust small coil Rd, reduced resistance until small coil oscillation stopped. about 3500R. Don't remember what circuit Rd was, maybe open with just Rin connected. Then adjusted circuit Rd with a pot(loop removed). Repeated with small coil Rd=calculated 2400R. Amplifier out looks the same with small coil Rd=2400 or 3500, don't know if making it higher would make the signal better. I'll try again with a higher small coil Rd and adjust circuit Rd to see if small coil oscillation stops and verify what I had for circuit Rd when looking for the small coil oscillation to stop.

Thanks for explaining the process you followed.
For me, the inner coil should be damped LAST.
I don't know for sure, but if I were doing it, I would damp the coil assembly with the external Rd first. I expect it would reach near critical damping somewhere in the hundreds of ohms. At this point, there is no Rd on the inner coil as yet, and the scope would be monitoring the preamp output.(you observed you must now change to the loop over the inner coil method for next step)
Next, with the external Rd set in place, I would damp the inner coil, I expect a high inner Rd certainly above 3K, based on the fact that the external Rd MUST have some damping effect on the inner coil in the first place, because the inner coil is in series with the outer coil, making in effect a single coil( the circuit "sees" only one coil, not two).
So the inner coil does not need much damping because the external Rd takes care of the majority of energy in the ringing of the entire setup.
Of course this is only conjecture by logic, but you must account for whites 4.7K!! They are the experts, and their dual field coil is ordinary bundle loop! So how come 4.7K????
I'm not sure the impact of PRF on damping, but isn't the MPP and TDI of similarly high PRF?

[dbanner]

In post #190, I erroneously stated the whites internal Rd as a 4 band, clearly it is a 5 band resistor.

[dbanner]

I have no problem with saying the inner coil is isolated from the circuit by the outer coil, and therefore needs its own Rd as is stated in the whites patent, but my feeling is that it is not completely isolated, and the external Rd does have effect on it.

[dbanner]

.....As far as me bailing earlier it just got too crazy and the belittling comments.

What the hell are you talking about??? What belittling comments?

If any of my comments were percieved as "belittling", then I apologize.

[green]

Whites formula Rd=pi*L*C=pi*136uH*3.9pf=.00000000000000167 ohms
Rd=pi*L*SRF=pi*136uH*6.9MHz=2948 ohms
Rd=.5*sqrt(L/C)=.5*sqrt(136uH/3.9pf)=2952 ohms

[godigit1]

Whites formula Rd=pi*L*C=pi*136uH*3.9pf=.00000000000000167 ohms
Rd=pi*L*SRF=pi*136uH*6.9MHz=2948 ohms
Rd=.5*sqrt(L/C)=.5*sqrt(136uH/3.9pf)=2952 ohms

This formula,
http://www.freepatentsonline.com/7994789.pdf