THE ELUSVE 1us TC TARGET

[Gunghouk]

Typo - R14, k not p. Sorry

[Detectorist#1]

Hi Tinkerer,

Attached is the schematic of the frontend sensor board of one of my projects with 2.5us delay to first sample.
Maybe, this information will be useful to you in the efforts to design fast PI MD.

[Tinkerer]

Typo - R14, k not p. Sorry

Thank you.

[Teleno]

One thing I haven't seen mentioned before is the target response of the cable end right next to the coil. It can have a tc of a us or longer. But since it's present on both the ground and the signal cable I guess it has no effect on the preamp. I'm not sure about this though.

[Tinkerer]

One thing I haven't seen mentioned before is the target response of the cable end right next to the coil. It can have a tc of a us or longer. But since it's present on both the ground and the signal cable I guess it has no effect on the preamp. I'm not sure about this though.

Teleno, thank you for bringing up the subject.

Indeed, there are many additional factors that have significance in achieving a short delay. For several weeks now, I have been experimenting in trying the best compromise between ALL the bigger and smaller problems involved in very early sampling.

I found that the shortest delay does not always give the highest signal amplitude. Other factors also play a part.

As you have seen from the imperfection of the scope pictures I posted, I search for a short delay and at the same time reasonable sensitivity and reasonable stability. Nothing extreme.
The shielding and cable are next on the list.
Traditional PI used a coax cable. In my experiments, I found a shielded Audio cable to work better. A wrong cable is sensitive to the touch and movement of the cable. When you hold the cable in your hand you get a signal. A good cable configuration does not give a signal when touching and moving.
High current in the cable induces a (relatively) high current in the shield. The shield has inductive and capacitive (and resistive) characteristics. This means that the discharge of the current in the shield tends to oscillate. One of the reasons I decided to use a high inductance TX coil, (about 2000uH) is that I can reduce the coil current and still achieve a di/dt of about 30 Ampere-turns / microsecond, with less than 1A in the cable.

Yesterday I tried to achieve a better mechanical balance between the TX coil and RX coil in the DD coil assembly.
Why DD? should I explain?

I took the top shield off, cut the hotmelt glue fixing the 2 coils together and moved the coils 1mm at the time, while observing the signal at the output of the preamp, until I got the best balance. Then I fixed the coils again with hot melt glue and moved the ends of the TX and RX wires to get the best balance, Resoldered the connection of the cable shield to the coil shield.
After I closed the top shield of the coil, I found that all the work was for nothing, again there is one cycle slightly different than the other.
Why?????
Today I will work on the why.
I am aware that all this is boring for many. I am wasting my time to explain the process of developing a different (and hopefully better) PI for the few who might find it interesting.
Anybody interested should give some feedback. (like Teleno)
No feedback means no interest. No interest means I don't need to waste my precious time.

[ivconic]

Another good reason to start making a wireless coil!

[pito]

high current in the shield = wrong connection, the current in the shield should be 0

[Tinkerer]

high current in the shield = wrong connection, the current in the shield should be 0

Thank you for the feedback.
the current in the shield should be 0 Yes and no. I make the coil shield with graphite paint, in the hope that most of the eddy currents caused by the strong TX pulse dissipate (convert to heath). However, if the coil shield is not connected to ground, it does not have any effect. This shows us that some of the eddy currents must discharge to ground.
With the shielded Audio cable, I use the metallic cable shield to connect the coil shield to ground. The cable shield is very much affected by the high current switching pulses. These eddy currents must be conducted (current) to ground.
There is a great variety of shielded Audio cable on the market. Many are totally unusable as PI cables.
Some shielded Audio cables work more or less good. I tried to use many, many different cables until I got lucky and found the best one.

[pito]

this coil has 300 pF capacitance to ground, thick wall pvc tube,


to eliminate cable problem move Tx MOSFET and resistor to coil side = small metal box in first picture.


These eddy currents must be conducted (current) to ground. = but not through shield

0 current through shield

[IBGold]

Teleno, thank you for bringing up the subject.

Indeed, there are many additional factors that have significance in achieving a short delay. For several weeks now, I have been experimenting in trying the best compromise between ALL the bigger and smaller problems involved in very early sampling.

I found that the shortest delay does not always give the highest signal amplitude. Other factors also play a part.

As you have seen from the imperfection of the scope pictures I posted, I search for a short delay and at the same time reasonable sensitivity and reasonable stability. Nothing extreme.
The shielding and cable are next on the list.
Traditional PI used a coax cable. In my experiments, I found a shielded Audio cable to work better. A wrong cable is sensitive to the touch and movement of the cable. When you hold the cable in your hand you get a signal. A good cable configuration does not give a signal when touching and moving.
High current in the cable induces a (relatively) high current in the shield. The shield has inductive and capacitive (and resistive) characteristics. This means that the discharge of the current in the shield tends to oscillate. One of the reasons I decided to use a high inductance TX coil, (about 2000uH) is that I can reduce the coil current and still achieve a di/dt of about 30 Ampere-turns / microsecond, with less than 1A in the cable.

Yesterday I tried to achieve a better mechanical balance between the TX coil and RX coil in the DD coil assembly.
Why DD? should I explain?

I took the top shield off, cut the hotmelt glue fixing the 2 coils together and moved the coils 1mm at the time, while observing the signal at the output of the preamp, until I got the best balance. Then I fixed the coils again with hot melt glue and moved the ends of the TX and RX wires to get the best balance, Resoldered the connection of the cable shield to the coil shield.
After I closed the top shield of the coil, I found that all the work was for nothing, again there is one cycle slightly different than the other.
Why?????
Today I will work on the why.
I am aware that all this is boring for many. I am wasting my time to explain the process of developing a different (and hopefully better) PI for the few who might find it interesting.
Anybody interested should give some feedback. (like Teleno)
No feedback means no interest. No interest means I don't need to waste my precious time.

You are heading in the right direction Tony.

Regards, Ian.

[Carl-NC]

After I closed the top shield of the coil, I found that all the work was for nothing, again there is one cycle slightly different than the other.
Why?????

It is common for the shield to affect the balance of an IB coil. At White's, when nulling a coil using the "loose turn" method we would intentionally null it slightly off, so that when the shielded coil bottom was added it would pull it right in.

[Tinkerer]

You are heading in the right direction Tony.

Regards, Ian.

Ian, thank you for the encouragement.

[Tinkerer]

It is common for the shield to affect the balance of an IB coil. At White's, when nulling a coil using the "loose turn" method we would intentionally null it slightly off, so that when the shielded coil bottom was added it would pull it right in.

Why? Yes, my mistake was that I removed the top shield, adjusted the balance and then added the top shield again. I should have removed the top and the bottom shields for adjustment. Or measured the offset caused by the top shield and given allowance for that.

Mistakes are good. One can learn a lot through mistakes.

[pito]

my mistake was that I removed the top shield, adjusted the balance and then added the top shield again. = Leave it as it is and use the electronic nulling circuit (I described it earlier, check my posts)

[ivconic]

Why? Yes, my mistake was that I removed the top shield, adjusted the balance and then added the top shield again. I should have removed the top and the bottom shields for adjustment. Or measured the offset caused by the top shield and given allowance for that.

Mistakes are good. One can learn a lot through mistakes.

Than I am the ultimate champion of learning!