Regarding PI coils other than mono

[Nightryder]

Hello everyone,

I have a few questions regarding PI DD, OO(concentric), DOD, and Dual Field coils. I have been digging in the forum for the last 3 days to find answers with no luck. I still think I might find some answers if I keep digging but to be honest with you all, I started, with advancing in age, to have a nasty headache from reading for more than an hour. So here are my questions and I hope this will be helpful to everyone for future reference:

1- What should be the inductance values of Rx and Tx coils? (for each one)

2- In case of Oo (concentric), which coil is inside?

3- In case of DOD, are the two Rx coils supposed to have the right (final) inductance together? (since inductors are additive in series) Or each Rx coil should have the ideal inductance on its own?
(For example: Let's assume for the sake of clarification that the ideal inductance for any Pi Rx coil is 500 uH, Do I have to make a 500 uH coil for each Rx coil in DOD?)

4- For a dual field coil, does it matter if I put a damping resistor for each coil separately or one Rd for both coils?

5- What does the waveform of a damped Rx signal look like?

6- How on earth is the Garrett ATX coil called a DD coil on their website? I think the same is with the TDI original coil. (See attached image)


*Note: If you have answers for some but not all, please do not hesitate share.

Thanks!

[Carl-NC]

1 - Depends on the design. A common TX coil value (Minelab) is 300uH. The RX coil can often be a little bit higher, but it depends on how fast you want to sample.

2 - RX

3 - You would probably want the combination to be 500uH. There is some inductance cancellation since they are in close proximity and out-of-phase so you would need to experiment.

4 - Yes, the inner coil needs a separate damping resistor.

5 - Where? At the coil or at the preamp?

6 - This is what the windings look like:

[Pete61]

Hello everyone,

I have a few questions regarding PI DD, OO(concentric), DOD, and Dual Field coils. I have been digging in the forum for the last 3 days to find answers with no luck. I still think I might find some answers if I keep digging but to be honest with you all, I started, with advancing in age, to have a nasty headache from reading for more than an hour. So here are my questions and I hope this will be helpful to everyone for future reference:


5- What does the waveform of a damped Rx signal look like?


Thanks!

Ignore the yellow sample pulse, that was ch2.
ch1(turquoise) is the pulse from the coil, output of the 5534 IC on the Surf PI.
Left side is damped, right side is what it looks like with lots of spikes from ringing.
Too much resistance will make the pulse width larger, which you dont want. So basically have a short a pulse width as possible with no ringing spikes.

[Nightryder]

Hello Carl-NC

As always, I can not thank you enough!!

5- well, I only know how to adjust damping resistor for mono coil as in figures (11-3) and (11-4) in ITMD book. Which can be done at the preamp output.

So, I'm really not sure where or how for other than mono.

[Nightryder]

Hi Pete61

Thanks for sharing!

It looks to me as if it's a preamp output signal with a mono coil, is it not?

[Pete61]

Hi Pete61

Thanks for sharing!

It looks to me as if it's a preamp output signal with a mono coil, is it not?

Yes it is, my noob thought was the basic waveform would be the same for all coil types.

[Nightryder]

Yes it is, my noob thought was the basic waveform would be the same for all coil types.

I'm not sure either, but if it's the same regardless of the coil type then how is it possible to adjust damping resistors for both Rx\Tx coils using the same TP (preamp output)? I mean that's why I wanted to ask before I start making\experimenting on different coil

[Pete61]

I'm not sure either, but if it's the same regardless of the coil type then how is it possible to adjust damping resistors for both Rx\Tx coils using the same TP (preamp output)? I mean that's why I wanted to ask before I start making\experimenting on different coil

The way Ive understood it from reading(no practical experience) is that you wire the Rx coil temporary to the TX point to test for Rd values.
Then both Rd's are soldered to their respective coils in the coil housing.
I usually write the values of everything on the coil via a bit of masking tape, cos I tend to forget when Ive been juggling 3 different coils around.

[Nightryder]

The way Ive understood it from reading(no practical experience) is that you wire the Rx coil temporary to the TX point to test for Rd values.
Then both Rd's are soldered to their respective coils in the coil housing.
I usually write the values of everything on the coil via a bit of masking tape, cos I tend to forget when Ive been juggling 3 different coils around.

I think this works on dual field coils. Worth trying it with a DD coil.

Thanks!