How To Null Pi DD Coil and a Concentric Coil with bucking coil?

[autitch]

Question for the brains trust.

I?ve researched high and low throughout the forum and the best I?ve found in answering my knowledge gap is the following quote from the following topic post #4

https://www.geotech1.com/forums/show...ight=null+coil

bbsailor wrote regarding balancing a Pi DD coil ?You can use a signal generator on the TX coil and a scope on the RX coil to try to get close to a null, but this is not too critical, however it helps.?

I was wondering if someone could explain this technique a little further and how it would be applied to DD and a Concentric Coil with bucking coil.

What magnitude of signal does one need to input into the TX coil frequency and voltage? The typical SRF of the TX coil in in the range of 500kHz and 1300kHz depending if the coils are shielded or not.

Should this process be conducted after shielding or is it still valid to perform prior to shielding?

What phenomenon is one looking to observe on the scope to achieve a null. ( for a DD coil the TX & RX are overlapped to a greater or lesser extent and for a Concentric Coil with bucking coil the bucking coil turns are added or removed.)

Thanks in advance.

autitch

[waltr]

I do use and like IB coils on my PI detector. I run either an 20cm concentric or a 25cm DD. My thread goes over how I did the balancing with scope pictures. Look here:
https://www.geotech1.com/forums/show...ake-on-the-HH2

Balancing is not critical on a PI detector since these typically do not try to obtain any Phase (X) information unlike a VLF detector that extracts both X and R components from the signal.

There is theory that there is phase (X) info during the TX pulse if the coil is not perfectly nulled. See my thread for more.

My method is to connect the coils to the detector. connect scope to the Pre-amp output and turn on the detector.
Adjust the coils while watching the scope paying attention to the TX ON period.
When perfectly balanced the TX ON signal at pre-amp out will be flat and Zero Volts.
I adjust for slightly under balanced- that is at the beginning of the TX ON pulse the pre-amp rails but the signal decays to near zero Volts before the TX pulse is finished.
Look at the scope pictures in my thread.

I do play with the Damping resistor value before and after adjusting null. Not sure how much the effects the null but does effect the the tx off decay signal.

[WM6]

I do use and like IB coils on my PI detector.

Assume that with IB coil on PI detector, you are limited to fixed pulse rate (pps) or IB is gone?

Do we need to take in consideration circuit preheating time, to stabilize pulse rate (if not quartz driven)?

[waltr]

Assume that with IB coil on PI detector, you are limited to fixed pulse rate (pps) or IB is gone?

Do we need to take in consideration circuit preheating time, to stabilize pulse rate (if not quartz driven)?

I am not sure of that.

What I see on the scope is only during TX ON.

Look at my scope pictures here:
https://www.geotech1.com/forums/show...237#post238237

When coils are nulled or even almost nulled the pre-amp output Voltage is at Zero towards the end of the TX pulse.
Changing the Pulse Rate would NOT affect this nor would changing the TX pulse time (width).

I can believe that in a VLF detector that the Oscillator stability will effect the coil balance (Nulling) but not in a PI detector.

[autitch]

Hi waltr. Thank you for the detailed explanation above, however I’m unable to use the approach you describe. At this stage, I’m looking to build a DD coil for a Minelab Pi detector and since the circuit boards are painted white I’m not able to access or identify the Pre-amp on the detector. Furthermore I don’t particularly feel confident to put my hands into that little box. The generally described method by bbsailor “You can use a signal generator on the TX coil and a scope on the RX coil to try to get close to a null, but this is not too critical, however it helps” appeals to me as the detector isn’t required in the process so I’m seeking guidance on how to achieve a null without the use of the detector.

Autitch

[waltr]

That makes it difficult to determine if you achieve the proper Null between the coils.

You could try driving the TX coil with an oscillator and look at the RX coil on a scope.
Never tried this so unsure but you should be able to null the the coils. Then try another frequency, is it still Nulled?

The other issue may be is obtaining the proper Damping. Since you can not look at the pre-amp output with a scope it is a guess as to the damping.

The only thing I can think of to suggest is setup the coils on a cardboard box, away from metal and connect to the ML PI detector.
Then listen to the audio out while passing various targets over the coil. Play with the coil over lap (very slightly) to try and determine what works best.