Balancing DD Coil(s)

[Rick S]

Carl,
It seems that I am the only one right now that is doing anything on the this forum. I would hope others would join in on the fun.
I have built two DD coils and this is the result of what happened when I hook them up. I found out that one coil is on the positive side and will not balance with ground. The other one is negitive and will not balance to ground. I have tried different capacitance in parallel to see what the difference would be in getting the coils to react correctly in the DD confriguration. I haven't gone over the 50khz limit from the transmitter, but have tried from about 47 to 50khz. I still can't get the DD to work right. I have tried bending the Rx coil over the Tx coil and just laying the Rx coil on top at the proper spacing. Still no control over the ground balancing. Would this make any difference balancing of what plane the fields would be in? I have checked and rechecked the inductance of the two coils. I have put them right on the money each time. I thought that if there is error in the inductance meter I would make adjustments for that which I have done. When I pass my hand over the coil there is to much response as compaired with the stock coil. What I have basically is two Lc circuits and the only thing I haven't really played with alot is the resistance. I have tried a resistor in series and I still don't get any ground balance. I have looked at the impedance, Q, and bandwidth. Any hints would be helpful. Thanks for the bent ear Carl. Thanks, Rick S

[Mark Pauls]

Re: Balancing DD Coil(s)

Hi Rick

I expect you have balanced the coils as good as possible and fixtured them so they cannot move.

One thing I have noticed is that often you can adjust a small im-balance by connecting a small capacitor from one of the sides of the TX coil by maybe 20-1000 pF to the most hot side of the RX coil.
This fine tunning can balance the coil 100 % and the good part is this you can do it at the end of the cable where you connect it to the PCB.

Hope this helps.

Mark

[Rick S]

Re: Balancing DD Coil(s)

Thanks Mark for the info. I will try it later today.

[Carl]

Re: Balancing DD Coil(s)

Participation comes and goes... tinkering with detectors seems to be what people do when all other odd jobs are done, which is almost never.

What do you mean "on the positive/negative side"? Do you mean that ground effect is always positive and there is nothing you can do to make it neutral or negative? If so, I suspect that the coils themselves are OK, and that you don't have the overlap quite perfect enough. A little too much overlap might make the GB always positive, and too little overlap might make the GB always negative (or vice-versa).

The GMV design (and maybe any IB design) might be incredibly sensitive to coil balance, and you have to get the balance near-perfect. Trying to adjust a big multi-turn coil for balance is like driving a brad with a sledgehammer. What you might try next is take a single strand of wire in series with the RX coil and loop it over one side or the other of the TX coil. In other words, get as close to balance as you can with the normal RX coil overlap, then add a single-turn tweaking loop that you can move around and which will have far less sensitivity to position. I know this technique is sometimes used by manufacturers.

- Carl

[Rick S]

Re: Balancing DD Coil(s)

Carl,
You answered my question. Now to find time to apply these techniques. Thanks Rick S