Help with setting phase shift on a coil....

[John in NC]

I am working on my own coil for my White's MXT. I found a sacrificial stock coil that I could look into. I found the TX coil uses 22 gauge wire and the RX coil uses 32 gauge wire. I matched the inductance, resistance and capacitance of the stock coil. I am building and experimental type of double D coil. I have it made and when I hook it to my MXT, it detects great and I can null it for silent search. The problem I am having is the VDI numbers are not correct. Also I have not tried to discriminate yet. I think the MXT discriminates by using a 90 degree phase shift. Can I set up by using a function generator set at 15K (MXT run freq.) with about 10 volts on the TX coil and use an r-c setup to get the 90 degree phase shift while looking at both the TX and the RX on my dual channel scope to insure it's 90 degrees?
John

[Carl-NC]

How far off are the VDIs?

[Altra]

Something I found important for this coil, is the TX series resistance. Try matching the series resistance with the factory coil. My notes show 570uh at 2.48ohms. This includes the cable. Maybe Carl can give more accurate figures. Try adding 0.5 to 1.5ohms in series and note the change in VDI. This assumes you have the correct wiring and inductances.

Mark

[mikebg]

John, most important for phase shift is the capacitance across RX coil. You should adjust RX capacitance for proper GND elimination. Use as test object for ground mineralisation a ferrite rod (from AM antenna) and adjust RX capacitance to eliminate the rod when GND control is set near to one end of its range. If the VDI numbers are not correct, set GND control near to other end and change again RX capacitance for elimination of ferrite rod. No need of function generator and oscope.

[John in NC]

Good ideas. Thanks guys....

In my notes I have 555uH and 2.9 Ohms and I've matched those values. I'll give your ideas a try.
Thanks, John

[John in NC]

Altra, you hit it on the head!

Thanks for all the suggestions. It took a bit of experimenting, but Altra's suggestion of series resistance on the TX coil did the trick. Perfect VDI's every time.
Thanks a bunch, John