GMVSAT coil DD

[Rick S]

Carl, What is the best way to check that the coil(DD) I made works before I hook it up to the GMVSAT electronic circuit? Also, what is a simple way to null the DD coils? I am still learning this area of electronics. Would I use some of the Rx circuit in the electronics with a scope attached to see if the coils haved nulled and are at frequency. Has to be away or a circuit that could be built for trial and error process. Any help would be greatly appreciated. Rick S

[Carl]

Re: GMVSAT coil DD

You either need to hook it up to a circuit that is substantially similar to the GMV, or just go ahead and plug it into the GMV. You first need to resonate the two coils at the operating frequency of the GMV, 50kHz I think. For this you really need it hook to the GMV so you will have the proper capacitive loading, also need to have the final cable on the coil.

Start with the TX coil. You need an oscope hooked up across it, although a DMM might work, never tried it - I suspect the 50kHz might be too high for most DMMs. You want to put a capacitor across the coil to kill high-frequency noise, but not affect the TX signal. Try different values until you've found the highest value cap that does not decrease the signal level more than a couple of percent. Now move the oscope to the RX coil, and with the coil intentionally un-nulled do the same thing. This one might definitely require an oscope as the signal level will be much smaller.

An alternative to all this is to try and match the resonance of your coils to that of the original. You can take the original coil and hook it up to a frequency generator, first the TX then the RX, and sweep it around the 50kHz region. This will tell you the resonant frequency and the resonance bandwidth, which you can duplicate in your new coil. With this method it doesn't matter if the coil is not connected to the detector.

Nulling the coils involves sliding the overlap around to minimize the RX signal. You need to do this in all-metal mode, which the GMV already is - may also need to switch off the SAT, not sure if that will affect things.

I suppose you are placing the coils on some sort of disk, so get them roughly placed for a minimum response, well away from any other metal. Now glue the coils down everywhere except where they overlap. Now you can fine-tweak the overlap, maybe with a higher sensitivity setting (which I don't know if the GMV has).

[Rick S]

Re: GMVSAT coil DD

Thanks for the info and will let you know what happens. Rick S