RX Coil Signals

[Dante]

I have a DFX but I dont think that matters for this question. Using an o-scope I can view the TX pulse train on one wire of the coil cable. I am unable to find any trace of a signal on what I presume to be the RX wire. There are four conductors in this cable. The wire that I believe to be the Rx conductor will show a signal if I use the TX as probe ground but that just seems to me that what I think is the RX wire is at ground like the other two wires. I am baffled.

[Ken H.]

Re: RX Coil Signals

Do you have a piece of metal placed near the search coil for this test? Aluminum foil is a good test metal.

[Beachcomber]

Actually if the coil is designed and built

properly you shouldn't see anyhing on the receive winding until a piece of metal is brought into the detector's range. Coils are tuned so that the recieve section doesn't see the tranmit signal. Also there may not be anough of a signal to see on a scope at the receive winding even with a target in front of the coil. You may need to place your scope probe after the first amplifier to have enough of a signal to see.

HH

Beachcomber

[Cody]

Re: RX Coil Signals

The two coils are in a state of inductive balance. The receive coil is shielded from the transmitter coil.

An interesting test would be to lay the detector on a wood surface. If the detector is in an all metal non motion mode type then you can measure the TX signal and RX signal by placing a piece of metal at various distances from the coil starting within about and inch of the coil. If the detector is a motion type of VLF then you need to move the metal object in the coil radiated electro magnetic field. I think you most likely know this since you are using an O scope to measure the coils parameters.

The metal object is something like the iron core of a transformer between the primary and secondary winding. This is just an example and not to be takes as the "way it is".

Cody

[Charles (Upstate NY USA)]

Re: RX Coil Signals

I have a Minelab Explorer and its RX signals are tiny, with my scope on the next to smallest volts division waving a coin across the coil at say 5 inches gives me a rock solid response in the detector headphones yet RX is flatlined on the scope. If I place the coin very near the coil, 0-2 inches I then see a response from RX on the scope, try a popcan and send the coil into overload, that should show up. I'm assuming you are using the DFX to drive the coil and not a signal generator of some type. A continuity test should define the correct pair of wires in the cable, TX might have less resistance than RX at least thats true for the Explorer, certaily TX will have a signal easily seen on the scope.

Re probes, I connect channel 1 to TX and the probe ground to the detector ground which on my machine is one of the RX wires, I connect channel 2 probe to RX main and don't bother grounding the probe.

[Dante]

Re: RX Coil Signals

thank you all. I feel better to know that I was correctly testing this unit. I knew I had the correct pair of wires and was waving a target under the coil. As charles said the DFX sounded off loud and clear but I was unable to view the signal on the scope. I did find a small trace on the lowest volts/div scale but was unsure if I was just seeing EMI or the actual signal. I am glad to know that indeed I have found the signal. I must admit I had expected it to be a larger signal but it makes sense at least.
I only have a 1 channel 10 MHZ handheld scope and was afraid it was not up to this task.

Dante

[Dave Emery]

Re: RX Coil Signals

The output of a well nulled receive coil such as the one on a DFX will be about 6 - 8mV.