Mono Coil VS IB Coil Target Response

[Mr.Jaick]

I've been working on PI Mono Coil's X signal to see the possibilities if any
at the moment I use current sense resistor.
I have used this method of measurement and succeeded to get accurate DISC at around 70% of the depth.

so basically you have a comparator with hysteresis that has a threshold level, whenever the current passes that threshold level output goes high
with metal close(in theory) the coil inductance changes and the output pulse gets wider or narrower depending on the Fe/Non-Fe
but that experiment was with a Ferrite cored 200uH/5A small inductor which has a concentrated field, nothing like a real air cored coil
with a 22cm / 360uH / 550kHz (connected to TX) it shows almost nothing!
I say almost because if you put a huge 10cm*15cm aluminum plate on the coil windings it will show something, useless.
anyways there are better ways to extract that signal
now I have a question that I simply can't find the answer for, not with my oscilloscope nor simulators.

as you know it is easy to get good DISC out of an IB coil and because you don't have the huge air-signal to block your view you can see the small changes at TX on with the Rx coil(using O-scope)
for example: DD IB coil, coin at 5cm gives 1mv change in signal level looking at RX coil when TX on.
Question : does the TX coil get the same effect as RX coil when metal close (but we can't see it) ?
And if not, Why not?

[Carl-NC]

Any changing magnetic field applied to any coil will induce an EMF in the coil. In the case of an RX coil one side is connected to a relatively high impedance circuit so the induced EMF is easy to see. The TX coil is driven, usually with a relatively low impedance circuit, so the induced EMF is not as easy to see. However, the induced EMF in the TX coil will create a small coil current that adds/subtracts from the driven TX coil current and that effect can sometimes be seen as a variable loading effect on the TX drive circuit.

[Mr.Jaick]

Hello Carl and thank you for your collaboration.

The TX coil is driven, usually with a relatively low impedance circuit, so the induced EMF is not as easy to see.

That's correct , but when you say "not as easy to see" do you mean the induced EMF is there (in TX coil) no matter how small it is?
it's just our measuring technique that does not allow it to be seen?
for example I was using a 0.1R sense resistor to sense the current, so if I change it to 1R I can amplify those small changes by 10 as I increase the TX impedance, but there is a tight limit to this

[Carl-NC]

The target-induced EMF in the RX coil is 100% of the signal in the RX coil, assuming perfect IB. The target-induced EMF in the TX coil creates a current in the TX coil which is a very small fraction of the total current in the TX coil. Let's say it's 0.1%. Therefore if the normal TX current is 100mA then the target-induced current will be 100uA. Whatever resistor you use to sense the current, the target-induced portion will always be 0.1%. You cannot change/amplify the target portion without also changing the main TX portion.

[Mr.Jaick]

You cannot change/amplify the target portion without also changing the main TX portion.

Yes indeed.
I didn't mean to say only the target portion gets amplified, both get the same amplification but the TX portion is just a DC offset which we remove and after that amplify the small change
my only concern was that does the smallest target eddy still effect the low impedance TX coil? which is apparently yes
I mean when the target is far away from the coil not when close to winding and saturated

I'm still not sure if this theory works or not because my current setup is not adequate so I need to build another one
maybe at the end I'd go with the IB coil

Thank you as always for your help.

[Carl-NC]

If the target induces an EMF in the RX coil, then it also induces an EMF in the TX coil, even when the target is deep. It's just really really hard to see anything in the TX coil.

[Mr.Jaick]

If the target induces an EMF in the RX coil, then it also induces an EMF in the TX coil.

That makes sense.

It's just really really hard to see anything in the TX coil.

yes it is very hard.
imagine the induced EMF of a target far away from the coil magnetic field adds 100nA to the total coil current then you pass that 100nA through a 1R sense res and get 100nV change
now if prefect world and no noise, then you can bring up your 8 1/2 digit Keysight and measure it.

what are the other methods of achieving DISC for mono coils?
If you direct me to some related threads and posts I would greatly appreciate

[Carl-NC]

what are the other methods of achieving DISC for mono coils?
If you direct me to some related threads and posts I would greatly appreciate

In PI it might be possible to look at the decay curvature and determine not only target tau (that's definitely doable) but also ferrous. Ferrous should give a response that is not a simple exponential so it should be distinguishable from non-ferrous. This will require a minimum of 3 samples.

Garrett made a discriminating BFO by combining frequency shift effects with transmitter loading effects (what I mentioned on post #2). This used a mono coil.

Off-resonance designs from the early-mid 1970s used a mono coil and had a variable discriminator. But they were not very deep.

I don't know how much any of these have been discussed or where to find it, you'll have to search.

[Mr.Jaick]

I'm looking for X signal discrimination, DISC based on R signal is not reliable enough, ground condition, different shape Fe material and composite exponentials all effect DISC reliability.
I either have to use an IB coil(last option) or investigate current sensing methods on the TX.
but all and all I'll give it my best shot.
thank you for having me.

[Carl-NC]

Yes, I agree, you need to look at the X signal. TX current sensing won't achieve the sensitivity of the RX side so you might be able to achieve discrimination for the first 3-4" but after that it's just R signal. That's why we have IB coils.

[Mr.Jaick]

yeah exactly
specially when coil gets large and TX field opens up