Targets frequency response

[Aziz]

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Note that this classification is subject to received frequency band only, not to the transmitted by own TX spectrum. If the spectrum radiated by transmitter contains frequencies that the receiver does not handle and does not need, it means that the TX wastes energy.
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Exactly! PI is wasting a lot of TX power. On the other hand, PI makes simple GB possible.
Aziz

[Tinkerer]

Cutoff frequency and timeconstants of ground

Addition to post #206:
Conductive ground also has series of timeconstants as described above in post #98:
http://www.geotech1.com/forums/showt...9656#post99656
John Corbin measured enough exact two of ground timeconstants. Note that measured by him ratio is almost 9 times
http://www.geotech1.com/pages/metdet...byn/corbyn.pdf

Looking at the TC2 of the ground response above, I just realized that a problem I have with a damping resistor R5, might be turned into an advantage.

Below is a simulation circuit. The inductor L3 with Resistor R5 form a high pass filter. Choosing the right cutoff frequency it might be used to mitigate the T2 response.

A frequency problem.

Any help?

[mikebg]

Exactly! PI is wasting a lot of TX power. On the other hand, PI makes simple GB possible.
Aziz

At what ground, conductive or permeable? Please explain something more.

[Aziz]

At what ground, conductive or permeable? Please explain something more.

GB in PI architectures (off-time sampling) is overall simpler than other GB architectures. It doesn't matter, what type of ground you have. Only the magnetic viscous susceptible matter is the challange (time delayed magnetisation response). The magnetic relaxation effect is quite trivial. Conductive ground is trivial.

Aziz

[Davor]

You can observe both systems' GB as very similar in terms of GB if you only observe a GB point as a combination of offset and a slope with zero crossing in time. Same goes with targets as well.

I'd say that the biggest problem of GB is not GB per se, but the offset that screws everything else and acts as if ground response is not sufficiently attenuated.

In case of IB, offset is dictated by residual unbalanced Tx field (air field), and once it is removed, you can observe Rx zero crossing at some point in time, a delay. Fun part is that this zero crossing delay is strongly related to the target tau, and less so with VLF frequency. At specific frequency it translates to a certain phase, but essentially it is a consequence of delay, you know the famous e^jwτ=cos(wτ)+j*sin(wτ) Being first order system the angles can stretch only within -90° and +90°. Finding zero crossing determines target ID. Provided there is no offset involved.

In PI you have essentially the same thing. You have exponential decay at rate e^-t/τ and half life at τln(2), and there you have it - your zero crossing... provided you can determine what the curve does in infinity. A first approximation would be subtracting EF.

Now something completely different. Main difference between CW and pulse systems is that in CW systems the residual target response is truncated in the opposite going half period, thus eliminating offset in a process, and reducing the amplitude of extreme long taus. Hence the viscosity effects are happening in the same phase playground as everything else.

In PI the zero crossing of viscosity affected materials happens beyond the EF point, thus screwing the offset and also zero crossing reading of other materials. A perfect nemesis.

There are two nemeses in CW systems though. Ground response may happen due to the two contributors, e.g. ferrites and salts, and rigs seldom attenuate both, but the other one is one completely overseen: 2nd harmonic. It effectively shifts offset between positive and negative going half periods.

Solutions? In CW systems you can fight 2nd harmonic and introduce ground balance that is separate from discrimination criteria. In PI you can introduce bipolar pulsing, and perhaps use some weighting function that would result in half life that is different from τln(2).

[mikebg]

Looking at the TC2 of the ground response above, I just realized that a problem I have with a damping resistor R5, might be turned into an advantage.

Below is a simulation circuit. The inductor L3 with Resistor R5 form a high pass filter. Choosing the right cutoff frequency it might be used to mitigate the T2 response.

A frequency problem. Any help?

Tinkerer, please explain the problem, how to simulate it and where is the output point. I see only butiful TX pulses.

[Aziz]

Davor,

brilliant!

[Tinkerer]

Tinkerer, please explain the problem, how to simulate it and where is the output point. I see only butiful TX pulses.

Thank you for the interest. I attach the picture of the traces. On top you can see the formulas I use.

L1 is the TX coil
L3 is the Bucking coil the sum of TX+BU is the balanced coil.

Adding the target response to the balanced coil trace (red) we see what the actual output of the RX coil is.

This is an actual real circuit that works. I try to reproduce the traces I see on the oscilloscope.

When we balance the coils, sometimes we get a little movement when potting the coils and the balance is upset. I have used a pot at R5 as current bypass to correct unbalance.

R5 has several functions. It is damping L3, to avoid HF oscillations.

However, when simulating R5 for best damping, I noticed that it also influences the response of long TC targets, as HP filter.

You got the Flyback pulse on your picture. I prefer to look at the currents as this is what I see at the RX pre-amp input.