PI limit to early sampling

[Phiphi]

Now I continue. I have forgotten to give the parameters I have used : L : 300 uH, C : 250 pF and Rs : 0.3. That are the values I have measured on a 11" Minelab coil I have purchased on eBay. The current at Tx off is about 1.5 A.

Usually, to limit the voltage at the preamp input, it is necessary to add a resistor and diodes like that :



The same but with the inductance :



With these components there is a decay time increase as seen on the following images :




Now we have 6.0 us without inductance and 4.4 us with inductance. Once again, the damping resistor is optimized to get the shortest decay time.
So I had the idea to add diodes serially with the damping resistor instead to have R + D in parallel with the damping resistor. Here is the result :




There is a 22 ohms resistor in parallel with the diodes to prevent oscillations. The related waveforms are the following :




Now we have 4.3 us without inductance and 3.5 us with inductance. Not so bad isn't it ?
The result is a very short time and an output low impedance very good to enter a low noise amplifier.

What do you think about these ideas ?

Next time, I will present you my ideas for the Rx part !

[bbsailor]

Now I continue. I have forgotten to give the parameters I have used : L : 300 uH, C : 250 pF and Rs : 0.3. That are the values I have measured on a 11" Minelab coil I have purchased on eBay. The current at Tx off is about 1.5 A.

Usually, to limit the voltage at the preamp input, it is necessary to add a resistor and diodes like that :

[ATTACH]37708[/ATTACH]

The same but with the inductance :

[ATTACH]37709[/ATTACH]

With these components there is a decay time increase as seen on the following images :

[ATTACH]37710[/ATTACH]
[ATTACH]37711[/ATTACH]

Now we have 6.0 us without inductance and 4.4 us with inductance. Once again, the damping resistor is optimized to get the shortest decay time.
So I had the idea to add diodes serially with the damping resistor instead to have R + D in parallel with the damping resistor. Here is the result :

[ATTACH]37712[/ATTACH]
[ATTACH]37713[/ATTACH]

There is a 22 ohms resistor in parallel with the diodes to prevent oscillations. The related waveforms are the following :

[ATTACH]37714[/ATTACH]
[ATTACH]37715[/ATTACH]

Now we have 4.3 us without inductance and 3.5 us with inductance. Not so bad isn't it ?
The result is a very short time and an output low impedance very good to enter a low noise amplifier.

What do you think about these ideas ?

Next time, I will present you my ideas for the Rx part !

Phiphi,

Lower TX circuit capacitance allows a higher value of damping resistor (Rd) to be used. This has the effect of lowering coil current turn off time constant (coil L divided by Rd) to better stimulate smaller, lower TC targets. A turn off TC 5 times faster than the target TC then fully stimulated it. As an experiment try shorting out or eliminating the diode is series between the MOSFET and the coil and see what the optimum Rd value is. It should be lower because the MOSFET COSS is now in the circuit. The diode isolates most of the COSS from the TX circuit during the discharge and damping time thus allowing a higher value of Rd.

How did you measure the 250pf of the coil capacitance?

Joseph J. Rogowski

[Qiaozhi]

What do you think about these ideas ?

The reason you are getting a faster decay is because the 47uH inductor is connected in anti-phase to the TX coil. This has the effect of reducing the TX coil inductance from 300uH to 253uH. In fact, if you run a simulation with three scenarios:

1) TX with 300uH coil
2) TX with 47uH coil added
3) TX with 253uH coil

you can readily see that scenarios 2 and 3 provide substantially the same result.

You should also study this thread -> http://www.geotech1.com/forums/showt...PI-TX-Settling
as the setup in your simulation still has the coil current rising rapidly at switch-off.

[Phiphi]

How did you measure the 250pf of the coil capacitance?

Joseph J. Rogowski

I have measured the inductance and the resistance with a LCR meter. Then I have measured the self resonance frequency and, by deduction, the capacitance.

[Phiphi]

In fact, if you run a simulation with three scenarios:

1) TX with 300uH coil
2) TX with 47uH coil added
3) TX with 253uH coil

you can readily see that scenarios 2 and 3 provide substantially the same result.

I have simulated the third case with 253 uH, 510 ohms critical damping resistor and Tx width shortened to 42.2 us to start from the same current.
The voltage peak is lower (about 500V instead of 600V) and the time to reach 100 uV is 4.4 us instead of 3.8 us in case 2 so the results aren't exactly the same.
And it's impossible to reduce the inductance of a given coil but easy to add an inductance as I have done !

You don't speak about the second idea (2 diodes serially with the damping resistor) that gives, combined with the added 47 uH, a gain of 2.5 us ! (6 us with the traditional solution and 3.5 us with my improved solution.) Moreover the impedance is very low (750 ohms compared to some kohms) so it's easier to connect a low noise amplifier behind.

May I have your feeling about this improvement ?

[Sean_Goddard]

has anyone tried Active Damping, and by that I mean using a FET to "slug" the first part of the decay using severe overdamping then in the region of interest, switching to a lower damping coefficient (resistor)?

I wonder if this would provide and benefits.

[Teleno]

has anyone tried Active Damping, and by that I mean using a FET to "slug" the first part of the decay using severe overdamping then in the region of interest, switching to a lower damping coefficient (resistor)?

I wonder if this would provide and benefits.

I have done some experiments and simulations. One big problem is maintaining the optimum damping point once achieved. Balance is quite critical and it's thrown off by light inductance changes such as those caused by targets and temperature changes, which make it even worse as FETs are hard to compensate for T and unpredictable in their characteristics.

In the end this approach can't get even close to what can be achieved by using a separate, induction-balanced Rx coil.

[mickstv]

You don't speak about the second idea (2 diodes serially with the damping resistor) that gives, combined with the added 47 uH, a gain of 2.5 us ! (6 us with the traditional solution and 3.5 us with my improved solution.) Moreover the impedance is very low (750 ohms compared to some kohms) so it's easier to connect a low noise amplifier behind.

May I have your feeling about this improvement ?

One of my detectors uses a similar method to your two diodes in series with the damping resistor. The main difference is I use a 5.1v zener in series with the damping resistor and a variable resistance in parallel with the zener, set to about 90 ohms. The amp connects directly across the zener and I can sample under 8us with a 120us tx pulse.

[mickstv]

Here's a screen shot showing the output of the preamp running about 140x gain. Sample pulse actually starts just after 6us.

The coil is 291uh 0.5ohm

[Qiaozhi]

You don't speak about the second idea (2 diodes serially with the damping resistor) that gives, combined with the added 47 uH, a gain of 2.5 us ! (6 us with the traditional solution and 3.5 us with my improved solution.) Moreover the impedance is very low (750 ohms compared to some kohms) so it's easier to connect a low noise amplifier behind.

May I have your feeling about this improvement ?

Have you tried connecting it to a preamp?

[mickstv]

Here's a basic schematic of my detector frontend.

[Phiphi]

Have you tried connecting it to a preamp?

Yes but only in a simulation. Here it is :



I use a low noise but fast opamp AD797B with a gain of 10 in order to not saturate the amplifier.

Below are the waveforms at input and output of the amplifier :




The output reach 1mV (100 uV input with 10 gain) at 3.65 us after Txoff command. So it is possible to sample at less than 4 us.

Next time, I'll present my ideas for the receiver following the preamp.

[moodz]

I have done some experiments and simulations. One big problem is maintaining the optimum damping point once achieved. Balance is quite critical and it's thrown off by light inductance changes such as those caused by targets and temperature changes, which make it even worse as FETs are hard to compensate for T and unpredictable in their characteristics.

In the end this approach can't get even close to what can be achieved by using a separate, induction-balanced Rx coil.

Try Oz patent AU2013101058 the induction balance coil is an order of magnitude worse .... feedback loops rule.

[Teleno]

Try Oz patent AU2013101058 the induction balance coil is an order of magnitude worse .... feedback loops rule.

The tecniqie in that patent works best in the Tx coil of an induction balance arrangement. In monocoil it results in measuring the decay of the coil on a low R rather than the target's signal. I talked to you about that effect in another thread.

[moodz]

The tecniqie in that patent works best in the Tx coil of an induction balance arrangement. In monocoil it results in measuring the decay of the coil on a low R rather than the target's signal. I talked to you about that effect in another thread.

With respect I think your original analysis may be incorrect. It can be shown ( with this cct ) that the coil decay is proportional to the inductance and the current in the coil ie being representative of the energy stored in the H field at turn off and relatively insensitive to coil resistance. In fact its only dependant on the resonant frequency of the coil ( ie L and C ) ...
The OP was asking if an active damping technique had been tried ... this is an effective active damping technique that is insensitive to temp and mechanical coil changes ... ie damping will always track optimally.