Ground Balance Theory

[Davor]

I don't think Aziz meant anything bad on your account. It is a matter of unfortunate word positioning that makes it sound bad.

[Ferric Toes]

Still doubting my capability and potential?

Yeah, I like the doubters, denigrators, forum clowns and trolls... They are motivating me to break the limits..

Aziz

Science is all about asking questions and getting answers that stand up to testing in the real world. To state that you have the world's best GB you have to prove that it balances on real magnetically mineralised ground. Otherwise all you have is the WBTGB (T being theoretical). If you have done this testing with physical electronics, then that is fine. If not then you have WTG before WBGB. I have seen many good paper designs, mine included, which have fallen at the first hurdle as a result of some factor that wasn't, or couldn't be, considered in the design stage. I do not doubt your capability and potential but you must expect that statements you make will be questioned, particularly by others in the detector design business, as at the moment there is a scarcity of detail as to how this can work.

Denigrators, clowns, and trolls are not fitting descriptions for those whose criticism is constructive.

Eric.

[Aziz]

In a field tested detector?

Eric.

Still doubting my capability and potential?

Yeah, I like the doubters, denigrators, forum clowns and trolls... They are motivating me to break the limits..

Aziz

[Davor]

Never used a transformer as front end. The only thing I have tried is separate RX with higher turns, and centre tapped bifilar using half as TX. What would be the difference?

Transformers are happily performing as BALUN devices, and by means of impedance transformation have a great potential of improving input S/N. You know that one of my objectives is using shieldless coils, and transformers are happily attenuating common mode signals if you ask them politely.

I am considering reconfiguration of Rx topology a bit. I'd omit a preamp, and use a transformer instead, just to improve S/N. Then follows sampling/switching, but in a chopper amplifier configuration. So instead of a baseband/integrator/LPF it will be a kind of intermediate frequency amplifier that will take care of the most of the amplification here. Integration/LPF comes next, etc.

I realised that the remaining problem with nowadays rigs (after everything else is fixed) comes as 1/f noise. Professional rigs fix this problem using a high resolution A/D, thus avoiding 1/f fluctuations, but in full honesty the chopper amplifier will do the same job with a fraction of price and power consumption, using only garden variety components.

I already made some drawings and presented the idea at LF topic, but point is that the same topology is equally valid for PI as well.

There is an appealing feature of this approach in a form of sample-to-sample differential amplification. Say you take two samples after each pulse, A and B. If you SAH and differentiate a stream of A's separate from stream of B's, you are losing the static component, and you can apply much more gain. This results in a motion compensated detection, but HPF cutoff can be very low, as there is no 1/f trouble in amplification chain. Sub 0.1Hz seem feasible.

[Ferric Toes]

BTW, the real WBGB does not have a response hole at all and it is already existing.

In a field tested detector?

Eric.

[Ferric Toes]

So actually this whole unholy business of who stolen whose (expired) IP is actually off the imaginary table now. Ground balance in every case is a mere ratiometric primary school algebra, and by no means some obscure formulas or silly Tx timing play. The big deal from now on will be obtaining target information with best S/N approach.

So how about eliminating EF without reverting to extremely late sampling, as it clearly does not bode well in S/N calculations.

OT: Eric, have you ever tried using a transformer in a front end instead of an active preamp? The idea becomes increasingly appealing to me.

EF elimination is not a problem and it is at least as good as with "normal" PI. However as a preference I would use bipolar pulsing as EF does not rely on a dedicated late sample.

Never used a transformer as front end. The only thing I have tried is separate RX with higher turns, and centre tapped bifilar using half as TX. What would be the difference?

Eric.

[Aziz]

...
One aspect of the constancy of the SPM decay is that it is possible to devise a GB system where every thing else can be changed (TX pulse width, delay etc) while still maintaining GB, with the consequence the response holes can be sequentially time shifted and eliminated. The real WBGB.

Eric.

Hi Eric,

you very likely have seen the recently demonstrated response hole for a three sample GB scheme on the other 1A forum.
I think we need either more parallel GB schemes at the same time or sequentially different GB schemes to overcome the response hole problem.
BTW, the real WBGB does not have a response hole at all and it is already existing.

Cheers,
Aziz

[Davor]

So actually this whole unholy business of who stolen whose (expired) IP is actually off the imaginary table now. Ground balance in every case is a mere ratiometric primary school algebra, and by no means some obscure formulas or silly Tx timing play. The big deal from now on will be obtaining target information with best S/N approach.

So how about eliminating EF without reverting to extremely late sampling, as it clearly does not bode well in S/N calculations.

OT: Eric, have you ever tried using a transformer in a front end instead of an active preamp? The idea becomes increasingly appealing to me.

[Ferric Toes]

I have just purchased some 20cc sample pots that fit the Viscosity Meter chamber (standard pot is 10cc). That way I can measure double the quantity of a soil sample and get twice the signal. This helps with soil samples that have a low susceptibility/viscosity index as it doubles the S/N at late times. 10gm of Red Hill soil give a viscosity reading of 140, while 20gm of the same give a reading of 272. This slightly less than double result is probably because the additional height of the 20cc container takes the top of the sample outside of the uniform field pattern inside the solenoid coil. It was good to see that the decay exponents were similar -1.041 for 20gm and -1.040 for 10gm. The 3rd decimal place fluctuates +/- 1 digit anyway due to pickup noise. A future project is a noise cancelling sensor with a longer solenoid chamber.

One aspect of the constancy of the SPM decay is that it is possible to devise a GB system where every thing else can be changed (TX pulse width, delay etc) while still maintaining GB, with the consequence the response holes can be sequentially time shifted and eliminated. The real WBGB.

Eric.

[Ferric Toes]

My intention is not to dissolve this thread by posting pointless stuff, this well may go to off-topic or somewhere else, not directly related to ground balance, but indirectly can be.


Sorry for hand writing, this is from old scrapbook, i'm not too good for making documentation, but can be interesting. Built long time ago just for waveform observation, now something similar rebuilt to take some measurements, very interesting for experimentation.


Circuit TX part is based on constant peak current control of fixed inductance coil, turning it off when preset value is reached. Now just varying drive voltage, pulse width can be varied over very wide range, 50:1 or 100:1 is easily achievable, keeping energy stored in the coil and magnetic field during this time constant. Energy is 1\2 L x I sq. considering I is fixed, resultant pulse width needed to ramp up to predetermined value is I\ (1\L) x Vcoil. Circuit is built using UC3844 SMPS chip, driving power mosfet or IGBT. R\C constant at pin 4 sets operating freq. around 4.2kHz with given component values, but adjustable over wide range. Pin 3 is current sense input, with precision 1V comparator, sensing voltage across resistor from mosfet source to gnd to monitor peak current. Without coil or coil voltage applied, will produce 50% square wave drive, but when coil current ramp up and pin 3 voltage reaches 1V cycle is terminated. In this case, 0,5R resistance gives 2A peak.


This on picture is “high voltage” version built time ago, present setup is bit different, intended for lower voltage operation for convenience. Coil is 8in speaker wire flat spiral, about 160uH, 25 turns, connected using ordinary wire (same as coil is made of) not coax, but it consists of two identical halves, strongly coupled. Very convenient, RX side and dumping resistor can be galvanically insulated otherwise everything have to be floated on high voltage side. Value of dumping resistor must be adjusted for each pulse width, so this is still point by point method (some sort of dumping network will be needed, not resistor). With this values, 1uS is reached at somewhat above 110V, 3uS around 36V, 50uS at only few volts (i'm running out of PSUs). UC3843 is needed for 12V operation, original HV circuit was built using rectified tube filament voltage, and variac\HV rectifier to supply coil voltage (not good, ripple voltage can interfere with integrated measurements). Separating TX from rest of the circuit, just to contain damage in case of “big bang”.


I assembled another similar device, but for differential measurement, based on SG3525 chip and two coils from flat spiral, to generate one fixed long, and another variable pulse, to measure just difference between them. Can be connected for unipolar or bipolar pulsing, can be used for actual detector TX. Not finished timing and integrating part yet. Want to try another modification, “transformer coil” multifilar, say 5:1 turns ratio, most probably UTP, giving large inductance ratio to achieve two different pulse widths using single battery voltage. Naturally, field straight is lower for less turns, but fixed and can be compensated later. I wish if I have more time for this...

The high voltage rig is something that I would like to play with, being brought up in the tube era. I still have some 300V psu's for my vintage radio power. I also have a 6800uF 385V working electrolytic capacitor that is looking for a job. However, it is probably best to avoid potentially dangerous experiments now and stick to 24V at most. I will try the current limiting driver at some stage as an alternative to my existing current limiter which works on the average current drawn by the TX circuit.

I have put a damping resistor across the shield gap as a way of damping at low voltage.

Eric.

[Ferric Toes]

Eric,
A couple points from the VCA810 data sheet.

When the control voltage reaches -1.5v you reach the the "Teeple point". Never heard of this term before. From 0 to -1.5v you have smooth gain control. Less than -1.5v,internal high gain amps kick in and create a bump in the response curve. I am thinking of limiting my control to 0 to -1.5v to avoid this anomaly. Not sure if this will defeat the important part of the log response?

Another thing I found interesting. When the control voltage is greater than Vref(0v) the amp becomes an autenuator. Maybe this could be used to our benifit? For example during Tx On and early flyback?

Mark

Hi Mark,
The route that I will take is to have a linear amp with a small amount of gain, say x20, which is followed by a gate which is off during the TX period, and then on for all the read period up till the next TX. The gated signal will then be applied to the log amp input. This removes all the TX transients from the log circuit.

Eric.

[Altra]

Eric,
A couple points from the VCA810 data sheet.

When the control voltage reaches -1.5v you reach the the "Teeple point". Never heard of this term before. From 0 to -1.5v you have smooth gain control. Less than -1.5v,internal high gain amps kick in and create a bump in the response curve. I am thinking of limiting my control to 0 to -1.5v to avoid this anomaly. Not sure if this will defeat the important part of the log response?

Another thing I found interesting. When the control voltage is greater than Vref(0v) the amp becomes an autenuator. Maybe this could be used to our benifit? For example during Tx On and early flyback?

Mark

[Ferric Toes]

Well, I have just found my long lost VCA610's, so let the log amp investigation begin. I see that the data sheet says that the log application is temperature compensated, 250kHz bandwidth and the device itself is low noise (2.2nV rt Hz). I remember once trying one of them as a gain controlled front end in a PI. Seemed to work OK, and BW in linear mode is flat to 15MHz at 40db gain if you want it.

Eric.

[Kev]

The other benefit of a log amplifier is that it compresses the target signal right at the front end and reduces the large range of signal that has to be processed by the following circuitry.

Eric.

Hi Eric,
I'm really enjoying your contributions to this thread.
I wish I could fully understand the implications you are hinting at, I do though see the possible benefits to digitising early time samples with the log reduced dynamic range.

Thank you for sharing your results with us!
Best Regards
Kev.

[Tepco]

Breaking the math

Hi guys,

you are getting huge complexity with the log-amp re the GB issue.
Try to solve the integral of the log(signal responses):
Integral log (Exponential induction decay + Magnetic induction decay + Const signal) dt =
Integral ( log( A*e^(-t/"TC") + B/t + C) ) dt =

(If you are doing the integration windows as usual in a PI configuration of course.)

A

OK, let's mess up mathematics even more. Inspired by VCA810 chip, capable of doing many unusual things. Maybe not best for front-end log-amp, but... Using it later, in integrator stage can produce integrator gain and time constant variable over sampling period. Considering linear or exponential ramp voltage is already present in RC circuit determining sample time, using it to modify integrator parameters over that time is easy. What math will look like then?
(Posting something truly stupid here is not forbidden by forum rules, but think twice about this proposal. May sound like reinventing analog computing in 21^st century, only for purpose)