AMX TX

[Carl-NC]

...which is kind of against the requirement of a cheap detector for artinsal gold fossicking

"Cheap" was not a stated requirement, Tony's comment was that it should not cost more than an ounce of gold. Keep in mind that these markets are currently paying Minelab prices so even a detector priced at 2 ounces might sell well. But for an $1800 (USD) detector you want a total production cost of $450, or 25%, for decent margins. A $100 PCB would easily fit this requirement. A direct-sampling approach, even with a $30 ADC and a $10 micro, is still quite cheap, no doubt cheaper than a TDI board was to make.

That said, I'm very interested in learning more about your scheme.

[moodz]

This is the description of initial conditions and results of the second simulation in POST #45 (typical but powerful CC-based PI system).

• XMIT Coil dia 8”, 50 turns resistance 1 ohm, Inductance 850µH
• RCV Coil dia 4” resistance 2 ohm, Inductance 300µH
• MOSFET VDS : 800V RDSON : 360mOhm (not good)
• XMIT battery voltage : 2.45V
• Pulse period 200µsec = 5Kpps, two half-periods of 100µsec = 10K decays / sec
• Differential Receive chain : gain 50x to ADC input

Results

• Coil Current : +1Amp to –1Amp = delta 2 Amp.
• 2 Amp x 50 turns = 100 Amp.turn
• Flyback Voltage : 900V
• Resonant Frequency : 160KHz, Flyback Width : 3µsec
• Battery Power Consumption : 4.7W
• Pulse Delay : 1.2µsec

BUT

• Signal Offset at ADC input : + 2.4V to –2.4V
  • If more receive gain than 50x, Saturation
• XMIT Coil Current Ramp : 33mA over 100µsec
• RCV Voltage Ramp : 380mV

THUS,

• Needs for Automatic Compensation of energy losses.
• This keeps the XMIT coil current CONSTANT
• Dramatically reduces the signal OFFSET at the ADC level

--> This woiuld allow for a much higher amplification gain without saturation.

...getting back on track. One way of adjusting for losses is to boost the loss with a parallel ride on inductor inside the detetor with a variable resistance.



By varying R3 the "ramp" of the CC pulses in L1 will be adjusted. Of course this doubles the power but this particular circuit with 1.2 volts has 500 volt flybacks and uses 1 watt.

This will only be good to adjust for certain types of losses and only to a certain level.

moodz

[moodz]

I haven't built anything because these ideas were triggered by Carl's post #52 from two days ago.

I expect that the TX energy would need to vary by a tiny fraction since the point of equilbrium is so sensitive. I might be wrong though. Too early to say but I'm certainly planning on building this.



Here's an example:



Draft22.1.zip

My concern from Carl's post #52 was that the AMX design is specifying high speed sampling ADCs and complex dsp schemes ( which is kind of against the requirement of a cheap detector for artinsal gold fossicking ) whereas if you take the basic idea of #52 combined with the bipolar TX but OC / open circuit the RX coil during TX flybacks ( target charge times ) then using the long L/R of a shorted coil RX times you have plenty of time to sample small ( and big ) targets.

There is absolutely no need for high speed ADC / DSP.

I do have a ZP design built up and it has about 20 bits worth of sampling integration and gain in the front end and 12 bits of "real" ADC in the back end to give around 30 bits of real sensitivity. It damps in around 1.4 microseconds and can detect 0.1gram nuggets with ease.

I am going to post the schematics ... soon ... documentation has not been tracking hardware

moodz

[Teleno]

Have you built a real circuit ? ... I put in some extra features to control the TX energy and what was noticed in practice is that this caused muting of target sensitivity.

moodz.

I haven't built anything because these ideas were triggered by Carl's post #52 from two days ago.

I expect that the TX energy would need to vary by a tiny fraction since the point of equilbrium is so sensitive. I might be wrong though. Too early to say but I'm certainly planning on building this.

And if desired, a decaying signal with time constant (R1 + Rdson + R3) / L, generated by an RC circuit synchronized to the TX pulse, can be subtracted at the preamp to refer the target signals to a zero baseline.

Here's an example:



Draft22.1.zip

[moodz]

Yes it's similar, however it has elements that make the adjustment less critical:
- constant energy to drive the coil (fixed charge in C1 independent of Vcc)
- Rdamp to preadjust the discharge (speed not a concern because of the spreading in time of the shorted coil's response)
- a source resistance to bias the depletion mosfet according to the discharge current (soft transitions)
- coil ends up connected to a virtual ground.

A feedback loop only has to slightly vary the energy in C1 (by changing Vref) in order to compensate for ground effects and/or temperature variations.

And if desired, a decaying signal with time constant (R1 + Rdson + R3) / L, generated by an RC circuit synchronized to the TX pulse, can be subtracted at the preamp to refer the target signals to a zero baseline.

Have you built a real circuit ? ... I put in some extra features to control the TX energy and what was noticed in practice is that this caused muting of target sensitivity.

moodz.

[moodz]

Here is the LTSPICE sim for the previous post ... ZPDAMPER.zip

moodz.

[Teleno]

Interesting use of the depletion FET ... this scheme has some to my ZP damping scheme.

Yes it's similar, however it has elements that make the adjustment less critical:
- constant energy to drive the coil (fixed charge in C1 independent of Vcc)
- Rdamp to preadjust the discharge (speed not a concern because of the spreading in time of the shorted coil's response)
- a source resistance to bias the depletion mosfet according to the discharge current (soft transitions)
- coil ends up connected to a virtual ground.

A feedback loop only has to slightly vary the energy in C1 (by changing Vref) in order to compensate for ground effects and/or temperature variations.

And if desired, a decaying signal with time constant (R1 + Rdson + R3) / L, generated by an RC circuit synchronized to the TX pulse, can be subtracted at the preamp to refer the target signals to a zero baseline.

[moodz]

Here is a damping scheme ... based on the original patent ... it works whether you short the coil or not.

The current sink current value has to be set to the integral of the TX coil current over the flyback time ( approx 1 us ) ... or approx half peak coil current.

In a real circuit a feedback loop will adjust the value of the current sink.

The "target stretching" simply works because an open circuit coil cannot store energy and has a very low TC ... a short circuit coil does store energy and has a long TC ... the timing of when you switch the coil from OC to SC that matters.
You have to OC the RX coil during flyback current events ( ie target storing energy from TX ) and SC the RX coil during constant TX current events ( including zero current ) .... ( target releasing energy to RX)

The other important thing to remember that a current sink / source has a high source impedance ( an ideal current source has impedance of infinity ) so when its connected across the coil the coil sees a high impedance.

The diode across the current source stops it going bananas when S3 turns off.

S1 is the TX switch ... S3 switches in the current sink during flyback ... S2 shorts the coil ( not necessary for damping function )

[moodz]

Because you need to dissipate the energy of the B-field before shorting the coil (in a monocoil) or balance it out in a Tx/Rx combo. In other words, the coil has to be shorted when the current approaches zero. Only then the target energy becomes visible.

Here's a concept circuit. Rdamp in parallel with the depletion mosfet (at high Rds with Vgs < 0 due to the current in R3) dissipate all the energy, then as I(L1) approaches zero the depletion mosfet is at Vgs ~ 0 and the coi sees a resistance (R1 + Rdson + R3) ~ 15 ohm.

The blue trace is the response to a 1us taget, the green trace is a 10us target, the red trace is no target. The responses are extended far beyond the time constant of the targets.





Draft22.zip

Interesting use of the depletion FET ... this scheme has some similarities to my ZP damping scheme.

[Carl-NC]

“I have had my results for a long time, but I do not yet know how I am to arrive at them” — Carl Friedrich Gauss

[Teleno]

I agree, in a current mode coil the current is
In other words, the current is exactly the same waveform as the incoming B-field. You simply lose the derivative. I've built a Magnetic Field Probe using this concept and it works exactly as the math suggests. I don't see any targets getting stretched out in time.

Because you need to dissipate the energy of the B-field before shorting the coil (in a monocoil) or balance it out in a Tx/Rx combo. In other words, the coil has to be shorted when the current approaches zero. Only then the target energy becomes visible.

Here's a concept circuit. Rdamp in parallel with the depletion mosfet (at high Rds with Vgs < 0 due to the current in R3) dissipate all the energy, then as I(L1) approaches zero the depletion mosfet is at Vgs ~ 0 and the coi sees a resistance (R1 + Rdson + R3) ~ 15 ohm.

The blue trace is the response to a 1us taget, the green trace is a 10us target, the red trace is no target. The responses are extended far beyond the time constant of the targets.





Draft22.zip

[ivconic]

[Carl-NC]

I agree, in a current mode coil the current is



but ε(t) is -N*A*dB/dt where N=turns and A=coil area. Therefore the current becomes



In other words, the current is exactly the same waveform as the incoming B-field. You simply lose the derivative. I've built a Magnetic Field Probe using this concept and it works exactly as the math suggests. I don't see any targets getting stretched out in time.

[Teleno]

I don't understand this. Even with a current-mode RX coil the turn-on response should be instantaneous, not an integrated response.

In an ideal shorted coil (zero resistance) the current is the integral of the EMF divided by L

The step response is not instantaneous but a linear ramp of slope 1/L. A pulse response is a ramp ending at a constant current plateau... to t = infinity.

Now if we take into account the coil's resistance the ramps turn into exponentals, the plateau current decays at a rate R/L. We don't have infinite time to measure but successive samples along the slow decaying R/L exponential can be added to improve the S/N ratio. Integrate the leaky integral.

The tau of a target determines the position in time of the peak Rx current.

[moodz]

Both of the following waveforms can be considered "constant current" because the pulses have constant current amplitudes:



Maybe the former should be CCC-PI: continuous constant current.

we could call it bipolar slightly ramped almost constant current BSRACC-PI



I don't understand this. Even with a current-mode RX coil the turn-on response should be instantaneous, not an integrated response. In the past I have intentionally slowed down the RX preamp which can drag super-fast targets out in time, allowing you to see something that otherwise may be too fast. But the no-free-lunch scenario applies: stretching out the target response also weakens its amplitude.

It takes a bit of thinking to unpack it ... the target stimulation during flyback proceeds at the rate of di/dt in the coil during flyback .... but if the TX coil current falls to zero after the flyback period and is "shorted" .... the target is now the transmitter and the tx coil ( ie our detector coil ) is the target if you get what i mean.

see pic below ... the target current rises during the TX flyback and then decays in about 5 us ( L/R = 1us ) .... the TX coil stimulation drops to 0 in a much longer time ( more than 30 us ) .... ie the stimulation lasts much longer than the target decay. !!

its no magic during receive its as if we were using the target as the detector coil and the 300uH 0.5R is the target ... simple reciprocity in physics. Because we damped the coil to "zero" the target energises our coil to some value .... and it will decay at its own L/R not the targets !! ( remember the damping system is not connected at during this time )

in that pic below the target integration would be over 30 usecs !! ( for a target with a time constant of 1 usec ).

moodz