Induction Balance Stuff - Single/Multi Frequency Response, GB, Disc, Measurements, Ideas, Fun, etc.

[Aziz]

Hi all,

now the promissed pics and proof of work. It is a messy work at the moment however.

Look, I have reached the optimum operating point.


The flyback voltage is approx. 340 V, the Vc node at approx. 14 V. I have operated the TEM2 transmitter with another battery source (6*1.2 V) during searching for the issues.

What a mess!


And the FFT spectrum of the TXref output:

Well, the SNR (signal-to-noise ratio) is approx. 75 dB. Could be better. It is still noisy.
The choke L1 on the bread board is disabled.

Cheers,
Aziz

[Aziz]

Hi all,

I'm thinking of an another KISS test board, which follows the off-resonance principle using the critical energy balanced double tuned TEM2 transmitter.
Let's call it simply Off-Resonance TEM2 transmitter.

We don't need much power to the TX coil. We don't have extremely high flyback voltages (100 V max should be enough). We don't need more battery power or battery voltage.

The LM393 should run according to the data sheet from +2 V to 36V.
How about a 2-cell battery version? This is 2*1.2 V = 2.4 V (NiMH battery) or 2*1.5 V = 3 V (standard battery)
Yes, I would like it. That's a good suggestions. Accepted.

No mosfets? Ok, accepted.
Simplified circuit? Accepted.
Still TX-current up to +100/-100 mA and above? Yes.

Ok, I'll give my best.

[Aziz]

BTW, do we have logic level mosfets? Yes!
Well, ok, in this case, I'm going to use only one mosfet. It will greatly improve performance and efficiency.

[Aziz]

Hi all,

I have not decided yet about the selection of the transmit transistor: BJT or logic level n-ch mosfet
BJT:
good: simple, fast, wide range of transistors available, operating voltage down to 2 V possible (due to LM393 comparator), I have convenient transistors at home
bad: requires relative high base current to fully switch-on, bad power efficiency, requires high voltage protection zener-diode due to thermal runaway of the BJT

Mosfet:
good: simple, fast, good power efficiency
bad: logic level below 2 Vgs(th) is probably difficult to find and I don't have any convenient at home. Minimum operating voltage could be higher than 2.4 V.

Well, power efficiency is not going to be relevant here. We are burning a lot of battery energy to limit the transmitters current either. That's the reason for, why I like to use low operating voltage.

I will look, which appropriate n-ch mosfets I can buy. Otherwise, I could even use the NPN BC337-40 transistor (max 45V, 500 mA).
Cheers

[Aziz]

Look,

the circuit simplifies a bit. This is the simple low voltage BJT version, limitting the flyback voltage below the break-down voltage of the transmit transistor (BC337-40). The TX-coil current will swing between approx. +100 and -100 mA. The TXref signal between 0 and 2.3 V. Enough to feed it into the sound card input.
Operating voltage 2*1.2 V, 43 mW power consumption, 18 mA battery current. Max flyback voltage is 40 V.

This is a nice circuit. Ready to burn out some parts?

Cheers

PS: There is no choke anymore. The heater resistor has taken the job for.

[Aziz]

Oh man!,

forgot, that the NiMH battery cells can have more than 1.2 V voltage, if they have been freshly charged. I also want to use the standard 1.5 V cell batteries.
I have to limit the flyback voltage to max. 39 V (I have found the zener diode for). The heater resistor rised up to 330 Ohms to limit the transmitter current further. Or I may increase the driving resistor to lower the base current of the transmitter transistor.

So the finaly specification for a two cell battery: operating voltage down to 1 V each cell (or min 2 V total) and max 2*1.5 V = 3 V (total).
I'm going to heat up my soldering iron today. I don't want to wait for parts to get them.

[Aziz]

Hi all,

the low power, low voltage off-resonance TEM2 is finished and is working as specified.

I had to change the diode D2 1N4148 into a Shottky diode 1N5817. This lowers the voltage drop and the transmit driver comes close to the power supply rails. This ensures operating voltages down to 2 V (I have tested the circuit with flat batteries).

Next step:
Starving the transmitter a bit of its power by increasing the R2 (The Heater). Looking at the R and X response change. R response is the resistive response caused by targets eddy current. Targets "steal" and "burn" energy out of the transmitter. R response should cause the TXref signal go down therefore. Going towards off-resonance.
X response is the ground caused response (iron oxide, ferrite, etc. ) . R response change should be much bigger than X response on the TXref channel.

Then:
Connecting a receive coil (RX) in IB configuration. The RX coil will see the X and R response changes too. But it detects more X response because the induction balance will get out of balance by so called "reactive" materials nearby the coil (ferrite, iron oxides, iron, .. ) .

We can eliminate these X responses in the RX-channel by processing both TXref and RX signal channels. And this process is called ground balance. But we don't want to lose much target signal by ground balancing. This is the reason for having the TXref channel in this off-resonance configuration.

Aziz

[Aziz]

Bingo! Time to celebrate!



I have increased the R2 upto 1 kOhm so the transmitter is starving of energy. The TXref channel got pure R-response channel now. X-response not detectable. No phase change either.
This is very good news.
Cheers

[Aziz]

If the TXref channel is out of X-response, we can fully identify the R and X response on the RX channel now. We can solve the simple linear equation.
Without losing any target response. And we have additional TXref channel for target signals.
Win-win.

I will look for logic level mosfets. And I have a bunch of missing parts. I'm also short of soldering wire. Time to spend some money for the parts.

[Aziz]

Hi all,

there are many nice logic level n-ch mosfets available. I will try different ones. A third off-resonance TEM2 transmitter test board for mosfet version will be done, if I get the mosfets.
The gate source threshold voltage Vgs(th) is specified for 1 .. 2 V. It should be enough to get them switched on with 2-cells (2.4 - 3 V). Even they don't fully switch-on. It doesn't matter.
My BJT version with BC337-40 has approx. 1 Ohm switch-on resistance at approx. 5-6 mA base current.

The mosfet version will reduce the power consumption further.
I have at the moment 3 mA for standby (no transmit signal) and 15.3 mA for full operation with two Ni-MH cells. Most of the energy is lost for switching the transistors and the heater resistor. Hater resistor? Yep, I hate it.

Let's make the low voltage, ultra low power, ground breaking and rocket-science gizmo.
Cheers

[Aziz]

This is so nice. Metal detecting on a mono coil with no X response. A big bunch of Australian hotrocks won't even be detected.
I have not connected the RX coil yet. This will give us the turbo boost.

[Aziz]

Hi all,

this is very very interesting now. Watch out, we are entering a novel break-through technology now.

I have checked the simple parallel LC resonance (TX-coil + capacitor) on the sound card output/input lines with a high series resistor to get into the off-resonance effects. On a single frequency (low and high). Well, it shows the off-resonance effects for both frequencies. But with both X and R responses, phase shifts, etc.
So it's not the same.

The way I'm using the Off-Resonance TEM2 method seems to cancel the X-Response at appropriate high series resistor (The Heater resistor) on the TXref channel.
So what's going on there?

I love my heating resistor now. My "Hero Resistor".
My RX channel is still offline.
Cheers,
Aziz

[Aziz]

Congratulations to us.

We have invented a novel method to cancel X-response.

The "Heater Resistor" R2 just moves my optimal operating point (and therefore my optimum operating frequency).
It's all about critical energy balance in the L-Ct1/Ct2 network.
So it is independent of the resistor R2.
But the operating point is critical. There is a point of level (the optimal), where all the X-responses cancel out.

I adjust it with selecting the optimum operating frequency. At the optimum operating frequency, all X-responses cancel fully out. Only R-responses remain in the TXref channel.

Cheers,
Aziz

PS: I hate my heater resistor again.

[Aziz]

Hi again,

I have checked the other harmonic frequencies of the wideband response signal (2x, 3x, .. of the fundamental operating frequency). Some harmonic responses pass through very low X-response. So I have not reached the real optimum operating point yet.
But the multi-frequency responses are all consistent with the fundamental frequency.

As it is independent of the heater resistor R2, we can push more current through the TX-coil later.
Oh yes, you probably don't know, why my resistor R2 is called the heater resistor.
When I power up my test board, it takes approx. 20 - 30 seconds to warm up this resistor. During this time, I can observe/measure the temperature rise up by the decoded response value.
It is heated so far until the temperatur of R2 becomes constant.

Aziz

[Aziz]

The Heater heats up my brain.
We need the high impedance path of the LC network to the battery source. It must be good decoupled from each other. And this is best done with a high inductance choke. No heat, no problems, high current to the TX coil.