Balanced VLF Tx

[ODM]

It is well suppressed in the synchronous detector MD (about nonlinearity of soil - keep silent…)

The second harmonic "culprit" meant here is generated by nonlinearities in the oscillator itself, and part of the TX coil's field, interfering with phase comparator to give non-50% duty cycle.

[Sergey_P]

interfering with phase comparator to give non-50% duty cycle.

OK

[Davor]

Exactly. A net result is as if offset is roaming wildly all over the place. Driving a Tx from a digital source gives always clean 50% duty cycle, and I expect this balanced oscillator to behave just as nice.

Aziz, nice catch.

[Aziz]

Hi all,

BTW, the circuit simulation can tell you some more facts. You just have to play with it a bit.
(Discovering, learning and "inventing" by playing and experimenting. A "Trial-and-Error-Engineering" at best. )


For instance:
If you vary the resonant frequency (by varying L and C), you can observe two facts:
- High frequency: high target response, less TX coil current (high XL and Z), higher efficiency (less battery drain).
- Low frequency: less target response, high TX coil current (low XL and Z), less efficiency (higher battery drain).
(XL: inductive reactance, Z: impedance, L: TX coil inductance, C: TX tank's resonant capacitance)

So a high TX coil current isn't all and everything. A high current isn't a good style in the electronics as the power losses becoming relevant too (P = I²*R) .

Nevertheless, I like our TX oscillator.
Cheers,
Aziz

[Davor]

A "Trial-and-Error-Engineering" at best.

Well, not in my case. I knew what I was after, and a balanced Tx is a result. You can't say component tweaking is trial and error, because there are no errors to begin with. You could calculate everything, but even there you'd have to start from some assumptions and build your calculation from there.

I'd say this is a nice platform to make standalone detectors as well. Having Tx voltage in abundance and in perfect counter phase begs for a simple and reliable no-brains I-Q phasing network. That would make it complete.


How about this one, it has a pretty accurate phase, but not amplitude ... which is not a problem for supplying a comparator:

[Aziz]

Well, not in my case. I knew what I was after, and a balanced Tx is a result. You can't say component tweaking is trial and error, because there are no errors to begin with. You could calculate everything, but even there you'd have to start from some assumptions and build your calculation from there.

I'd say this is a nice platform to make standalone detectors as well. Having Tx voltage in abundance and in perfect counter phase begs for a simple and reliable no-brains I-Q phasing network. That would make it complete.


How about this one, it has a pretty accurate phase, but not amplitude ... which is not a problem for supplying a comparator:

(Hey, that was an offer for playing with spice simulations. One can really learn a lot with it.
BTW, you would laugh at me how I came to the emitter inductors... *LOL* That was a result of playing, incident, forgetting to put the bypass capacitors at the V+/V- node, looking at different issue, ... )
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Nice, really very nice to have the balanced quadrature clock signals (after a comparator stage) for demodulating the RX like a true I/Q lock-in amplifier.

Aziz

[Davor]

Yeah, just add a LNA, a switcher and a rectifier ... you have a nice all metal detector.

[Davor]

A small step forward, a Tx with phases manipulation for GEB and Disc. With these values any comparator would have some negligible offset error, so this is just about it. See how both Disc and GEB are shifted left and right from the ideal I and Q phases by rotating the pots left or right from the centre? That's the beauty of it - it makes the most logical phases control I can think of right now. Red trace is a "normalised" Tx signal for reference, blue traces are GEB, and grey traces are Disc.

Boy! I love this oscillator.

[Carl-NC]

OK, now it's time for "further comment."

I have been working intensely for most of this year on some new transmit circuit designs. I worked from two angles: VLF, starting with the very same circuit Davor posted; and PI, starting with the very efficient Fisher Impulse circuit. I have developed a variety of (what I think are) novel methods that bridge both VLF and PI/TD. In fact, my end goal was a TX circuit that could be run-time configured to one of several radically different TX modes.

I reached my end goal a few months ago and began writing an extensive patent application. As happens so often I kept exploring more new ideas and putting off the patent app so I could include more stuff. Davor's post of this circuit prompted me to get the durn thing finished before someone figured out the really cool things this circuit is capable of.

Ergo, I got the app finished and filed. Obviously, the basic cross-coupled TX circuit will not be part of any claims and, to be honest, I considered it to be known prior art anyway and had no intention of claiming it. But novel derivatives of it are claimed, and the basic circuit is in the body as prior art.

I wanted to post this disclaimer so that when the patent is eventually made public certain people don't explode with claims of IP theft.

And, no, I had never explored the supply inductance twist, nor some of the other angles mentioned here.

- Carl

[Aziz]

Hi Davor,

good job!

I personally don't need the I and Q signals as they can be extracted digitally (software) from the TX reference signal.
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Hi Carl,

we expect fair play. Otherwise you will be tarred and feathered. *LOL*

Cheers,
Aziz

[Davor]

Isn't the patent system patently absurd?

IMHO there is nothing to steal here. Maybe this particular oscillator that I designed here has some unique features over the prior art, I don't know - I can't be bothered to examine that in detail. As far as I'm concerned all uniqueness of this circuit is a free contribution from me to the public at large. It is published here at this forum, and it can be syndicated anywhere else now.

Almost two months ago I kind of announced that I recognise unbalanced regime of the Tx as a problem, especially for unshielded coils, see http://www.geotech1.com/forums/showt...417#post156417
So this is my answer to that problem, and frankly, I'm quite happy about it. And I intend to use it.

I think there are many derivatives that can be produced from a balanced sine voltage source. It is the next best thing to the floating source, and a very clean from harmonics too. Only I fail to distinguish those as non-obvious. It seem patent examiners do not share my vision though.

E.g. an obvious application of this source would be applying the Haven's technique to obtain perfect quadrature (where needed), as per the attached picture. From left to right you have a semi-quadrature source, a pair of limiters that are not needed if the amplitude is kept reasonably equal (see picture). It is followed with a pair of summing and limiting devices that are safely replaced by comparators - summing and limiting action happen at zero crossing. You may observe v1 as I+ on my schematic above, and v2, -v2 as Q+ and Q-. So all you need are two comparators hooked (Q+, I+) and (Q-, I+), and voilà, perfect quadrature is on their outputs. Same as vout1 and vout2 below.

[Davor]

I've plotted the GEB control voltage phase V(Q+,PotI) against the I+ phase V(I+), and here is the result ... derived in spice as phase of V(q+,poti)/V(i+). Blue trace is the Q+ phase by itself (PotI at 50%). I'd say it is very nice for a circuit of this level of complexity.

[Carl-NC]

Isn't the patent system patently absurd?

I'm not a big fan of patents, but we're being forced to play the game.

One thing about VLF is that perfect quadrature isn't needed. In fact, non-90 quadrature has advantages.

[Davor]

Yeah, I know, the Haven's principle is just for the academic point of view.

As shown in the post #128 both GEB and Disc phases can be set independently, and the beauty of doing it this way is that no active phase shifters are needed, the setting is linear within reason, and can work nicely in a large frequency span.

So my future VLF rig will have very few components on the board

[Aziz]

More Magic

Hello friends,

you will find my new kiss-masterpiece. *LOL* (I like to exaggerate, it is probably invented already, but in case it isn't, it is now. )
You will find two different solutions. Either Vcc or GND referenced. Use whatever you prefer.
Notice, there are only two transistors required (cool switching = low loss, low power). And one magic emitter inductor.

The TX coil is a center-tapped coil. This oscillator is ultra high efficient and gives ultimate bang to the targets. It works even at lowest voltages. You can increase the supply voltage, if you use high voltage rated transistors.

Enjoy.


PNP-Variant:


NPN-Variant:


Cheers,
Aziz