Balanced VLF Tx

[WM6]

f0 = 1/(2*3.1415*sqrt(300µH*587.545nF)) = 11,987.8 Hz
Yep, that is the correct value.

Aziz

So calculated period duration is 83,42us.

[Aziz]

Hi all,

I'm yet missing the guru's yippie ya yeah gangnam dance of this great oscillator "invention".

(Ok, you need more time to recognize the genuine magic. )
(In fact, I have found it by pure chance - there hasn't been any IQ involved. *LOL* )

Enough jokes.

BTW, the oscillator (not the amplifier part of course) is working down to 1.5 V supply voltage (+/- 0.75V). At this supply voltage, the TX coil current achieves almost 190 mApp in the showed coil configuration.
One can operate the oscillator even from a reference voltage generator. It must deliver enough current for this purpose however. Example: Vref=2.5V, Iref=5 mA (required oscillator current for the coil configuration), TX-coil current: 330 mApp. Not that bad.

The laptop/tablet/netbook sound card project can even be powered by the USB interface (5V @100 mA). You need a different op-amp which is working at +/-2.5V.

Cheers,
Aziz

[Davor]

Isn't USB limited to 1A per socket? Anyway, 100mA is a true overkill for this kind of project.

If not for a real MD for field purposes, this kind of setup may as well serve as a learning polygon, algorithm testing, and a coil comparison one.

[Aziz]

Isn't USB limited to 1A per socket? Anyway, 100mA is a true overkill for this kind of project.

If not for a real MD for field purposes, this kind of setup may as well serve as a learning polygon, algorithm testing, and a coil comparison one.

The 100 mA current limit is available without the software negotiation with the USB host. 100 mA is still an overkill.
A good working USB powered controller (@5V) would require approx. 15 mA (including everything). And the TX coil current would still have 600-700 mApp. That's a nice idea & toy to start with the USB VLF controller.

Aziz

[Aziz]

*BLING* *BLING* *GRIN* (my brain is working right now.. *LOL* )

I've forgot to mention, why I don't need a current source/sink, power control of the magnificient TX oscillator and supply voltage regulation (diminishing battery voltage, etc.).

Answer: I am tracking and processing the TX reference voltage too.

That is one of the excellent possibilities to make complex things easy with software coding.
Any modulation of the TX reference can be detected and compensated by the software.
The demodulation of the RX is based on the demodulation of the TX reference of course.

That's going to be a very very ultra high efficient VLF detector controller.

And I've forgot the USB power issue in previous post:
The virtual ground (splitted voltage point), which is connected to the sound card input ground is colliding with the USB ground, which actually would short the virtual ground and USB GND (=Vee). We need additionally a negative voltage or we can even exploit the LC oscillator to get a negative/symmetrical power supply for the amplifier. As the TX reference voltage is tracked and processed, it wouldn't have a negative effect in this case.

Cheers,
Aziz

[Sergey_P]

I've forgot to mention, why I don't need a current source/sink

Parallel circuit feeds current rather than voltage…

Oscillator changes its parameters under the influence of feedback from the ground ,targets - a mistake. We lose the ability to separate signal ground and targets ...

[Aziz]

Parallel circuit feeds current rather than voltage…

Oscillator changes its parameters under the influence of feedback from the ground ,targets - a mistake. We lose the ability to separate signal ground and targets ...

No. *LOL*

It's going to be a K.I.S.S.*-masterpiece. The Ultimate VLF Detector. *LOL*

* K.I.S.S.: (from Wikipedia)

Keep it simple, stupid.
Keep it small and simple
Keep it sweet and simple
Keep it simple and straightforward
Keep it short and simple
Keep it simple and smart
Keep it strictly simple
Keep it speckless and sane
Keep it sober and significant
Keep it simple and stupid
Keep it safe and sound

Aziz

[Davor]

Sergey, you may observe a Tx signal as a reference and build conclusions from there. Ground proximity will change tank parameters for sure, but will that influence the rest of the rig ... I'd say not very likely in case of a competent design. Let's see what changes we may expect.

Tx amplitude ... not a problem. VLF IB is capable of handling signals in a wild span of several decades of amplitude, working mainly on phase comparison, so a dB or two of amplitude change is of minor concern.

Phase shifters ... depends on a type of a shifter and a signal source. A delay type shifter will compensate for a phase shift in a wide frequency span, provided there is no wandering 2nd harmonic in a Tx signal, so these are OK - there is no 2nd harmonic in a balanced oscillator. A direct phase shift, as with PLL does not compensate. For a constant frequency rig such as Verator this is a non-issue.

Rx front end ... highly dependent on many things. Sure bet is an aperiodic front end where Rx tank Q<0.5, such as in Aziz' solution, Tejon etc. Quasy resonant solution is OK with a free running Tx oscillator, but critical with constant frequency Tx source. Nexus=resonant Rx and resonant Tx, both cancel each other shifts and live happily ever after.


Don't get me wrong, a constant frequency Tx source is a fine solution, provided you fix your Rx front end properly. Digital source by it's symmetry is free from 2nd harmonic, and if a coil is sourced in a bridge it becomes balanced. Quadrature phases for Rx can also be sourced digitally. However, I'm itching to see a free running balanced Tx oscillator in Verator, as apparently it does not depend on the digital part at all. Verator hides some extra jewels yet to be discovered by the wider audience, and I'd be glad to see some videos of it in action.

[Sergey_P]

Step 1 - measurement of signals from ground, targets
Step 2 – subtract, compensate ground
Step 3 - determine the parameters of the target
…
Oscillator - a tape measure made of rubber (Step 1 – bug)

[Davor]

sorry, no idea what you wish to say

You could also put it in Russian, and add some kind of translation to it so that Qiaozhi does not scream "ENGLISH!!!". Too short aphorisms do not make much sense.

[Aziz]

Hey guys,

let's start with facing full of problems. If ain't work, we can make the circuit more complex later (a regulated oscillator using a voltage controlled current sink/source).

BTW, the base resistors Rb at the transistors can be increased to 10k .. 22k. This will improve the power efficiency further. With Rb=4.7k (approx. 17 mW each), they get warmer than the transistors (approx. 2-3 mW each).
The most energy will be burnt in the TX coils resistance (~200 mW). Oh yes, the power splitter resistors eating ~35 mW each.

I remember, if I use a receive resonant LC tank, I can even leave the amplifier and the power supply splitter. The signal is strong enough to be above the noise floor level. So connecting the coil directly to the sound card input would suffice too. And everything gets easier too.

Cheers,
Aziz

[Sergey_P]

sequence of signal processing in the MD

[Aziz]

sequence of signal processing in the MD

Sergey, you have to know, that I'm processing all the signals in digital domain (using DSP algorithms).
There are many many ways to combine.
Aziz

[Davor]

Oscillator - a tape measure made of rubber (Step 1 – bug)

This part is a complete mystery to me. I have no idea what you meant. The rest is clear to both of us.

[ODM]

WM6: I was referring to scavenged toroid cores for chokes. Some manufacturers like to use conflicting colors, and then there's some manufacturers that seem to just use whatever paint was available at the time