VLF MD with digital signal processing : Bee-Buzz 1

[Aziz]

Well ok,
look where the RX source signal current is really flowing:




The RX signal current path is correct. All the RX signal current is flowing into the virtual GND. This is at the negative input of the op-amp. This is a "black hole" for currents. That's for sure.
Aziz

[moodz]

Pito,

this is the current (signal) path to GND. Depending of the sign its a differerent path.



This is what I meant with acting like a TIA (transimpedance amplifier). RX coil current to voltage output. It's still a moving coil (MC) amplifier.
Aziz

umm ...the CB configuration is often used where low voltage battery operation is required. The path to ground is not strictly correct as the CB amp has a lower input impedance than the buffer amp ( several ohms for CB amp vs approx 330 ohms for the buffer amp. In this case the least resistance path to ground is the low input impedance of the CB amp. The CB amp is low noise in part because of the low input resistance.
However connecting an RX coil to a low impedance in proximity of the TX coil allows currents to flow in the RX coil and thus it looks like a target even if it is mostly balanced ( because eddy current is flowing in it ) This "spoils" the target response of the system. Unless you dont want the worlds best detector system in which case its fine.

[Aziz]

Pito,

this is the current (signal) path to GND. Depending of the sign its a differerent path.



This is what I meant with acting like a TIA (transimpedance amplifier). RX coil current to voltage output. It's still a moving coil (MC) amplifier.
Aziz

[Aziz]

In your drawing the battery and capacitors will short the signal = so how is possible that your circuit is working (good just only on paper )?


This is a different story, you have R304 and R305

No, it won't short. The input impedance of the pre-amp is very low and thus the current through the RX coil makes the deal.
The MC pre-amp you have shown is in principle the same as my modified pre-amp circuit.
Aziz

[Aziz]

That is on paper, in real word with Rx coil connected ?
Picture of the bread board ?
not clear how Rx signal (green ) is getting to opamp

Hi pito,

RX coil will be connect to RXin and GND. The latest version has not been built. But today evening/night or tomorrow could be ready.
Note that not voltage get into the op-amp. It's the current from the first stage through the resistor R5. Negative op-amp input will be regulated to positive op-amp input and that is GND. So the current flows through R5 to (virtual) ground.
Note that the amplified signal from the first stages rides about the levels of Vpp and Vee (battery connection). This is somewhat confusing until you make the circuit simulation to understand the operation.
Aziz

[pito]

In your drawing the battery and capacitors will short the signal = so how is possible that your circuit is working (good just only on paper )?


This is a different story, you have R304 and R305

[pito]

That is on paper, in real word with Rx coil connected ?
Picture of the bread board ?
not clear how Rx signal (green ) is getting to opamp

[Aziz]

Hi all,

the is the next final pre-amp I want to use. It's a buffered version of the previous pre-amp with 6 V battery now. The NE5534 needs more voltage. The ouput driver is an inverting unity gain stage shown below. You can add more gain in the output stage if you want.



Oh well, it's only 0.228 nV/rtHz above 10 kHz. But hey, I can live with that. Really.

Cheers,
Aziz

[Aziz]

can you show PCB of it ?

I don't have a PCB. It's my old ultra-low-noise pre-amp in a bread board design. I will show you an improved new version soon, which is much more quiet, If I get it finished.
Cheers,
Aziz

[Aziz]

Until you lower the coil to the ground, then you get a huge reactive signal. And this will vary rapidly as you sweep the coil, so you can't just cancel it on-the-fly without affecting target signals. This is why companies don't strive for super-nulled coils, it doesn't matter.

Hi Carl,

yep. This effect can be observed. It is no good to the keep residual RX voltage at the noise floor level. The decoded phase is so much noisy. Decoded amplitude is stable however.
We don't need perfect IB-balancing. But a correction into the right direction and avoiding pre-amp overload is ok with active IB nulling. This is a nice feature and I am going to like it.

BTW, I like the idea of buffering the output of the pre-amp. Sound cards have often different input impedances (10k .. 20k .. how knows how much as this isn't often specified) and this affects the gain of the pre-amp.
I'm intending to add a second stage buffer/amplifier as you shown in the schematic. But in the inverting configuration with NE5534. And band limitting the pre-amp so the huge low frequency noise won't get much amplified.

If my operating frequency is above 20 kHz, I can get down to 0.112 nV/rtHz input voltage noise density with Rs=1, Gain=123, with NE 5534 (G=1). This is the ultimate ultra low noise solution. I think, I should leave that now.

Oh well, I have to make another bread board now with output driver stage (discrete or op-amp).

Cheers,
Aziz

[pito]

Hi all,

I have connected a pre-amp of gain 100 to the RX-signal path. Yep, pre-amp and almost perfect IB nulling gives the ultimate kick to the detector. I had to reduce the TX power.
.

Aziz

can you show PCB of it ?

[Carl-NC]

I have adjusted the amplitude and phase and have achieved the perfect IB. I mean really perfect as it can be done up to the noise floor level.

Until you lower the coil to the ground, then you get a huge reactive signal. And this will vary rapidly as you sweep the coil, so you can't just cancel it on-the-fly without affecting target signals. This is why companies don't strive for super-nulled coils, it doesn't matter.

[Aziz]

Hello friends,

it is now time for the new 3 V pre-amp.
If I take an RX coil with 1 Ohm series resistor and a gain of 108 (so approx. x100), I get an input voltage noise density of 0.179 nV/rtHz. You see, that the 0.2 nV/rtHz can be achieved easily now.
This will produce at 10 kHz with 200 Hz bandwidth 0.273 µV (rms) noise. The sound card noise is much higher. I wouldn't see the pre-amp noise in the FFT spectrum or the decoded values.

For a RX=2 Ohm, it rises to 0.221 nV/rtHz. Still super-duper. Noise will be 0.34 µV (rms).

If I find some spare time, I will make the 3 V pre-amp.

Cheers

[Aziz]

Hi all,

I have connected a pre-amp of gain 100 to the RX-signal path. Yep, pre-amp and almost perfect IB nulling gives the ultimate kick to the detector. I had to reduce the TX power.

Why almost perfect IB nulling? If there is almost zero RX-signal, the decoded phase isn't stable at the noise levels. We need some minimal residual RX-voltage to make the phase decoding stable.

So, we have answered the long lasting question for the pre-amp need. Yes, we need a pre-amp.
Pre-amp gain?
What you prefer. 50 - 100, 100, 100 - 500. Everything is possible with the IB nulling technique.

Aziz

[Aziz]

Hi all,

I'm going to implement auto-perfect-IB-null function sometime. Yes, it is even possible to cancel out residual IB-RX-signal for complex wave-forms like the chirp-signal or multi-frequency signals. This is, what never has been done before.

It makes great sense to do this.
1. Low TX power saves battery consumption. We don't need much TX power.

2. Cranking up pre-amp gain to compensate TX power reduction. As much as needed. Not more. Pre-amp does not consume much power. But the TX driver.

3. Easy IB-coil making.


Yeah!, the ultimate VLF/LF detector is coming soon.