VLF MD with digital signal processing : Bee-Buzz 1

[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

[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]

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

[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]

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

[Detectorist#1]

Aziz,
Maybe you go far from serous logic - to amplify signals via power supplies lines - or I'm not right? Don't trust in every cases on software simulation.

[Aziz]

Aziz,
Maybe you go far from serous logic - to amplify signals via power supplies lines - or I'm not right? Don't trust in every cases on software simulation.

This is not my idea. It's been used for ages. And my first prototype is working too. But I need the improved version.

[pito]

[Aziz]

Hi all,

ok, it's really time to make the circuit simulation by yourself and investigate the pre-amp. Below is a working spice file for download. But be careful and don't fall into the current "black hole".

Below picture shows my first real working prototype (above) and the next in preparation with the op-amp driver (below).



Cheers,
Aziz

[pito]

original = prevented from short circuit for AC

[Aziz]

Look at the circuit simulation. Make the sim and inspect all the current flows. It isn't a short circuit nor is a "black hole".
Aziz

[Aziz]

original = prevented from short circuit for AC

No, it is for defining the collector current.

[pito]

No, it is for defining the collector current.

those resistors do 2 jobs the second one is AC separation from power lines, in your drawing they are missing

this may work,
100uF for 10 kHz ? = bad idea, should be in nF

[Olly]

Those are two different vesions. The first uses a battery to float the biasing supply and the second is a powered version where a more standard power supply is used. According to the originator of the design the floating battery version has superior noise performance. He says build the first then use it while you try for years to get a powered version to work as well.

It's a very, very clever and simple bit of circuitry...

[moodz]

The power supply vee and vpp are unreferenced to ground.