VLF MD with digital signal processing : Bee-Buzz 1

[Aziz]

Hi all,

I have revisited the Richard Lee's pre-amp and have achieved an input voltage noise density down to 0.18 nV/rtHz. Pure discrete solution operated at 3 V supply voltage with output buffer.
But!
The input impedance of the pre-amp gets down to effectively almost 1 Ohm (actually 0.96 Ohm).
The RX-coil is practically shorted to ground.
I have never thought, that it is so much low.

Even the circuit simulation get these values:
Rin = VT/IE,
Rin = input impedance
VT = thermal voltage (= 26 mV)
IE = emitter current = 13.5 mA (taken out of the circuit simulation)
Rin=26 mV/13.5 mA = 1.93 Ohm for single transistor, but I have two, Rin=1.93 Ohm || 1.93 Ohm = 0.96 Ohm.
see https://en.wikipedia.org/wiki/Common_base

This is no good. All the RX-coil sensitivity gets lost. But the pre-amp is the quietest pre-amp I have ever built.

Next time, I will try the other pre-amp with much higher input impedance (10 kOhm). This will take some time as I have to travel to Berlin this week.
Cheers,
Aziz

[moodz]

Hi all,

I have revisited the Richard Lee's pre-amp and have achieved an input voltage noise density down to 0.18 nV/rtHz. Pure discrete solution operated at 3 V supply voltage with output buffer.
But!
The input impedance of the pre-amp gets down to effectively almost 1 Ohm (actually 0.96 Ohm).
The RX-coil is practically shorted to ground.
I have never thought, that it is so much low.

Even the circuit simulation get these values:
Rin = VT/IE,
Rin = input impedance
VT = thermal voltage (= 26 mV)
IE = emitter current = 13.5 mA (taken out of the circuit simulation)
Rin=26 mV/13.5 mA = 1.93 Ohm for single transistor, but I have two, Rin=1.93 Ohm || 1.93 Ohm = 0.96 Ohm.
see https://en.wikipedia.org/wiki/Common_base

This is no good. All the RX-coil sensitivity gets lost. But the pre-amp is the quietest pre-amp I have ever built.

Next time, I will try the other pre-amp with much higher input impedance (10 kOhm). This will take some time as I have to travel to Berlin this week.
Cheers,
Aziz

An ideal current amplifier will have an input impedance of zero ohms. A well designed current amplifier will have a input impedance well below your 1 ohms.
You are designing a voltage amplifier with a low input impedance ( which is counter intuitive and possibly wrong ).

And theres no free lunch because maximum power transfer occurs at source impedance = load impedance.

Its an interesting question ... should the coil amplifier be a current amplifier or a voltage amplifier ?
Until you answer that question .. then you can move to designing the amplfier.

[Carl-NC]

The RX-coil is practically shorted to ground.

No, it's not. It's practically shorted to virtual ground. Here's a circuit to consider:

What's the gain of this circuit? How does it work? (Hint: you already have the answer.)

An ideal current amplifier will have an input impedance of zero ohms. A well designed current amplifier will have a input impedance well below your 1 ohms.
You are designing a voltage amplifier with a low input impedance ( which is counter intuitive and possibly wrong ).

And theres no free lunch because maximum power transfer occurs at source impedance = load impedance.

The RX coil is capable of producing either a voltage or a current. In the case of the Lee amplifier, the RX coil current is converted into an output voltage, therefore it's a transimpedance amplifer (G=V_out/I_in). It's a perfectly legitimate way to make an RX preamp whether the ultra-low noise is a benefit or not.

Also, maximum power transfer only matters when you want to transfer maximum power. In the kinds of designs we do, we avoid that because it's just wasteful.

[moodz]

No, it's not. It's practically shorted to virtual ground. Here's a circuit to consider:

What's the gain of this circuit? How does it work? (Hint: you already have the answer.)



The RX coil is capable of producing either a voltage or a current. In the case of the Lee amplifier, the RX coil current is converted into an output voltage, therefore it's a transimpedance amplifer (G=V_out/I_in). It's a perfectly legitimate way to make an RX preamp whether the ultra-low noise is a benefit or not.

Also, maximum power transfer only matters when you want to transfer maximum power. In the kinds of designs we do, we avoid that because it's just wasteful.

When a coil has a current flowing because there is some impedance across it and a changing magnetic field it it forms a magnetic circuit with other inductors / susceptibities in the vicinity.
When a coil has no current ( ie we just measure the voltage across it ... there is no magnetic circuit formed ( because there is no current ).
This is the key decision.

At the rate this simple "Kiss" project is progressing we will never get to the main detector.

[Aziz]

Hi all,

RX current sensing would produce much TX - RX coil interaction. Regardless of wether good or not good. My observation is: no good.
In case of RX voltage sensing, there is almost no current through the RX coil and hence almost no interaction with the TX coil.
So I tend to do the voltage pre-amp next time.
---
My choke L1 for the dual resonant frequency VLF/LF is getting too difficult to wind with the thick wire. Anyway, it must be done. 500 g 1 mm copper wire will be split for two different chokes. As tight as possible. As compact as possible for max. inductance per weight. This is the opposite way one would wind the TX coil of a PI detector.

I want to see the dual frequency response.
Cheers,
Aziz

[Carl-NC]

RX current sensing would produce much TX - RX coil interaction.

Not really, the RX current is miniscule compared to the TX current, and the coils are induction-balanced. That goes both ways.

[ArchibaldSTM]

500 g 1 mm copper

It's crazy...

[Aziz]

It's crazy...

I've got two big water bubbles on my fingers.
The final choke has 1135 µH inductance, 0.567 Ohms series resistance and weighs total 170 g (including the hansa plast plastic core). I don't to intend to get more water bubbles so I will take the choke for further experiments.
Cheers

[Aziz]

Hi all,

you know I like crazy experiments. Something a serious engineer would never do.
What about pure impedance measurements on the TX-coil side?
We have the choke L1 and the TX-coil at the transmitter side. A VLF/LF dual frequency detector without an RX-coil? No IB-design. No pre-amp required. Simple coil design.

Oh well, I have put the choke L1 at the center of the TX-coil and measured the impedance change on the TX-coil.
Indeed, not bad results. We could also take the voltage at the choke L1 and measure it's own impedance change too.
The coil coupling coefficient between TX-coil and choke L1 is low (k=0.1 .. 0.2) as the choke is smaller in diameter.

This brings an interesting application. A concentric TX-coil with smaller choke (half diameter) at the center position.
This is for sure interesting to investigate and invest some time.

Cheers,
Aziz

[Aziz]

Not really, the RX current is miniscule compared to the TX current, and the coils are induction-balanced. That goes both ways.

But I would require more gain at the RX-coil. This would produce more amplifier noise at the end and there is no free lunch.