What is the best frequency response for the preamp in a PI ?

[moodz]

I really do get it but I've been dragged down a rabbit hole, which probably should be in another thread, wondering if the amplitude of the TX harmonics should at least be all the same amplitude as far as possible and do particular harmonics 'excite' particular target types.
This is leading me to wonder about modulating pulse width, frequency sweeping and multi-pulse TX signal harmonic content.
Sorry for derailing your thread.

Are the teeth of this nit comb fine enough to catch those pesky sub gramme critters? (rhetorical)

It will almost certainly be an application of the Rumsfeld Matrix ...
The four quadrants of the Rumsfeld Matrix

1. Known knowns: These are facts or variables that we're aware of and understand. They form the basis of our knowledge and provide a solid foundation for decision making.
2. Known unknowns: These are factors we know exist, but don't fully understand. They represent gaps in our knowledge that we must address through research, investigation, or consultation with experts.
3. Unknown knowns: These are elements that we don't realize we know. They're typically buried in our subconscious, overlooked, or dismissed as irrelevant. Uncovering these insights can lead to surprising breakthroughs in decision making.
4. Unknown unknowns: These are factors that we're not aware of and can't predict. They represent the most significant source of uncertainty and risk, as they can catch us off guard and derail our plans.

To illustrate this the current Magpi Design can easily detect a 0.1 gram nugget using a 40 cm coil .... but the first sample occurs at 8 microseconds. Go figure.

[Gunghouk]

I like it. I'm done

[moodz]

Here is the preamp response for a PI operating at 10 Khz and gain of 1000.

This is a switched preamp and the required response is from the fundamental to at least the 11th harmonic.

By filtering at baseband all the problems with mains EMI earthfield etc ... just disappear .. attenuated by over 100 dB

[moodz]

Here is the preamp response for a PI operating at 10 Khz and gain of 1000.

This is a switched preamp and the required response is from the fundamental to at least the 11th harmonic.

By filtering at baseband all the problems with mains EMI earthfield etc ... just disappear .. attenuated by over 100 dB

So has anyone got an idea of wha the target tau for a 10mm x 10mm x .016 mm ( kitchen foil ) ?

[MOR_AL]

Today I already designed the pulse generator part (555), inverter, mosfet and back to back diodes.
I included a circuit that limits the drain current, regardless of the Ton time of the 555/inverter and the value of the coil.
It is imperative that the resistor value in parallel with the coil is optimized for each coil value.
I've been trying for some time now to use an operational amplifier (OA) for the next stage.
I used several of them as tests, always focusing on common components.
I reached the following conclusion.
OAs were designed to function in their linear region. The undesirable signal, coming from the back to back diodes, has a voltage of around 4Vpp or more.
When this signal is amplified by the OA, the OA inevitably reaches its nonlinear region. The exit from this region seems to be the problem.
Due to this fact, I am thinking of designing an amplifier stage such that it does not saturate.
It could be an anti-logarithm amplifier, but for the useful signal, there would be no need for precision, just that it would amplify low voltage levels a lot and a little for high voltage levels.
I even tried to find out what the frequency range of the useful signal would be, but due to the presence of the unwanted signal in diodes with much higher voltage, I was unable to see the difference.
I'm not dealing with that at the moment.
At the moment I'm trying to design a circuit for testing my purchased OA. As I said, I'm not a fan of expensive components and some of my OA are probably fake.
The tests refer to the slew rate and the band gain product (BGP).
For testing, a signal generator and an oscilloscope are required.
MOR_AL

[Teleno]

How to implement this high pass? The response to a step pulse is a decaying exponential that starts at saturation. For 1kHz transmit rate the time constant would be in the 100's of us. Sampling that then try to separate the target from the decay doesn't seem to be advantageous.

[MOR_AL]

I think the following.
Although the signal coming from the back to back diodes has strong components at the frequency of the pulses generated by the 555, this is not a useful signal. It exists all the time. The useful signal will occur when the coil passes close to a metal.
The useful signal will appear in increasing fashion for about half a second to a few seconds.
As this signal is non-cyclical, relative to the 555 signal, the frequency spectrum would be calculated using the Fourier Integral and not the Fourier Series, which refers to cyclical signals.
Leaving theory aside, I believe that the frequency spectrum of the useful signal has to do with how quickly the coil is handled close to the metal.
I think the useful spectrum would start at a few hundredths of a Hertz and extend to a few Hertz.
All AOs have high gain in this frequency range. But due to the presence of the undesirable signal in the diodes, the OA will always saturate. Leaving saturation is important, which is why I consider using a circuit whose OA does not enter its nonlinear region.
MOR_AL

[moodz]

How to implement this high pass? The response to a step pulse is a decaying exponential that starts at saturation. For 1kHz transmit rate the time constant would be in the 100's of us. Sampling that then try to separate the target from the decay doesn't seem to be advantageous.

The key words are ...response to a step pulse.

The step response of the tx coil being damped masks the target response. The target is isolated from the tx/ rx coil by air ...so intuitively there is already a high pass filter in place as you can't transmit DC through ac magnetic coupling. So the remaining task is to remove the tx decay from the coil but leave the target signal.

[Teleno]

So the remaining task is to remove the tx decay from the coil but leave the target signal.

Alright but we all know that is the problem.

My question was related to implementing the amplifier bandwidth, if instead of a DC amplifier you connect a high pass filter what happens is,

No filter



RC high pass filter, -3dB at 500 Hz

[moodz]

The average DC output of the waveform of your filter ..if it's working correctly should be zero. The output looks like it has a DC imbalance. I realize you have a single supply amp but shouldn't you have a vcc/2 reference to set the DC bias?

[Teleno]

The average DC output of the waveform of your filter ..if it's working correctly should be zero. The output looks like it has a DC imbalance. I realize you have a single supply amp but shouldn't you have a vcc/2 reference to set the DC bias?

reference is biased at 1.57V.

The problem is, as I pointed in a previous message, is that the step response of a high-pass filter is a decaying exponential, and much worse than the coil's for a 500 Hz cut-off frequency.

[moodz]

Ah yes I understand what you are getting at. The high pass filter is responding to a step change in DC at it's input .... The solution requires a high pass filter that only responds to target decays as the Eddy current in the target is zero before the step an d the decays to zero in a short time eg few microseconds. The high pass filter input capacitance must not be charged by the input step as the step is a tx signal not a Target signal.

[moodz]

The main problem with your filter though is that you are filtering the transmit step response ... You don't want to do that.