VLF MD with digital signal processing : Bee-Buzz 1

[moodz]

Hi Aziz ... there may or may not be a reason. Maybe it was a "bluebird" .. some designers choose to overdesign to maybe counteract a percieved deficiency somewhere else. VLF machines are fundamentally frequency domain and use ( or should use ) synchronous demodulation. If the mixers used are "leaky" ... ie the mixers are not balanced and port to port isolation is poor then low frequency noise at the input will impact the target frequency bandwidth. So if you lower the noise at the frontend you can offset this effect ... however you should really look at getting a better mixer as the demod.

Most schemes I see use chopper switches like the 4066 or 4053 in single ended mixer configuration ... these are not really ideal if high performance mixing is required.

Look up Gilbert Cell for ( you can get a chip NE602 NE612 I think ) ... they can achieve balanced mixing ... theres lots of schemes though even using 4066 in some circuits.

Of course with high spec ADCs this is all a moot point as nowadays the mixing can be done in the DSP and achieve near perfect results ( depending on the bit resolution and noise floor of your ADC ).

[Aziz]

Hi Moodz,

it would be interesting to know, why Fisher made such an effort on the input stage. I do not have any idea.
Cheers
Aziz

[moodz]

Just reading the data sheets for a "cheap" ne5534 shows the source resistance will have more effect on noise than your wallet will. LOL
( and it is random noise anyway .. non synchronous )

zz0.ntdhu6imfqzz

[Aziz]

You now design real working VLF metal detector with real tests or just ultra low noise amplifier?

Real tests. Digital decoding: FFT, Goertzel and Lockin-Amplifier. Just for fun and quriosity.
Without preamp and with preamp.

Forget Rocket-Science, this is Oreshnik-Technology.

Aziz

[Detectorist#1]

You now design real working VLF metal detector with real tests or just ultra low noise amplifier?

[Aziz]

Hi Aziz,
As Moodz said in #244, the problem with noise in VLF MD isn't in front-end stage. Ultra low noise solution not helps in real life of VLF MD - but if you have different results - all we will be happy to know this!

We will see, what is possible.
I will measure some resistors connected to the input and measure their noise at +40 dB gain.

[Detectorist#1]

Hi Aziz,
As Moodz said in #244, the problem with noise in VLF MD isn't in front-end stage. Ultra low noise solution not helps in real life of VLF MD - but if you have different results - all we will be happy to know this!

[Aziz]

Hi all,

I have found my ultra-low-noise preamp breadboard (PNP-Version with 3 x BD176 in parallel). Gain = 1 + 470 Ohm/4.7 Ohm = 1+100 = 101. Approximately 40 dB Gain. It should rise the signals by 40 dB.

Installed the sound card driver on the Windows 7 PC. Measured the noise floor without any input on the sound card. Plugged the preamp with coil -> Noise, a lot of noise! Shorted the input of the preamp -> down to noise floor.
Checked the initial noise floor of the sound card without any input connectors. Connected the ultra-low-noise preamp with shorted input: No rise of the noise floor. At least not measureable. Maybe less than 0.5 dB rise.
This is a good preamp. A super-duper-ultra-low-noise preamp.

It will take some time to make some real measurements. I have not found everything. Everything (connectors) have been corroded. I need my soldering iron.
Cheers,
Aziz

[Detectorist#1]

Hi dbanner,
Yes, you are right. If low noise OpAmps was pricey in old days, now LT1028 (maybe lowest noise amp for VLF designs still year 1992) haves price of 13USD. Interesting - how was the price of LT1028 at the time of designing of Fisher Gold Bug?

[Detectorist#1]

Hi Moodz,
Thank you for the fast answer! What are advantages of MAGPI project with remove the EF and low freq noise in comparison with bipolar TX PI with inherently eliminating of EF and low freq noise of front-end stage? Maybe saturation process in frontend stage involving additional modulation make the difference?

[moodz]

Hi Moodz,
Thank you for this detailed explanation! I understand that using of balanced mixers in analog treatment of the signals eliminates the need of special low noise OpAmps in frontend stage of VLF detectors. Using of direct sampling allows more simple hardware solution with help of firmware after ADC stage again without need of low noise amplifiers in frontend stage.

This discussion is pointed on VLF MD, but In case of PI detectors this analysis also is interesting. I know your post for "Filters - low-pass, high-pass or band-pass" in PI designs. In this discussion you also mention that useful signals are near of the TX frequency. Is this valid only in the case of single sample unipolar TX pulses. What happens in the case of two samples (main and EFE) in PI designs? I see many PI designs with high-pass filter for 15.9Hz ( 0.1uF and 10K) after front-end stage. If EFE sample have 100us delay after Main sample - filter with 1ms time-constant will change the condition for right substracting and sure elimination of Earth's Field signals or I'm not right?

Yes you are right but consider that the preamp ( before sampling - which is demodulation ) is either driven to saturation during flyback or switched away .... this is a form of modulation .... so this causes convolution of low frequency noise ( like mains Earth field etc ) to be modulated onto the "carrier" which actually is the pulse reptition frequency. So each sample you take is a separate demodulation process. ( whether one or many ). Considering that any waveform can be deconstructed to sine waves ... the filtering is altering the phase and amplitude. So in the MAGPI project the aim was to remove the EF and low freq noise by feeding the deconvoluted signal back to the input as negative feedback causing it to be cancelled. Easy to show in a simulator like LTSPICE ... but works in practice too.

[Detectorist#1]

Hi Moodz,
Thank you for this detailed explanation! I understand that using of balanced mixers in analog treatment of the signals eliminates the need of special low noise OpAmps in frontend stage of VLF detectors. Using of direct sampling allows more simple hardware solution with help of firmware after ADC stage again without need of low noise amplifiers in frontend stage.

This discussion is pointed on VLF MD, but In case of PI detectors this analysis also is interesting. I know your post for "Filters - low-pass, high-pass or band-pass" in PI designs. In this discussion you also mention that useful signals are near of the TX frequency. Is this valid only in the case of single sample unipolar TX pulses. What happens in the case of two samples (main and EFE) in PI designs? I see many PI designs with high-pass filter for 15.9Hz ( 0.1uF and 10K) after front-end stage. If EFE sample have 100us delay after Main sample - filter with 1ms time-constant will change the condition for right substracting and sure elimination of Earth's Field signals or I'm not right?

[dbanner]

Ok. thanks moodz, I hadn't installed the IDE yet.

[moodz]

CubeMX CubeIDE is c?, c++?
MX generates dot C file for chain?

[moodz]

Below is a diagram to help explain ... a wanted signal at 1 Khz ( much lower than other unwanted signals ) is mixed with an LO at 1 Khz and the wanted signal is translated down to near DC and the unwanted signals are translated to near 1 Kz or above. A low pass filter can now be used to remove the unwanted signals.