VLF MD with digital signal processing : Bee-Buzz 1

[Altra]

Yes, I didn't think it would work with direct sampling.

If you google search "stacking a/d converters " there is some information pro and cons.

[Detectorist#1]

I have modest experience in building of military radars and they have a very rapid change in the level of the received signal with distance as with metal detectors. There, logarithmic processing of the received signal is used. Trying to use linear processing of the received signal with multi-resolution analog-to-digital conversion leads to serious problems in my opinion.

[Repwoc]

thanks for bringing up this point.

zero crossing + peak detection is very susceptible to noise,

OK, thanks for the explanation.

Looks like I need to do more experiments.

[moodz]

The RX signal in the coil already contains some noise, which gets amplified in the first op-amp, along with the addition of the op-amp's own noise. The critical factor is the required SNR at the signal extraction stage to recover a reliable signal. To determine the allowable op-amp noise, you must work backward, accounting for all noise sources in the chain, ensuring the first op-amp's noise is low enough to maintain the target SNR.

Thumbrule : get the opamp with the lowest possible noise, at your frequency of interest, that you can afford.

The opamps you mentioned, are likely to have been used in this Garret detector.

Your proposed detector is a single frequency @ 6 Khz ... Op Amp data sheets quote broadband noise and its usually worst in the first DC to 1 Khz ... far away from 6 Khz. No need to spend money on expensive opamps based on CE ( Catalogue Engineering )

The only way that low frequency noise in the ( cheaper ) opamps is going to get into your 6Khz signal is if there is non linearity or overload ( ie convolution ) occuring through bad design.
Do you reckon they did moonshots back in the 60s with these uber opamps ... no they didnt.
moodz

[Atul Asthana]

Great conversation, I am learning

which low cost 24 bit codec would be suitable in this design?
and which direct sampling design should I study?

[Atul Asthana]

Your proposed detector is a single frequency @ 6 Khz ... Op Amp data sheets quote broadband noise and its usually worst in the first DC to 1 Khz ... far away from 6 Khz. No need to spend money on expensive opamps based on CE ( Catalogue Engineering )

The only way that low frequency noise in the ( cheaper ) opamps is going to get into your 6Khz signal is if there is non linearity or overload ( ie convolution ) occuring through bad design.
Do you reckon they did moonshots back in the 60s with these uber opamps ... no they didnt.
moodz

point to consider about using expensive opamps.
however, with 24 bit codecs, I'll probably need good opamps.

[Carl-NC]

Personally, I think you're original plan of using the internal 12b ADC makes the best sense as it minimizes the number of hurdles you have to jump just to get initial results. Plus, it will give you good data on what minimum # of bits are needed.

The first direct sampling detector was the original X-Terra (30/50/70). They used a Wolfson CODEC WM-something, don't have my notes right now. The next was the White's Prizm 6T which was used a 24b ADC (not CODEC) to quadrature-sample the RX. Then everything ML did -- Go-Find, Equinox, Vanquish, XT-pro, etc -- were all direct sampling. As are the XP Deus models, the newer Nokta models (Legend, Simplex, Score, etc), and the Garrett Vortex.I know that Nokta uses a more mainstream TI CODEC, the PCM-something. Look at the PCM1808 as an example, it's only 96kHz but is the only part number I know off the top of my head. Most everyone is using a 192kHz CODEC. Generally, you want to sample at 4x so 192kHz supports up to 40kHz which you might recognize as the highest that any of the MF models run. There are also a few 384kHz CODECs.

[ivconic]

Great conversation, I am learning

which low cost 24 bit codec would be suitable in this design?
and which direct sampling design should I study?

[ivconic]

point to consider about using expensive opamps.
however, with 24 bit codecs, I'll probably need good opamps.

[ivconic]

Carl is absolutely right.
You're going to open a can of worms if you go down that road.
The "trick" is not in the codec but in the DSP.
In the case of X-Terra it is a TMS320VC5402 DSP.

[Atul Asthana]

Personally, I think you're original plan of using the internal 12b ADC makes the best sense as it minimizes the number of hurdles you have to jump just to get initial results. Plus, it will give you good data on what minimum # of bits are needed.

.

I agree with Carl that lets keep things as simple as possible.

so, is 6 khz tx frequency the right choice or do we need to select some other frequency, and thr rrason thete of.

my next step is to analyse whether the signal processing methodology needs any improvement :
1. sampling strateg,
2. integration philosophy
3. amplitude and phase extraction,
4. ground and target response extraction
5. VDI calculation

And the administrstive functions management.

Please comment, describing logic behind your suggestion for change.

[Atul Asthana]

The first direct sampling detector was the original X-Terra (30/50/70). They used a Wolfson CODEC WM-something, don't have my notes right now. The next was the White's Prizm 6T which was used a 24b ADC (not CODEC) to quadrature-sample the RX. Then everything ML did -- Go-Find, Equinox, Vanquish, XT-pro, etc -- were all direct sampling. As are the XP Deus models, the newer Nokta models (Legend, Simplex, Score, etc), and the Garrett Vortex.I know that Nokta uses a more mainstream TI CODEC, the PCM-something. Look at the PCM1808 as an example, it's only 96kHz but is the only part number I know off the top of my head. Most everyone is using a 192kHz CODEC. Generally, you want to sample at 4x so 192kHz supports up to 40kHz which you might recognize as the highest that any of the MF models run. There are also a few 384kHz CODECs.

I’m interested in understanding the sampling and signal processing strategies employed by direct sampling detectors.

Where can I find detailed information about these technologies?

It would be helpful to know if any manufacturers or research papers provide insights into their design or algorithms.

[Carl-NC]

6kHz is fine. For the purpose of development, frequency doesn't matter and can easily be changed later.

[Atul Asthana]

Thanks ivconic for the schematics.

As per Carl's recommendations, if we stick to the original scheme, there isn't much hardware involved, as outlined in an earlier description.

Once the software design is finalized, we will focus on the appropriate hardware, which primarily consists of op-amps and filters. These components have already been calculated but will be revisited if the TX frequency changes.

Additionally, care will be taken to ensure the hardware supports efficient signal processing with minimal noise. Any changes in TX frequency will also necessitate adjustments to the filter bandwidth and response characteristics to maintain system accuracy.

[Carl-NC]

I’m interested in understanding the sampling and signal processing strategies employed by direct sampling detectors.

Where can I find detailed information about these technologies?

It would be helpful to know if any manufacturers or research papers provide insights into their design or algorithms.

That's all hidden in code. However, based on patent US7579839 I assume Minelab is doing narrowband synchronous quadrature demodulation in software. Likewise, Noka has a patent (US11914095) on an asynchronous demodulation method.