field test unit no 001 "model T"

[UrbanFox]

Just in case this was a genuine question, I will give a genuine answer.

With traditional PI, you can find the X response at any time that you have current flowing in the TX coil. This may be during the TX ON time as well as during the Flyback decay time.

Tinkerer

Thanks Tinkerer. I am looking at a previous post...

http://www.geotech1.com/forums/showthread.php?17287-field-test-unit-no-001-quot-model-T-quot&p=158003#post158003

Where it was stated...

"But the ground balance can be implemented (by separating X and R) and the relation of X/R (or vice versa) gives a good indication to discriminate iron/non-iron targets. It might work for most conditions. But can fool you too"

My understanding is that the statement can be applied to VLF, but GB with PI will not directly use X component. Am I incorrect?

[chudster]

Folks are welcomed to open up discussions on this topic under a new thread.

Note that the thread has been opened at:

http://www.geotech1.com/forums/showthread.php?19667-Open-Metal-Detector-Project-Charter

Chudster

[UrbanFox]

as if you have any idea
Why does every troll have to pose as a stuffy professor, or as a nagging woman :duh:

Hi Davor, could you please explain for me where the X component is coming from to do PI Ground Balance??
What is trolling about the question?? Aziz made a statement about X and R components being used for GB, and as the thread is focused around PI then I feel the question is appropriate.

[Carl-NC]

I agree. I as trying to get a definition of "direct sampling"

"Direct sampling" traditionally means directly sampling the carrier signal, with no prior demodulation. Amplification & filtering are fine, and normally done. There is also IF sampling and baseband sampling. Most detectors use baseband sampling. Minelab X-Terras and White's Prizm 6T use direct sampling. One of the projects in my book is a direct sampling PI.

In my experiments with traditional PI, I observed that the capture of very short TC targets can be enhanced by integrating on the pramp level. Although the peak amplitude gets diluted by the integration, the signal TC gets extended. The key word that needs to be added to integration is the integration time.

Interesting isn't it? Putting a ~5us pole on the preamp pushes fast target responses out in time so you can see them. I used this trick in my security walk-through PI design. It does, however, weaken responses, which is to be expected.

- Carl

[Davor]

I so love when you speak dirty

[Tinkerer]

Too bad input signal is not a linear decay so that this makes complete sense. It could be with a log amp though

Right, with even a crude log function in the preamp, I could read a 10usTC target still after 100us.

Tinkerer

[Davor]

The key word that needs to be added to integration is the integration time.

Too bad input signal is not a linear decay so that this makes complete sense. It could be with a log amp though

[Davor]

Hard kick (high dI/dt) = high frequency response = energy efficient kick

I think this is just partially true - the frequency part will remain the same in a given system, regardless of the peak voltage you obtain with the kick.
The way I see PI systems is a bit of a contradiction. In one hand it is proclaimed to be resilient against the fast ground and reactive responses, sea water and whatnot (near 90° responses in VLF) by virtue of delayed sampling, but in the other hand as the same principle also hurts detection of small gold a "somewhat" faster response is chased after. Being good and having fun at the same time in a way. Kinky and saintly. A contradiction.

Obtaining the faster sampling is actually brain-dead simple: just reduce the kick. Nothing is going to happen with the frequency response, exponential fall is going to be the very same.

[Tinkerer]

Don't talk about GB with me then, Aziz. Talk to the members of this forum and explain where, with a PI, the X component is coming from for Ground Balance.

Just in case this was a genuine question, I will give a genuine answer.

With traditional PI, you can find the X response at any time that you have current flowing in the TX coil. This may be during the TX ON time as well as during the Flyback decay time.

Tinkerer

[Tinkerer]

Not true. The preamp can provide a better SNR than you can get with an ADC alone, unless the ADC has a built-in LNA. In direct sampled radios there is always a LNA in front of the ADC. Also, a lot of these high-res ADCs are rather picky about how they are driven, and often need a preamp with a particular matching network to minimize sampling kickback. You sure don't want the sampling kickback to see a coil.

Also, an integrator establishes a "channel" that limits your processing, i.e., you need a separate integrator for every unique portion of the response you want to process, but a preamp alone does not limit processing.

I agree. I as trying to get a definition of "direct sampling"

In my experiments with traditional PI, I observed that the capture of very short TC targets can be enhanced by integrating on the pramp level. Although the peak amplitude gets diluted by the integration, the signal TC gets extended. The key word that needs to be added to integration is the integration time.

Tinkerer

[Aziz]

In PI, it does make sense. The BW of the TX pulse determines how hard the target gets kicked. The harder you kick it, the stronger the response. Once the target has been kicked, it is true that you don't need to maintain all that BW through the receiver. In fact, you can direct sample with only 100Hz of BW.

Hard kick (high dI/dt) = high frequency response = energy efficient kick
Yes, that's pretty fine.

Remember, the whole TX spectral energy is spread over from the TX pulse frequency up to the upper limit. That's a large bandwidth.
Now imagine, when someone is using due to limitations only 100 kHz BW of the let's say 800 kHz BW. 700 kHz BW TX spectral energy is wasted in this case. The energy could be used elsewhere.
Aziz

[Carl-NC]

Direct sampling:

The way I see it, direct sampling would be directly off the RX coil.
If we add a preamp, we might just as well add an integrator/anti alias filter.

Not true. The preamp can provide a better SNR than you can get with an ADC alone, unless the ADC has a built-in LNA. In direct sampled radios there is always a LNA in front of the ADC. Also, a lot of these high-res ADCs are rather picky about how they are driven, and often need a preamp with a particular matching network to minimize sampling kickback. You sure don't want the sampling kickback to see a coil.

Also, an integrator establishes a "channel" that limits your processing, i.e., you need a separate integrator for every unique portion of the response you want to process, but a preamp alone does not limit processing.

[Tinkerer]

yep I have seen the kickstarter.com site ...very interesting concept ...who is up for it ?

I understand you need to be a legal US resident to participate in Kickstarter.

Tinkerer

[Carl-NC]

Does it make sense to transmit at frequencies, you can't detect at all?
Clearly not. Just save the TX power energy or use it additionally in the frequency range, you can detect.
Aziz

In PI, it does make sense. The BW of the TX pulse determines how hard the target gets kicked. The harder you kick it, the stronger the response. Once the target has been kicked, it is true that you don't need to maintain all that BW through the receiver. In fact, you can direct sample with only 100Hz of BW.

[Tinkerer]

Hey Tinkerer,

Thanks for your response. Its helpful for the advancement of stuff with some real data to chew on.

In summary with a 2uS signal we need 1MSPS or higher to have a decent chance of detection or smaller targets. Moodz working at 2MSPS is 'overkill' but not unreasonable as its good to oversample (where is that antialiasing filter though Moodz). At 48KSPS, Aziz's soundcard seems OK for cannonballs with very long time constants (160uS) but not a tiny nugget.

Note we are talking about direct sampled solutions here. We can do all sorts of analog processing to have a half analog half digital solution (including integration) to mimic an analog detector with lower sample rates.

Chudster

Direct sampling:

The way I see it, direct sampling would be directly off the RX coil.
If we add a preamp, we might just as well add an integrator/anti alias filter.

However, the signal response of a 1us TC target is of really short duration, so to get the peak response, the sample should also be short, otherwise we dilute the peak amplitude with a long sample time.

One of the secrets of FE discrimination, with PI, is that the X response may have a TC of 2us, while the R response of the same target, has a TC several times as much.

Unless we are capable of sampling the X response separately from the R response, we will not be able to differentiate this FE target, as the sum of the X+R responses will mask the weaker response.

In other words, we need not worry about 2us TC targets with a traditional PI, since we do not have the discrimination capability.

With the TINKERERS TEM PI, however, that is capable of sampling a 1usTC target at it's peak, the capability of taking several samples of 1us in quick succession, can make a difference.

So, again, we need to specify which technology platform we are talking about.

Moodz has his own technology platform. Only he knows what the capabilities are and how to make best use of these capabilities.

We must not assume that all PI platforms are equal.

Tinkerer