PI Max Frequency?

[Ferric Toes]

The highest pulse rate I have used in a PI is 100K pulses per sec. This was for a detector to find broken off needle tips in clothing material. It would also find a 1mm diameter sphere of bronze which is a test standard in the food industry for metal detectors. To do this it is necessary to sample 1uS after the TX switch off. Search coil was a four layer pcb with integral shielding and inductance of 30uH.

Ferric

[Davor]

Hi Eric

[Ferric Toes]

Hi Davor,

The screen name is just an anagram which seemed appropriate to keep, having just returned from Australia where the ferric content of the ground is the worst on the planet. Also, this is the best metal detector technical forum on the planet and I want to be amongst all the good info being discussed.

By the way, the TX pulse width of the detector in my last post was 3uS!

Ferric.

[Tinkerer]

What are the limits of PI technology? I don't know, I've only begun to explore.



Then don't take a baseline sample.



Well then, maybe we're talking different "speeds". A target's step response is t*e{-t/tau}; the decay is fundamentally tied to the tau. I'll have to ask for more details, which you may not be ready to divulge at this time.

- Carl

No, we are talking about the same "speeds". This subject actually goes back many years and has been often discussed, between opposed opinions. I don't think the opinions ever came to an agreement. So basically I am just giving a new twist to an old problem.

I just love to turn a problem into an opportunity.

Thank you for giving me the option not to divulge at this time. Indeed, it needs to be given some thought, given the implications.

Then don't take a baseline sample.
I have not experimented with symmetrical bipolar PI, so I can not make a statement on this.

Tinkerer

[Tinkerer]

Hi Davor,

The screen name is just an anagram which seemed appropriate to keep, having just returned from Australia where the ferric content of the ground is the worst on the planet. Also, this is the best metal detector technical forum on the planet and I want to be amongst all the good info being discussed.

By the way, the TX pulse width of the detector in my last post was 3uS!

Ferric.

Hi Eric, fantastic to have you back here on the forum.

Tinkerer

[mschmahl]

Dear Forum,

Looking at PI detectors, they seem to all max out at a pulse frequency of about 1KHz- presumably due to technical limits. If a TX pulse time is 100uS and allowing 100uS for sampling and processing the pulse, then 5KHz would seem to be possible in theory.

1KHz is pretty fast already. Would there be any advantage in a 2-5KHz PI?

Is it technically possible?

Chudster

Hello Chudster.

I have changed the Timing of the Surf-PI-Pro. Now it has 5kHz pulse frequency. Look here http://www.detektorforum.de/smf/eige...html;msg448618

And here is a Testvideo http://www.youtube.com/watch?feature...&v=0_jgogQuSKc

[6666]

mschmahl

thanks for the link to the website, for anybody who only reads English and who is interested
you need to register to see any pictures, and the trick is you have to answer the GRASS test question with
grün

[chudster]

Hello Chudster.

I have changed the Timing of the Surf-PI-Pro. Now it has 5kHz pulse frequency. Look here http://www.detektorforum.de/smf/eige...html;msg448618

And here is a Testvideo http://www.youtube.com/watch?feature...&v=0_jgogQuSKc

Vielen dank!
(Many thanks).

I am not sure how much the higher pulsing frequency improved your depth?

Also how might it decrease your detecting time on one battery charge?

Did it heat up the components much?

Chudster

[mschmahl]

The higher pulsing frequency doesn´t improve the depht. But the signal is more stable and without false signals. And the Surf-PI is usable on land (lightly mineralization).

I have no idea about the detecting time, but i think it´s lightly shorten.
I have test it for one hour and didn´t heat up any components. All is very cold. If I change the pulse-on time, than i heat up the fet.
The disadvantage is that the tone is more a moskito-hum due to the high frequency, which should be further modified.

[xycy]

Hi Mschmahl,
I work also with DIY SM. I wonder what kind of coil is being used in the clip. I tried to register to that site but I haven't 'u' with points above. So you have an access to the pictures. Can you post the scheme with all changes in it? Would be appreciated!

Regards.

[mschmahl]

Hi Mschmahl,
I work also with DIY SM. I wonder what kind of coil is being used in the clip. I tried to register to that site but I haven't 'u' with points above. So you have an access to the pictures. Can you post the scheme with all changes in it? Would be appreciated!

Regards.

Hi xycy,
i don´t have a scheme with all the changes. But i will try to post all changes.
This is the original scheme that i use.
I use the delaypot 20k not 10k.
C2 is 3300 uF not 1000uF.

Than i made the Delaymod from ossamaussama http://www.geotech1.com/forums/showt...587#post158587, so that my delaypot worked.



The next is the Damping Resister. I have changed it to 270 Ohm


The next change is the op-amp NE5534 to LT1115 and playing a little bit with offset for max depth.
Now the signal is smoother.

Now changing Timing similar to what has described tepco.
For first sample from 33us to 22us change R26 to 33k. The same for second sample change R29 to 33k. Changing the intersample from 142us to 22us through R28 to 33k.
Change frequency from 596 Hz to 5,02kHz through R1 to 10k. Change pulse-on time from 46us to 20us through R2 to 1,2k.
Now the Pulse is changed from 1,6ms to 180us.

That´s all.


The coil is wound in a water pipe 27cm x 23 cm. It has 199 uH and 2,0 Ohm.

[xycy]

Hi Mschmahl,
Thank you for the answer. Is it better with 1uf fot C7?
Is LT1115 better then NE5534?
Have you more PCBs? I have difficulties making such. May I purchase one?
Also Im using batteries for -5V. I tried to make -5V without synchronizing it with 555 and had much interference. If you get ~30cm depth then using 7660 is OK, right?
Regards.

[Davor]

LT1115 is surely better than NE5534, but also 6 times more expensive. MAX410 is right in a middle in quality, price and availability between these two, and better suited for the 1kohm source resistance of a surf PI.

[mschmahl]

Hi Mschmahl,
Thank you for the answer. Is it better with 1uf fot C7?
Is LT1115 better then NE5534?
Have you more PCBs? I have difficulties making such. May I purchase one?
Also Im using batteries for -5V. I tried to make -5V without synchronizing it with 555 and had much interference. If you get ~30cm depth then using 7660 is OK, right?
Regards.

Hi xycy.
With C7 0,5 uF you have to swing faster than with 1uF. I like swinging slow, so i use 1 uF. I don´t have PCB´s.
Using 7660 is ok. I use TC7660CPA and TC7660S. Both are OK.
For LT1115 read Davor´s post.

[Tinkerer]

High frequency PI.
Rather we should say high pulse repetition rate (PRR) PI. Or high pulse per second PI (PPS).
Here is one way to look at it:
If I use 10,000 PRR, I get a signal response of: 1782 for a target of 10us TC
267 for a target of 100us TC
162 for a target of 500us TC
For large deeper targets, I want to increase the TX power, so I make a longer TX time and use 5,000 PRR
Now I get a signal response that is near double, at 3440 for a target of 10us
593 for a target of 100us
349 for a target of 500us
Keeping the same fast response time, at 50ms, I can integrate or stack 250 samples or 500 samples.
The more samples we stack, the better is the S/N. But this equation is not linear.
Would somebody be so kind, to calculate and post the mathematical formula that shows the “point of diminishing returns”?
What I mean to say, up to a certain amount of stacked samples, there is a considerable amount of improvement in S/N. After that, increasing the amount of integrated samples, produces an ever diminishing amount of improvement.

Tinkerer