NEW !!! Full differential PI front end.

[okantex]

Hi Moodz,

congratulations for your improvements.
I am an electronist , dealing as hobby,
I was inspection scope results.your last pictures are interesting , there is no cutting of high peak ( fly back cut-off).why there is no flyback ?
okantex

[moodz]

PI Discriminator results ...
Input to sampler stage.

GREEN = end of TX pulse to MOSFET gate
WHITE is sample to input of sampler - provided by sample gating.

NO TARGET - note sampling begins at 8 - 10 uS


COPPER TARGET ... note pulse peak is LOWER


FERROUS TARGET ... note pulse peak is HIGHER


The little glitch in the white trace between the sample peak and the end of the green TX pulse is the flyback pulse getting into my sampler .. it does not appear to affect the bits that count.

I would say the results above ar a basis for discrimination .... no ?

works on the bench anyway ...

PS the circuit this was grabbed from is an evolution of my schematic previously posted .... not ready to release yet.

moodz.

[moodz]

.... just an update ...

... started rebuilding the amp circuit. Pulse is sampling at 8 uS and descrimination is working. ( well the output goes up for copper down for ferrites and side cutters ).

Anyone got some spare gold I can test :-) ?

GB is on my todo list.

coil driven by 100uS pulse. 5 volts regulated.

moodz.

[Carl-NC]

Paul,

Keep in mind that using an image server like postimage.org means that eventually your images will disappear from this thread, rendering it useless. It would be better to post images using the forum's 'attachment' method; they are then permanent.

- Carl

[Aziz]

Here is an interesting low noise amplifier design:

Extreme Low Noise Preamplifier
0,4 nV/Sqr(Hz) and less - isn't that low?

http://www.beis.de/Elektronik/LNPreAmp/LNPreAmp.html

Aziz

[Aziz]

Hi again,

Thanks Aziz ... the only ( > 1 Mhz ) amp I have here is the 3130 ... what specific type would you suggest ?

The 1n5822 is alot quiter jitterwise than the 1n4148 and I agree with you on the analysis.

I will work more on the shielding and keeping the noise sources away .. like my desklamp ha ha.

thanks moodz.

the INA103 would be fine, but it needs more supply voltage.

http://focus.ti.com.cn/cn/lit/ds/symlink/ina103.pdf

Maybe other types could provide less operating voltage. The instrumentation amplifiers generally are destined for differential signals.
Anyway, it could also be implemented with conventional low-noise op-amps. It could be worth to try it out.

Aziz

[moodz]

Thanks Aziz ... the only ( > 1 Mhz ) amp I have here is the 3130 ... what specific type would you suggest ?

The 1n5822 is alot quiter jitterwise than the 1n4148 and I agree with you on the analysis.

I will work more on the shielding and keeping the noise sources away .. like my desklamp ha ha.

thanks moodz.

[Aziz]

Hi moodz,

using the 1N4148 makes the clamping faster than with high capacitance schottky diodes. That means, the schottky diodes are limitting the bandwith much (lowpass filter), which results in picking less noise and therefore less jitter.

The comparator is used in an open-loop gain (very high). Therefore it is quite prone to noise. I suggest using a high gain instrumentation amplifier with band limitting (lowpass filter). The instrumentation amplifier has a high impedance input on both inputs (should be better than the standard difference amplifier circuit). An active shielding could also be applied to the floating coil.

Regards,
Aziz

[moodz]

Wrong schematic above ... see this one.





moodz

[moodz]

Applied KISS principle ....
sensitivity restored.
o/p jitter very good.
components reduced.
Note using shottky again ... 1n4148 caused increased jitter.




NO TARGET


50 cent coin at centre of coil


The green trace is the end of the 100 uS tx pulse drive on the MOSFET gate.

The white trace is the o/p of the comparator ... note some jitter on the target pic.

cheerz from moodz

[moodz]

here is a version with reduced jitter on o/p pulse ... lost some sensitivity though.

[moodz]

SIMPLE PI front end ....
The circuit below is the simplest front end using a comparator at the output the output pulse will get wider as a target is brought near the coil. Ideal for simple CPU processing.
This might be a new way of processing PI front end signals. woohoo :-)
Tx pulse set to 100 uS



NO target



With 50 cent coin in center of coil



The back end processing would have to resolve increase in pulse width to determine presence of target.

This circuit can detect a large metal sheet at 1 meter.

cheerz from moodz.


This idea could do with more development.

[moodz]

saturated amp ?

The first 20 us is the trailing edge of the tx pulse , the next 10 is under the back EMF ... what exactly do you want the amp to do during this time ?

Well...

Hi moodz,

don't use the schottky diodes. They have a quite long recovery time. The 1N4148 has a much lower capacitance and they recovery from clamping very fast.

This can be seen on the circuit simulation.

Aziz

BAT81..3 seem faster then 1N4148 - they have a three times lower capacitace! And they ARE "shottkies"...

Well...

U ,unregistered guests ,need to buy yourselfs a cro ,an get your act together!Ha!an do some work of yur own! nitpicks!then post it here! so we can pick!Ha!Trolls!
We enjoy moodz test an experiment,I do,so register an share your knowledge,I want to learn more.

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