NEW !!! Full differential PI front end.

[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]

Wrong schematic above ... see this one.





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]

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 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

[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

[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

[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.

[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.

[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]

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

Thanks okantex ...

WHITE is the flyback pulse 175 volts peak .. you wont see the peak here obviously with 2v/div. We are looking at the base of the pulses here.

GREEN is the sample.

note the sample is not amplified .... it is o/p of sampling stage and is the input to my processing stage.

The peak you are looking at is part of the flyback. See the pic above ... the sample is taken from the falling ''wall" of the flyback. I have offset vertically the sample and flyback however you can see from the shape that the sample tracks the flyback exactly in level on the falling side. I dont use clamping diodes at all in the front end in my current scheme. The use of clamping diodes throws away the useful signals on the falling curve.

The other thing about clamping diodes is that they unclamp ( ie provide useful signal only when the forward conduction voltage is below 600mv etc )
This could occur at any time depending on the slope of the curve. However targets change the shape of the curve thus the point at which the diode unclamps ... start of sampling if you will ... is not fixed in time. My frontend sampler always begins sampling at a fixed time. ( not too early of course or I will dump high voltage into my front end ... i have not managed to do this yet.)

Note also ... some may think I am simply attenuating the flyback peak down to the sample level ... this is not correct the sampler is a switch that switches at a predetermined time after the tx pulse has ended. Approx 5 us here.

note also the peak of the sample intercepts the curve at the correct level.
( noting that I have moved the position of the flyback up slightly so that the sample trace is not obscured. )

note the 'soft' rise time on the rising edge of the sample ... this is good for me as there is less hf energy in the pulse to be processed and I can use slower op amps.

cheerz from moodz

PS I have noted your comments about referencing offsite resources Carl ... thanks.

[Carl-NC]

PS I have noted your comments about referencing offsite resources Carl ... thanks.

Yikes! Shrink those pics! I have auto-scaling turned off so pics come in at their full size. So you need to edit them for size before posting. A width of 800 works well:

[okantex]

Hi Moodz ,
thanks for explanationss.

I am confused about pictures.

at previous posts ,picture for copper,I choose this because it has lowest peak ,lower than also no target option.
at 10us peak is 2V
at 50us peak is 1,2V this is good.

but at your last picture
at 12us peak is 0,6V
at 15us ,it is 0, disappears.

maybe I am reading scope ,in wrong way, excuse me than.

regards/thanks for your understanding

[moodz]

Hi Moodz ,
thanks for explanationss.

I am confused about pictures.

at previous posts ,picture for copper,I choose this because it has lowest peak ,lower than also no target option.
at 10us peak is 2V
at 50us peak is 1,2V this is good.

but at your last picture
at 12us peak is 0,6V
at 15us ,it is 0, disappears.

maybe I am reading scope ,in wrong way, excuse me than.

regards/thanks for your understanding

Hi okantex ... the last picture was intended to show how the sample is taken from the falling edge of the flyback spike rather than being cut off by clamping diodes. Also the CRO set to 2 volts/div 5 uS/div rather than 1 volt / div 10uS/div in previous pics. I dont think there was a target in the coil at the time.
It is important that the fly back pulse effectively gets narrower in presence of good conducting target ( sample peak goes low ) ... wider with ferrous target ( sample pulse goes higher ) ... the base line will also lift in both cases.

moodz

[Unregistered]

Hi Moodz,
what about effecet of size of metal to hight of peak.
does the hight of peak change according to metal size.or size just effects width of peak?
Also what about effect of distance ( target to coil) to the hight of peak?

our variables are too much, we have to consider them all .

what do you use after this above circuitry, multiplexer and S&H caps then outer Ato C circuitry ( counter related to chip)?