RX Flyback Blocking for Best Op-Amp Performance

[moodz]

...I am good but I am not that good.

Aziz ... what you ask is impossible with my CRO .. there is only 10 bit resolution in the ADC and no offset facility so I cannot zoom in on peak. I would imagine that the peak voltage may vary ... however the descrimination response comes from magnetic and eddy current response of targets which is not going to happen right at peak time ..... the peak is only approx 2 - 3 microseconds after turn off.

What is your interest in peak anyway ???

Regards,

moodz.

PS I had dinner so now I am unpi$$ed.

PPS OK I will do three runs just to see if there is anything to spot.

[moodz]

Hi Moodz, you've got me and maybe everyone else stumped on how you've got negative "Q". It will be very interesting to maybe one day see your design.



Mick

Mick .. it is my term for want of a better word ... I obviously wont say anything just for now .. maybe I made a mistake somewhere. However if any companies out there are interested in a little mutual benefit I am listening ...

moodz.

[mickstv]

Moodz if it all works out I wouldn't mind betting ML will be very interested. And hopefully the $$$ will flow.




Mick

[der_fisherman]

Many thanks to all participantsfor an interesting blog.
Andy

[moodz]

... did the three runs

Moodz,

you didn't understand me.

Make the measurement for no target, copper and iron. And look at the peak flyback voltage. If you can zoom around the peak voltage, the better we could see the difference.

Cheers,
Aziz

PS: You had one drink too much according to the picture.

Aziz ... did the runs and saved the traces in the CRO but they all overlay each other on inspection ... there is one pixel difference between the peak no target and peak copper trace. No point publishing. At 50 volts / div the resolution is too course. The output from the preamp captures a "window" at the base of the flyback so you can do fine measurements on it and the information here is more relevant anyway. At one sample point you get target descrim and target size / strength. I am going to post some results with the amplifier gain > unity and smaller targets.

Regards from moodz.

[Tinkerer]

I think it is time that you give credit to the originators of the method.
http://www.geotech1.com/forums/showp...&postcount=819

Tinkerer

[Monolith]

Monolith, in theory all targets need only 12 Hz bandwidth in the preamp. The filter block (illustrated in the above block diagram) is bandpass. It should suppress frequencies below 3Hz because they represent modulation of GND signal, and frequencies above 12 Hz because they are 1/f noise of RX electronics.
http://www.findmall.com/read.php?34,129427
Excitation and receiving frequency spectrum above 25kHz contains too large GND signal generated by conductive soil. In time domain that means you should avoid sampling at small delays. However metal detectors for demining can operate at 300kHz because they suppress GND signal using two RX loops. Landmines with low metal content are as "very small targets". Note that each RX loop is in induction balance with TX loop.

Hi Mikebg,

thank you for the feedback.

When I talk about the bandwidth of the preamp, I do not mean an operating frequency. I mean the response signal from the target. Very small targets have a very short TC. The response of the target has a duration according it's TC. Whatever method you use to capture this response may distort or mask the original response, but the original response remains the same.
The preamp bandwidth I mention, was determined by trial and error. In the effort to reduce various noise at the output of the preamp, I experimented with the bandwidth. I could notice a reduction in the amplitude of the target response when the bandwidth went below 300kHz. At 150kHz the target signal attenuation was considerably more.
Using a bandwidth of 50kHz works fine for larger targets.
With traditional PI detector technology, it is well known that for short TC target detection, the coil needs to have a minimum self resonant frequency of about 300kHz. Higher self resonance allows for shorter target TC detection.
The rate of change, di/dt, in the coil current and it's magnetic field, generates the target eddy currents. This is the same with a step function like in PI, or with a continuously changing sine wave like in VLF, or any different TX wave form like in a hybrid.
Depending on the TX wave form, a method has to be devised to capture the target response.
The duration of the target response follows mostly (there are some exceptions) the exponential decay curve.
The generally accepted definition of a target TC is the time for the signal amplitude to fall about 64% from it's peak. A target of 1us TC will therefore best be detected a greater than 1MHz bandwidth.
I agree with the 3Hz and 12Hz you mention above, but this refers to the coil diameter and sweep speed.

Now, all this is my understanding of it. I am often wrong and greatly appreciate it when somebody corrects my mistakes.

Monolith

[moodz]

Thanks Tinkerer

I think it is time that you give credit to the originators of the method.
http://www.geotech1.com/forums/showp...&postcount=819

Tinkerer

I will tell you why later.

[simonbaker]

This is all very mysterious and exciting!

[mikebg]

Hi mikebg:

Can you define exactly what you mean by "air signal"? I think you defined these terms somewhere but I don't remember.

I keep hoping there is a way to make a great mono coil detector because so much easier to build mono coils. Or at least unbalanced two coil detectors.

What about electrically cancelling the air signal?

Regards,

-SB

Hi Simon. To increase sensitivity of RX (gain of preamp), we need electrically cancelling the AIR & GND signal.
AIR signal - voltage in input of RX preamp when search head is enough away from the ground.
AIR & GND signal - voltage in input of RX preamp when search head is in search position (close to the ground).
http://australianelectronicgoldprosp...2907/#msg12907
Unlike MONOCOIL, an induction balanced RX loop receives suppressed GND signal when it is generated by conductive soil. However there is no improvement when the GND signal is generated by ferrite soil. Only twin RX loop configuration can suppress the GND signal at all types soil.

[simonbaker]

Hi Simon. To increase sensitivity of RX (gain of preamp), we need electrically cancelling the AIR & GND signal.
AIR signal - voltage in input of RX preamp when search head is enough away from the ground.
AIR & GND signal - voltage in input of RX preamp when search head is in search position (close to the ground).
http://australianelectronicgoldprosp...2907/#msg12907
Unlike MONOCOIL, an induction balanced RX loop receives suppressed GND signal when it is generated by conductive soil. However there is no improvement when the GND signal is generated by ferrite soil. Only twin RX loop configuration can suppress the GND signal at all types soil.

Ok thanks.

However, I don't understand your point about "suppressing" the GND signal. Conductive soil should look like a conductive target, but if the soil is very uniform, the motion filter will ignore it. Ferrite soil will also be ignored by motion filter (if it is uniform), but it will also be ignored by the ground channel even if it is not uniform.

The twin RX loop looks interesting, but more for suppressing EMI than ground perhaps???

Regards,

-SB

[Alexismex]

Only twin RX loop configuration can suppress the GND signal at all types soil.
Thank Mike i agreed 100%

[mikebg]

Ok thanks.

However, I don't understand your point about "suppressing" the GND signal. Conductive soil should look like a conductive target, but if the soil is very uniform, the motion filter will ignore it. Ferrite soil will also be ignored by motion filter (if it is uniform), but it will also be ignored by the ground channel even if it is not uniform.

The twin RX loop looks interesting, but more for suppressing EMI than ground perhaps???

Regards,

-SB

SB, the sensitivity of a metal detector depends on gain of RX preamp. You can increase the gain until the AIR & GND signal starts to saturate output. The gain of preamp in an ideal designed metal detector is limited only by thermal noise:

http://www.geotech1.com/forums/showt...eferrerid=2910

Alexismex, unfortunately the suppression of GND signal by twin loop is far from perfect. Even if the operator try to keep both RX loops in equal height, the machine will detect small rocks buried in the soil. I use twin loop coil configuration to detect very valuable nonferrous meteorites, which can't generate signal at air test. However my friends call my meteorite locator "Rockfinder" becase until now it has not find such meteorite, but I have a collection of hundreds small rocks.

[simonbaker]

SB, the sensitivity of a metal detector depends on gain of RX preamp. You can increase the gain until the AIR & GND signal starts to saturate output. The gain of preamp in an ideal designed metal detector is limited only by thermal noise:

http://www.geotech1.com/forums/showt...eferrerid=2910

Alexismex, unfortunately the suppression of GND signal by twin loop is far from perfect. Even if the operator try to keep both RX loops in equal height, the machine will detect small rocks buried in the soil. I use twin loop coil configuration to detect very valuable nonferrous meteorites, which can't generate signal at air test. However my friends call my meteorite locator "Rockfinder" becase until now it has not find such meteorite, but I have a collection of hundreds small rocks.

I'm still not perfectly clear on your definition of AIR and GND signal.

Do you mean the residual "null" signal left over when we try to balance our IB coils but can't eliminate entirely?

I agree that if you could achieve a zero null signal you would have more head-room for your pre-amp.

However there are two considerations:

1. "Ground noise" : I define ground noise as fluctuations in the ground composition that modulate the RX signal phase same as a target. Untimately, this ground noise will mask weak targets and no amount of pre-amp gain will help anyway.

2. With a motion detector, the pre-amp only provides part of the gain -- before phase demodulation / target detection. After detection, a band-pass amplifier section can take out the air/ground signal and amplify the target signal to any degree you want, limited by circuit noise.

I'm still interested in the double RX coil and how it works. How does it work, i.e. what is it supposed to do?

-SB