Frequency Domain and Time Domain relationships

[Ferric Toes]

BTW,
I could (note: could) analyse the loop coils in respect to "antenna" features using the free NEC2 tool.
But it would require to become an "armchair engineer" first and a lot of work would be involved too.
It's far easier to plug the coil - I mean the loop antenna - into a LF radio as Eric did.
But you can't hear the VLF (and below) band region, which is of more interest.
Aziz

The frequencies 75kHz and up are of interest because they still fall within the bandwidth of the preamp. AM broadcast transmitters do cause interference to PI's as you can see, and hear, if you "tune" the TX frequency as a BFO. Fast sampling, higher rep rates, etc will not make things better. I have a dual 12in differential coil so I will connect up to a differential preamp, and hence to the radio and hear what happens. Will it be just the soft hiss of resistor noise, or Radio Beijing that I hear reporting how the economy is improving due to large scale manufacturing and export of metal detectors using foreign IP?

Eric.

[Carl-NC]

To Qiaozhi:
1. Every competent designer knows that resistance of RX coil generates noise.
2. Every competent designer should know that R28 also generates noise.
3. Every competent designer knows how to calculate resistance of R28
in order not to significantly increase the input noise: R28 is equal or < r_coil.
4. Every competent designer knows that sensitivity of a metal detector depends on gain of preamp.
5. Every competent designer knows that the gain of this preamp circuit is R29/R28 = 100k/13k=7.7 times.
CONCLUSION: The preamp of 1220B operates, but who will say that it is competent designed.

http://www.vbox7.com/play:c0b3781c
http://www.youtube.com/watch?v=yG5e1oaen-M

To WM6: See point 4.

I suspect if you take a C-scope 1220B and replace R28=100, R29=769, C6=13u, and C7=8.45n you will not get any performance increase at all. You'll just have a hard time finding the caps.

[mahavir]

Hi friends
Plz help me which metal detector is best from this.
. http://www.geotech1.com/cgi-bin/page...e=projects.dat

[Aziz]

BTW,
I could (note: could) analyse the loop coils in respect to "antenna" features using the free NEC2 tool.
But it would require to become an "armchair engineer" first and a lot of work would be involved too.
It's far easier to plug the coil - I mean the loop antenna - into a LF radio as Eric did.
But you can't hear the VLF (and below) band region, which is of more interest.
Aziz

[moodz]

...very nice radios there FT ... I use a "magnetic loop" on 3.5 Mhz .... its a single turn of copper water pipe 1.8 meter in diameter with 350 puff tuning cap. I remember from radio direction finding days using various coil configurations you could get deep nulls but not down to resistor noise levels LOL.

[Ferric Toes]

Aziz are you sure you have not turned into an armchair engineer by mistake .... did I tell you i design antennas for hf radio as well ... the coil does not exist which will have lower noise input than the input terminated amplifier ... in fact the noise figure for the system will be dominated by the first amplifier noise if you can ever get your shielding good enough ( you will need triple shielded mu metal 2.5 cm thickness each shield ).
When you have actually built a working detector off the bench then you will see that resistor noise is 30 db at least below your other problems.

I am currently using a double shielded 12in 300uH coil as a radio antenna. It works superbly and I can hear far more stations than with a 10m wire strung across my workshop. Reason is that there is a lot of e field em which the loop antenna rejects. It would be interesting to make a second indentical coil and use as a differential arrangement with a suitable preamp. I wonder what we would hear if the radio is then connected to the preamp output. Let's say between 2MHz and 75kHz, which is as low as the lower (black) radio will go. Who says I will hear nothing?

Eric.

[Aziz]

When I hear people talking about resistor noise as a problem ... they are either catalogue engineers OR they have solved all the other more important metal detector design problems ( give me a break LOL ).

I'm sorry, but I'm only a "trial & error engineer" (I'm not listed on the catalogue anymore).
Indeed, I have solved even more design problems. That's the reason, why I'm talking about resistor noise now.
*LOL*

Low-noise design is easy to understand and can be implemented quickly & cheap. Why shouldn't it be applied in the first instance too?
Aziz

[Aziz]

Aziz are you sure you have not turned into an armchair engineer by mistake .... did I tell you i design antennas for hf radio as well ... the coil does not exist which will have lower noise input than the input terminated amplifier ... in fact the noise figure for the system will be dominated by the first amplifier noise if you can ever get your shielding good enough ( you will need triple shielded mu metal 2.5 cm thickness each shield ).
When you have actually built a working detector off the bench then you will see that resistor noise is 30 db at least below your other problems.

Up to <3 war??? (turn your head left by 90 degree) Ok.
I very likely designed and built more RF antennas than you!
I even programmed a tool for 4NEC2, which made the antenna geometry definitions parametric via variables and made the antenna optimization easily possible.
I programmed an antenna programming language.
I even programmed a booster for 4NEC2 to use more CPU-cores.
I invented some other antennas.
I know of a lot of different RF antenna types.
I even made radar & satellite antennas.
I made antenna feeders.
And I have already used maghemite and AL-foil shielding for a MD coil (maghemite painted over Al-foil).

(Ok, this <3 battle makes no sense... I'll give it up. )

Of course, you're right. We have a lot of EMI noise, which dominates over the resistor noise in some cases. The anti-interference configuration however can reduce this EMI noise pickup by 20 dB - 30 dB (or even more).
Then there will be a measurable difference, whether you use a 4nV/sqrt(Hz) or 1nV/sqrt(Hz) amplifier.

Cheers,
Aziz

[moodz]

beeeks is right ...the end to end system is where the design focus should be ... the wright brothers did not try to build a jetfighter when they built their flyer ... I have 20 year old hewlett packard test equipment ( obviously using 20 year old technology ) that works better today than most present day engineers could design using latest components available ( ie catalogue engineers ) mainly because analogue design is poorly taught nowadays.

When I hear people talking about resistor noise as a problem ... they are either catalogue engineers OR they have solved all the other more important metal detector design problems ( give me a break LOL ).

[moodz]

With the use of anti-interference coil configurations, resistor (amplifier) noise does matter.
Aziz

Aziz are you sure you have not turned into an armchair engineer by mistake .... did I tell you i design antennas for hf radio as well ... the coil does not exist which will have lower noise input than the input terminated amplifier ... in fact the noise figure for the system will be dominated by the first amplifier noise if you can ever get your shielding good enough ( you will need triple shielded mu metal 2.5 cm thickness each shield ).
When you have actually built a working detector off the bench then you will see that resistor noise is 30 db at least below your other problems.

[Aziz]

Really? This means that you are able to make such induction balance that AIR signal to be commensurate with the level of noise. I can not believe until you show a picture of the output voltage of the RX preamp.

I didn't say this. You have misunderstood it.

Perfect induction balance nulling isn't required.
Aziz

[mikebg]

With the use of anti-interference coil configurations, resistor (amplifier) noise does matter.
Aziz

Really? This means that you are able to make such induction balance that AIR signal to be commensurate with the level of noise. I can not believe until you show a picture of the output voltage of the RX preamp.

[Aziz]

With the use of anti-interference coil configurations, resistor (amplifier) noise does matter.
Aziz

[vbeeeks]

Although I am not master /guru of electronics but what MikeBG is referring to are design problems that can be well understood by peeking in Application Note in the following link - as Moodz rightly points out - we have to look at the system noise as whole from input to following signal chain till the end, maybe I am wrong or seriously lacking in my observations, it maybe even off topic. Here goes the link -
http://www.analog.com/static/importe...es/MS-2066.pdf.

[moodz]

To Qiaozhi:
1. Every competent designer knows that resistance of RX coil generates noise.
2. Every competent designer should know that R28 also generates noise.
3. Every competent designer knows how to calculate resistance of R28
in order not to significantly increase the input noise: R28 is equal or < r_coil.
4. Every competent designer knows that sensitivity of a metal detector depends on gain of preamp.
5. Every competent designer knows that the gain of this preamp circuit is R29/R28 = 100k/13k=7.7 times.
CONCLUSION: The preamp of 1220B operates, but who will say that it is competent designed.

http://www.vbox7.com/play:c0b3781c
http://www.youtube.com/watch?v=yG5e1oaen-M

To WM6: See point 4.

.... noise from the resistors are the LEAST of your problems to be solved in detector design ... you would be pretty close to the UD if that was a problem in your system.