Target response tester

[green]

First paragraph: I can simulate a target in spice with an inductor and resistor in parallel. Double inductance or halving resistance doubles time constant. A 1inch square of aluminum foil has a time constant near 1.75usec. Time constant doubles if width doubles(2inch square)or thickness doubles(2 layers). With the foil, is inductance doubling or resistance halving with the two examples? Or a combination of inductance increasing and resistance lowering? What is causing or why is the inductance and or resistance changing?

Found some discussion about the subject. http://www.geotech1.com/forums/showt...229#post238229
With the foil I think I can see the resistance reducing to half by doubling the thickness or width if resistance is measured edge to edge not across the face. Don't know if that makes sense.

[green]

I've thought about the thick lead disc vs. the 1 ounce coin ....

the circulating currents in the flat coin will be predominantly in the one flat plane; they will be that way initially, and will stay that way as they decay, hence the uniform 'slope'. The bulky lead disc may start off with currents circulating mainly in the plane, because of the way the detectors' coil / magnetic field pass through the sample. But it seems likely that they may not stay circulating that way, and could easily end up flowing in loops in a perpendicular plane, ie. through the bulk thickness. The currents will still decay (as the lead has electrical resistance)... but the magnetic field they generate will not be picked up (or picked up much less) because their plane is perpendicular to the detectors coil. It's the "coin-on-edge effect", on a microscopic scale.
This is just a thought .... I'm not sure what particular experiments you could try, to understand the behaviour. Would a second coil, at right-angles to the flat disc/ 1 ounce coin give the answer? A drawing would help explain ... the second coil is on the same plane as the flat coin, but oriented so it sees the coin as 'on-edge'. Edit , attached pic.

------------

And regarding your response tester: I was thinking of other ways to clean up the noise on your charts.
It's not hard to implement a simple ( or not-so-simple ) digital filter in Excel.
A simple one to try is: Filtered sample value = (0.25 x Previous sample value + 0.50 x Sample value + 0.25 x next sample value)

Example: if you have samples s₃, s₄, s₅ etc chronologically
then your new samples would be: S₄ = 0.25 x s₃ + 0.5 x s₄ + 0.25 x s₅
You could create dummy values s₀ = s₁, and s_last+1 = s_last to make the first/last S data points less duff.


( did you try the low-pass filter on the 'reference current' input of the log-amp, by the way?)

First paragraph: I can simulate a target in spice with an inductor and resistor in parallel. Double inductance or halving resistance doubles time constant. A 1inch square of aluminum foil has a time constant near 1.75usec. Time constant doubles if width doubles(2inch square)or thickness doubles(2 layers). With the foil, is inductance doubling or resistance halving with the two examples? Or a combination of inductance increasing and resistance lowering? What is causing or why is the inductance and or resistance changing?

[Skippy]

Interesting, roughly what I was expecting: affects the fastest responses a bit too much, but quite acceptable for the longer TC targets. That's why I suggested the twin-output option, I felt that a little bit of LPF would clean up the short TC, and more filtering would be useful on some of the longer TC targets. Another option is to have a pair of sockets (eg. turned-pin IC sockets) on 0.2" spacing, and plug in an 'appropriate' leaded cap to suit the test target TC.
I was originally contemplating using the second opamp just like the first, see attached cct sketch. But you could simply add the two pin sockets across the existing 16K opamp feedback resistor. That would then leave the second opamp free, for different gain-scaled outputs etc.

Looking at those three charts, I would think a filter time-constant of 0.5 microsec would be more appropriate for the 10 usec Nickel coin. And filter TC of 0.1 usec for the foil (about 1.5 usec TC ? ). And hence, based on "Filter TC = 0.05 x Target TC", the 25c 'quarter'coin (TC about 100us) could suit a filter TC of 5 usec.
TC = 0.1 usec is 10k & 10pF, or 16k & 6p8
TC = 0.5 usec is 10k & 47pF, or 16k & 33pF
TC = 5 usec is 10k & 470pF, or 16k & 330pF

Just some ideas to play with. Maybe drop your 10k filter resistor to 4k7 or 3k3, then C values would be 2 / 3 times greater.

[green]

Re: low-pass filtering, you have the option of using the second uncommitted op-amp in the log IC, so you could have dual outputs, such as unfiltered & filtered, or 'mild filtering' and 'heavier filtering', to suit the test / test target in question.

Used the second amp. 10k to +input, 150pf +input to common, -input and output connected together for output. CH1(first amp)not filtered, CH2(second amp)filtered. CH zero's offset 1 major division. Y axis, log out, .4v/decade, 25mv in=0v out.

[6666]

Thanks, and yes I have been taking vitamin C with added zinc; oregano oil, olive oil pressed by a friend in Cyprus, turmeric, ginger, garlic, and various other things to frighten it away. Next year I might opt for a conventional 'flu jab.

Eric.

Ok, thats a good start,
there are some other pre cautions you can take to stop getting re infected and prevent it hanging on,
change pillow case often
keep your tooth brush soaking in an preferably alcohol based mouth wash,
only drink from disposable cups and only use them once
use disposable plastic knife and fork and only use once
sneeze into a tissue
only use disposable tissues to blow your nose, and only use once
when you blow your nose make sure you close your eyes
after blowing your nose immediately wash hands with soap and water every time, most important !
dont touch your eyes with out covering hand with tissue
And get a quad strain flu jab, here in Australia this winter they are making an extra strong flu jab
I hate getting sick

[Skippy]

Re: low-pass filtering, you have the option of using the second uncommitted op-amp in the log IC, so you could have dual outputs, such as unfiltered & filtered, or 'mild filtering' and 'heavier filtering', to suit the test / test target in question.

[Ferric Toes]

Try those zinc tablets

Thanks, and yes I have been taking vitamin C with added zinc; oregano oil, olive oil pressed by a friend in Cyprus, turmeric, ginger, garlic, and various other things to frighten it away. Next year I might opt for a conventional 'flu jab.

Eric.

[Skippy]

"What was your reason for splitting R5?"
To reduce power supply noise entering the + opamp input of the logamp. I saw you weren't using the 2.5 Volt internal reference, (due to only using +/- 2.5V supply rails), so I thought the direct use of the supplies to inject the reference current and fine-tune the offset null, may be a possible source of noise. I hadn't seen any specific details about the power supply ... 3-terminal linear regulators? Bench PSU with 60cm long leads to the board? The purpose of using 27K & 27K is to make the capacitor more 'effective'. If it was across the 100R resistor, the cap would need to be large to do anything. Similarly, from the pot wiper to ground would also be low-ish resistance. But ( 27K + C ) would make a decent low-pass filter.

However, this is a bit academic, as I hadn't really studied all your charts, and failed to notice the tests with a simple capacitor-discharge were noise-free to very low input levels, a fair way below where you would notice it, due to the larger detector-related noise.

The 220pF across the 16K resistor was likely to be too much, intentionally. I was thinking that it would be better to start of with a value that did some filtering, but may upset the response, then reduce it progressively. Rather than start off with 10pF and keep increasing the value until it got too high. I didn't come up with 220pF in a very scientific way, I just guessed that a decay time 0.693RC of 2 microseconds would not be too excessive.

[green]

I made an error in my previous post, it wasn't the 'reference current' input, it was the offset trimming circuitry, specifically splitting R5 in two and decoupling the tap to ground.Example 27K & 27K, with 10 uF cap.
Also try a low-pass filter with the output opamp, add a cap across the 16K (15+1), probably something quite small, 220pF may be a place to start, it will be obvious if it mucks up the output curves, particularly the fast decay ones.

Hi Skippy,
Let me try this again. When I started with the log amp I picked 1 meg reference input resistor. If a 1k input resistor can cause a noise problem on the preamp maybe a 1 meg input resistor could cause a noise problem with the log amp. The reason for adding 10k in series with a 1u capacitor from junction to ground. Hoping to reduce resistor noise. Wasn't thinking of other benefits like reducing PS ripple and maybe others. What was your reason for splitting R5? I haven't been calculating noise level, not sure I know how. Let me try a couple of examples. Someone can correct me or give an example on how it should be done. Preamp: 1MHz response, gain=450, input noise=1nv/rt Hz. Input noise=1000000^.5*1E-9=1uv RMS or 6uv p-p. 6uv*450=2.7mv p-p output noise. A 1k input resistor would have 4 times the noise. Log amp: 1meg input resistor, noise=1000000^.5*140E-9=140uv RMS or 850uv p-p. If I wanted 4 decades with 2.5v input full scale, 250uv to 2.5v the noise level would be greater than the minimum scale. With 200k input resistor I ended with, 1000*60nv*6=360uv p-p still higher than the minimum scale. Don't need 4 decades because of preamp and coil noise so maybe adding the capacitor and resistor to the input wasn't necessary. Not sure any of this is correct or makes sense. How much lower than the minimum scale should the p-p noise level be?

Haven't tried the 220pf capacitor yet but I would expect 48kHz cutoff to effect the decay.

[6666]

The 'flu is still hanging on so no further progress on my log amp. I'll get back to it as soon as I can. Soil magnetics ought to be a separate subject anyway as the responses are quite different from conductive objects.

Eric.

Try those zinc tablets

[Ferric Toes]

Are we going to move back to the original "detection depth" thread, now you seem happy with your log-amp setup?

http://www.geotech1.com/forums/showthread.php?24020-detection-depth/page6

This thread can still run, Eric is working on some projects, including a logamp.

The 'flu is still hanging on so no further progress on my log amp. I'll get back to it as soon as I can. Soil magnetics ought to be a separate subject anyway as the responses are quite different from conductive objects.

Eric.

[green]

Some pictures of 1x1inch regular strength aluminum foil. Did a test the other day where I was thinking operating the scope on average was helping, didn't think so before. Average does work on the scope screen but not storage unless I'm doing something wrong. If I were to purchase another scope I would look for one that averaged storage.

[green]

I made an error in my previous post, it wasn't the 'reference current' input, it was the offset trimming circuitry, specifically splitting R5 in two and decoupling the tap to ground.Example 27K & 27K, with 10 uF cap.
Also try a low-pass filter with the output opamp, add a cap across the 16K (15+1), probably something quite small, 220pF may be a place to start, it will be obvious if it mucks up the output curves, particularly the fast decay ones.

I was thinking R4 and R5 are in parallel so the input resistance is less than 100 ohms and splitting R5 wouldn't make a difference. The noise is coming from the preamp and coil, no noise with capacitor input decay(4 decades)

[Skippy]

I made an error in my previous post, it wasn't the 'reference current' input, it was the offset trimming circuitry, specifically splitting R5 in two and decoupling the tap to ground.Example 27K & 27K, with 10 uF cap.
Also try a low-pass filter with the output opamp, add a cap across the 16K (15+1), probably something quite small, 220pF may be a place to start, it will be obvious if it mucks up the output curves, particularly the fast decay ones.

[green]

I've thought about the thick lead disc vs. the 1 ounce coin ....

the circulating currents in the flat coin will be predominantly in the one flat plane; they will be that way initially, and will stay that way as they decay, hence the uniform 'slope'. The bulky lead disc may start off with currents circulating mainly in the plane, because of the way the detectors' coil / magnetic field pass through the sample. But it seems likely that they may not stay circulating that way, and could easily end up flowing in loops in a perpendicular plane, ie. through the bulk thickness. The currents will still decay (as the lead has electrical resistance)... but the magnetic field they generate will not be picked up (or picked up much less) because their plane is perpendicular to the detectors coil. It's the "coin-on-edge effect", on a microscopic scale.
This is just a thought .... I'm not sure what particular experiments you could try, to understand the behaviour. Would a second coil, at right-angles to the flat disc/ 1 ounce coin give the answer? A drawing would help explain ... the second coil is on the same plane as the flat coin, but oriented so it sees the coin as 'on-edge'. Edit , attached pic.

------------

And regarding your response tester: I was thinking of other ways to clean up the noise on your charts.
It's not hard to implement a simple ( or not-so-simple ) digital filter in Excel.
A simple one to try is: Filtered sample value = (0.25 x Previous sample value + 0.50 x Sample value + 0.25 x next sample value)

Example: if you have samples s₃, s₄, s₅ etc chronologically
then your new samples would be: S₄ = 0.25 x s₃ + 0.5 x s₄ + 0.25 x s₅
You could create dummy values s₀ = s₁, and s_last+1 = s_last to make the first/last S data points less duff.


( did you try the low-pass filter on the 'reference current' input of the log-amp, by the way?)

Present schematic of log amp. The 2 diodes(D1)on output prevent a problem with the scope over ranging with .2v/div when log amp out goes to -2.5v. I'll try some of your other suggestions.