DEEPER PI DETECTION DEPTH

[Aziz]

True, but early sampling means high signal and no problem with noise.
Temperature drift is a slow one, and it would matter if you need to read signal strength in decibels without calibration. Otherwise it is a non-issue.
I'm in a process of obtaining an oscilloscope at the long last, and my lab will then start to look up. I'd be very glad to do my own tests. You can't imagine how it is like to be lab-less. Like a thirsty person in a desert without a drop of water in my case.
If you observe the curve obtained by the log part of the plot, you'll see quality curve splitting starting at ~4us (~2V @ coil in this particular case). If I start sampling at ~5us (~350mV @ coil) and onwards I'm out of the woods, and supposedly it will work great for detection of small nuggets. It is not that high up at all, and simultaneously it is far away from noise as well.

Yep, I know what you're meaning with to be lab-less (oscilloscope and other instruments).

We can reduce the input impedance and the feedback resistance further to minimize noise. But we need more current drive of the op-amp. An additionally installed emitter follower transistor booster stage could deliver more current in this case.
And we can allow the sampling at much higher coil flyback voltage levels (that's the benefit of the inverting amplifier configuration).

I'm sure, the temperature drift would be the biggest drawback. I should mod one of my inverting pre-amps to test this.
Aziz

[Davor]

Thanks for finding that, not much said. Yes complex, but thats what users are demanding in performance.

You mean they demand it for their money. It is difficult to sell a few bells and whistles for a bag of money.
What I like more with that design is that it uses simple solutions to do complex tasks. Otherwise it would not make sense. Complexity and reliability were never best of friends.
I also like the way they harnessed a piece of information - voltage zero crossing to gain ground information. Information was laying there and they picked it. Brilliant.

[Davor]

True, but early sampling means high signal and no problem with noise.
Temperature drift is a slow one, and it would matter if you need to read signal strength in decibels without calibration. Otherwise it is a non-issue.
I'm in a process of obtaining an oscilloscope at the long last, and my lab will then start to look up. I'd be very glad to do my own tests. You can't imagine how it is like to be lab-less. Like a thirsty person in a desert without a drop of water in my case.

There is a lot of response energy in the early decay time. But the reactive response (ground signals) are huge at this stage too, which makes ground balance difficult.

If you observe the curve obtained by the log part of the plot, you'll see quality curve splitting starting at ~4us (~2V @ coil in this particular case). If I start sampling at ~5us (~350mV @ coil) and onwards I'm out of the woods, and supposedly it will work great for detection of small nuggets. It is not that high up at all, and simultaneously it is far away from noise as well.

[Aziz]

Hi all,

ok, limitting the feedback loop impedance via resistor limits the gain and noise of the amplifier. But the high input impedance is still too much and is generating a lot of noise.

The biggest issue is the temperature drift. That's the reason, why professional log-amps do cost more as they are temperature compensated.

I suggest to test the simple log-amp and let's see, what is obviously predictable.

Aziz

[Altra]

Thanks for finding that, not much said. Yes complex, but thats what users are demanding in performance.

[Davor]

It started at this very topic at page 17, post #408

Truly impressive design. OK, a tad too complex for my taste, but impressive nevertheless.

[Altra]

Davor,
Do you have a link to where the above patent application was discussed? I know its a continuation of Earle's 2002 application.

Thanks

[golfnut]

Id of thought a log amp was a good choice in a detector ... Because one minute your getting a signal marinally above noise floor and need to detect it. The next moment your getting a booming great signal from a flat cola can 10mm down and saturates your system for a period.

Log amps are used in commercial detectors already..


S

[Davor]

We've seen it already. It is a brilliant design. It uses two high voltage sources to generate a bipolar kind of TEM, detects a phase at the top, and then cuts it short just before it reaches zero to obtain a short PI pulse. Just brilliant.

Are you sure Davor ? .... Log of 1.0000 = 0 and log of 1.0001 is 0.00004342 ... In other words by logging the value you made the difference even smaller. ... A log amp is a compression amp. .. Not good for seeing small differences ....

Regards moodz

There is another view of this very example you put here, and it goes:
log 1.0001 = 0.00004327
log 10.001 = 1.00004327
log 100.01 = 2.00004327

That's why it is so good for low index AM detection.

[Altra]

Just found this application by Whites, looks pretty slick.

I posted it here because Carl's and Tinkerer's first few
posts in this long thread.

[Davor]

There are several good points to it, and I'll try to elaborate a bit. My gut feeling says that PI demodulation can only benefit from applying some appropriate weighting function to a Rx signal that will flatten its response over time. My first thought was a ramp or a sinus. There are some good points to these. Ramp is feasible via VCA, and its family, while sinus is feasible via LC tank. Trouble with linear ramp is that it will not help much beyond, say, one decade of signal span, and it is not flattening the signal a lot. Non-linear ramp is readily available as a log-amp, and here you go. The sinus weighting was already patented and as far as I know did not fare too well, partly because the time constant of a tank was IMHO far too short. I'll get back to that solution eventually.

Common feature of all log amps is flat top and bottom where the log curve hits its limitations. On the bottom side there is a (mostly) linear part that is usually buried in noise .. but not necessarily so. With the design I posted previously I can decide how deep the curve follows the log law. By addition of a single resistor in the existing design in a feedback path I can assure a fixed gain for low signals, and detection as usual. 2.2Meg fixes gain to 100 which is same as the "Surf" counterpart below. I traced the response without the resistor for comparison (dotted line). There is something else apparent there: the response is not symmetrical against "Surf" response, and "Surf" response (for no apparent reason) quenches much faster.

I could speculate about the "Surf" result falling short, but it seem as if the 1k in series with limiting diodes in effect damps a coil a bit too much at higher signal levels, so there's not much energy left for it to do its thing. One may guess its role in noise, but I don't expect much difference with larger one because it goes to a hi-impedance non-inverting input.

Anyway, here is a log-lin amp with max gain of 100.

[Aziz]

Lol. Not that I'm pro or con logamp (honestly I'm well out of my depth). But if you start applying Occam's razor to the effect of "The simplest reason that no one has ever done it, is because it doesn't work" Then you've pretty much put a halt to ALL innovation.

"Common man everyone knows lightning is the only way to get fire. If it was as simple as rubbing two sticks together don't you think someone would have tried it by now?"

Midas

*LOL*

I was only showing the simple principle of questioning things of course. Why, why, why..?

However Davor principle tried to get max. target response energy. Nothing wrong with it. There is a lot of response energy in the early decay time. But the reactive response (ground signals) are huge at this stage too, which makes ground balance difficult.

BTW, the induction balanced coil types reduce the unwanted flyback decay voltage and the ratio of the signal/flyback voltage increases heavily.

Anyway.

The main core issues are:
- low noise designs
- avoiding pre-amp saturation/overloading
- synchronous demodulator
- filtering the output of the demod

Aziz

[Midas]

....
If log-amps would offer more benefit, it would have been used long before.
(Occam's Razor fact)
...

Lol. Not that I'm pro or con logamp (honestly I'm well out of my depth). But if you start applying Occam's razor to the effect of "The simplest reason that no one has ever done it, is because it doesn't work" Then you've pretty much put a halt to ALL innovation.

"Common man everyone knows lightning is the only way to get fire. If it was as simple as rubbing two sticks together don't you think someone would have tried it by now?"

Midas

[moodz]

I'd go along with Earl's observations, except that I'm not too much into the noise story. Yet....which you can only benefit from the log amp. Log amp detectors are famous for AM detection of low modulation indexes.

I still think the whole story about a nasty noise is far too much exaggerated because I can decide NOT to sample deep in noise but half a decade above. It is the slope that matters, and simulation shows nice splitting of curves waaaay above noise. Besides, even a traditional PI Rx does not go that deep into noise. There is a tendency to sample a bit earlier.

I think the perfect way of harnessing data from such a setup is by means of window comparators. Level reference instead of time, and time result instead of level. Two such windows could yield discrimination. Voltage reference for the windows can be set by diode forward voltage drop, hence a temperature compensation. Easy.

Log amps are a different pack of animals altogether, and they need different care and feeding. But they are not saturating. And they are darn fast.

Are you sure Davor ? .... Log of 1.0000 = 0 and log of 1.0001 is 0.00004342 ... In other words by logging the value you made the difference even smaller. ... A log amp is a compression amp. .. Not good for seeing small differences ....

Regards moodz

[Aziz]

Hi Davor,

why do you insist on the log-amps? There are better solutions available.
1. VCA (voltage controlled amplifier)
2. PGA (programable gain amplifier)
3. Fixed gain amplifiers (more cascaded stages of them)

The latter one offers you the best performance (noise & stability). If you need different gains, you could use more amplifier stages (2 or more). The VCA is critical and does not offer the benefit of the fixed gain amplifier. Note, that fixed gain amplifiers offer a stable loop-back regulation.

The problem with the MD signals is, that the ultra tiny response signal is "riding" on the high flyback voltage. Best to detect the response signal is to integrate over a window time and look at the change of it. The tiny integrated signal change can be detected easier at the end.

Why bother with high flyback voltage? You can use the IB coils.

If log-amps would offer more benefit, it would have been used long before.
(Occam's Razor fact)

Cheers,
Aziz