ground balance capability

[mohalothman]

mohalothman what you describe is perhaps something like this (by Eric Foster), would love to hear more!

http://www.geotech1.com/forums/showt...880#post224880

thanks eclipse
it is funny when you see the other people have already had your thoughts and ideas it seem like this people are stealing from the future
anyway this means you are in the right way

[mohalothman]

perfect result! wow! that i waited ))))

thank you

dude could be generous and point out the interesting things with the result (soil response) i try to learn here thanks

[kt315]

i try grasp why Ferric-Eric did run to high inductivity coil (1mH) and why this class of PIs is more productive from his position.
yes, you decrease much TX power (in coil)... but then you can increase PPS (to 10000).
this class must work with the soil... but i do not have GOLDQUEST SS, C-SCOPE 4PI etc to give you more analysis.

[Davor]

PI-s in general have less problems with 1/f noise and drift that cause chatters in VLF-s. So to reduce chatter, you decrease sensitivity, and there you have it. Sources of 1/f noise are Tx transistors operating in linear regime (emitter degeneration decreases 1/f noise, but it is there), and opamps in gain blocks, especially FET input ones.
A PI has no 1/f noise in Tx, and every decent PI has an EF pulse, which in effect turns a gain block into an auto-zero amplifier, and it eliminates 1/f noise and drifts.
Fix 1/f noise, and a VLF will beat a PI.

[mohalothman]

PI-s in general have less problems with 1/f noise and drift that cause chatters in VLF-s. So to reduce chatter, you decrease sensitivity, and there you have it. Sources of 1/f noise are Tx transistors operating in linear regime (emitter degeneration decreases 1/f noise, but it is there), and opamps in gain blocks, especially FET input ones.
A PI has no 1/f noise in Tx, and every decent PI has an EF pulse, which in effect turns a gain block into an auto-zero amplifier, and it eliminates 1/f noise and drifts.
Fix 1/f noise, and a VLF will beat a PI.

when i would talk about noise immunity , something about PI devises will bother me all the time that is the PI in general suffer the high frequency noise which its frequency range f_tx/2 and above and that according to Shannon sampling condition.
the sampling rate in PI devises is the transmit frequency (f_tx) and so to overcome the high frequency must use antialiasing filter which is not possible because we will lose the useful signal. of course digital signal processing with UC is a choice you can achieve some digital filers you can't do it in analog space.
maybe there are some techniques overcome this problem but i can't figure out how. what do u thing i'm right or not?

[Davor]

You are about half way there.
The useful signal is a metal target response, which has exponential delay when excited with a pulse. Its spectrum is largely irrelevant, and in effect what you see is a time window you take a sample with. Its frequency response is ~1/t, where t is its duration. Its frequency response is not directly related to PI repetition rate.

Interferers are various ground effects and also radio sources.

The main radio source, and the one related to PI repetition rate, is the mains EMI in form of many harmonics of 50Hz in Europe and 60Hz in US (etc.). Thwartingit is achieved by synchronising the repetition with a certain mains harmonic, and thus there are no beats due to the frequency difference. When synchronised we say such phenomenon becomes cyclostationary.

Ground responds either very quickly in cases of water and ferrites, or very slowly in case of viscous minerals such as maghemite. The quick ones burn out before you take the first sample, so PI eliminates the regular ground even before its response enters the Rx, and no GB is needed for regular ground. The viscous ground is a completely different animal. Various GB schemes exist, but I'll not go into details on that.

But I was talking about 1/f noise...

The 1/f noise is a consequence of various effects present in active components, e.g. FETs and BJTs. It manifests as rapid rise in noise as you go lower with frequency. It has a distinct "knee" and it is around 1kHz for FETs and 100Hz for BJTs. Trouble with 1/f noise is that target response amplitude falls to this range. You may safely say that all target amplitude responses fall below ~10Hz. Now imagine a Rx that produces a strong noise below 10Hz - it appears that there are targets popping all about, and even when your coil is static, you hear something going on. We call it chatters.
There are a few approaches to fix 1/f noise, but classic VLF devices use none. The solution with classic VLF-s is to reduce sensitivity until chatters become bearable, and that's where you lose depth.
One of the methods to fix 1/f noise is auto-zero amplifier. When you analyse a PI with EF pulse, you realise such PI is in effect an auto-zero configuration, and quite immune against 1/f noise and drift.

[green]

when i would talk about noise immunity , something about PI devises will bother me all the time that is the PI in general suffer the high frequency noise which its frequency range f_tx/2 and above and that according to Shannon sampling condition.
the sampling rate in PI devises is the transmit frequency (f_tx) and so to overcome the high frequency must use antialiasing filter which is not possible because we will lose the useful signal. of course digital signal processing with UC is a choice you can achieve some digital filers you can't do it in analog space.
maybe there are some techniques overcome this problem but i can't figure out how. what do u thing i'm right or not?

Tried a spice simulation for an integrator. Frequency cutoff about 5Hz each stage. Sample rate 1000pps. Sample time 100usec. Play with different sample times and different input frequency. Try multiplies of sample rate+5Hz. Frequency>1/sample time(10KHz) seem to get filtered. Something to play with. I need to play more to try and understand what is happening.

[green]

Wondered if I could calculate MPP amplifier out noise(TP3). Using National Semiconductor Application Note 104(Noise Specs Confusing). I calculate about 21mvP-P amplifier noise at TP3(Tx and Rx disconnected, Tx pin2 jumper to Rx pin2 removed, jumper Rx pin2 and pin3). Multiplied calculated RMS volts x6 to get P-P volts. Don't have a MPP, wondering if someone with a MPP could measure P-P volts with a scope at TP3 to see if my calculation is in the ball park or calculate amplifier noise to see if my calculation is close to correct. Been playing with 1C integrator, think I'm getting closer to calculating integrator out P-P noise volts for a 1C integrator. Still don't understand the MPP integrator, maybe someone could explain how to calculate P-P noise out knowing P-P noise in.

[green]

Found this noise thread. http://www.geotech1.com/forums/showt...9331#post39331

States there is little difference in noise, single stage vs dual stage. Would the P-P noise be the same if I scoped the output of each amplifier?