Ground Balance

[Mr.Jaick]

Hello everyone
my question is:
How can we implement Auto ground balance/Tracking in PI realm?! I mean the concept of it not the actual circuit.
for sake of simplicity let's say we have one target channel and one ground channel
how do we continuously subtract just enough of the GEB signal from the Target signal without loosing sensitivity or getting false ground signals at the output of the target channel?
and can we do something to not track out the good targets?
the logic of it just isn't clear to me
any help is appreciated, if it's not a proprietary thing

[Carl-NC]

Hi,Carl

Can this be understood as the duration of each signal on RX will be different?

EF>GB>Target

Target samples too close to GB samples will affect sensitivity, and too far away will affect GB, right?

Correct.

[lc516]

Just the opposite... mineralization decays rather quickly. Here is how the TDI does it (this is all public knowledge, so NBD):

Turn TX on for 100us
Shut off, enter flyback
Wait 10us, take the Target sample
After the end of the Target sample, wait a short delay, take the GB sample
After the end of the GB sample, wait a long delay, take the two EF samples
Use a diff integrator to subtract Target-EF1
Use another diff integrator to subtract GB-EF2
Subtract A1*(Target-EF1) - A2*(GB-EF2)
Adjust A2 for ground balance

In TDI, all the samples have identical widths. I could tell you what to use, but you would learn more trying a range of settings, say 5us up to 25us. The delay between Target and GB samples as well; try 5us to 25us. Technically, this same delay should be used to space the two EF samples. The spacing between GB and EF1 samples should be 50us or more.

There are other ways to do GB, even simpler, but this one has particular advantages that TDI uses to produce distinct audio responses.

- Carl

Hi,Carl

Can this be understood as the duration of each signal on RX will be different?

EF>GB>Target

Target samples too close to GB samples will affect sensitivity, and too far away will affect GB, right?

[6666]

You can invert the polarity of a signal by swapping the timing, such as swapping SW2 and SW4.

Hi Carl
Thanks for that tip, could come in handy in near future.
Been messing with some code for a bipolar pulsar
does it look sorta right ?

Sequence is
TX1 100uS , first transistor
wait main delay (variable ) 10-20uS
SW1-A 10 uS main sample pulse out
wait 20 uS to 1st GB pulse
SW2-A 100-200uS pulse out
wait 250 uS to process signal

TX2 100uS, second transistor
wait main delay (variable ) 10-20uS tracking first main delay
SW2-B 10uS main sample pulse out
wait 20 uS
SW1-B 100-200uS, GB pulse out, width tracking same width as SW2-A
wait 250 uS to process signal

Repeat at TX1

[Carl-NC]

What matters is that you subtract the ground signal from the target signal, and there are several options. If they are both positive signals you subtract them, or invert one of them and then add. If they are opposite polarity already you just add. You can invert the polarity of a signal by swapping the timing, such as swapping SW2 and SW4.

[6666]

Hi Carl
another question please, with your 4 sample duel integrator GB circuit (attached) does it matter if the variable gain amplifier after the integrator is inverting or non inverting ? thanks.

[Carl-NC]

Yep, that's it.

[6666]

Thanks Carl
yes the sequence is sw1-sw2-sw2-sw1, and I now have second sw1 tracking the first sw2 WIDTH, and moved the EF pulses out further.
The video shows the two long pulses tracking in width. Does this look a bit better.
https://youtu.be/J8MGv5RZg6U

[Carl-NC]

I assume the sequence is SW1-SW2-SW2-SW1. As Davor said, the 2nd SW1 width needs to track the first SW2 width. Also, I would move the EF samples as far away from the main samples as possible, to minimize depth loss.

[6666]

Thanks Davor

[Davor]

This seem to be correct, provided EF pulse duration is balancing the previous pulses. This variation is easier to implement in a micro than my analogue solution with initial delay variation.

[6666]

Hi Carl
is this something like what should be happening with sw1 and sw2 ? for the single int
you will see the 100uS tx pulse at the top and sw1 and sw2 at bottom
the width of the first long pulse on sw2 is being altered with a pot the rest are fixed ?
The long pulse is going from 100uS to about ~180us.
dont take any notice of the distortion of the bottom of the wave form
this is caused by combining the sw1 and sw2 waveforms with diodes and resistor for cro.
The camera is not square onto the cro screen.

https://www.youtube.com/watch?v=NvmH...ature=youtu.be

[6666]

Thanks Carl for reply, will give it a try.

[Carl-NC]

Yes, that's correct. The exact timing requirements depend on a lot of things, like coil inductance, damping, and preamp compensation, but here is a rough example:

Target sample: turn on at 15us, turn off at 20us
Ground sample: turn on at 25us, turn off at 40-50us

[6666]

Hi Carl
do I understand your GB description correctly , that when you use a single ended integrator
the first pulse on switch 2 has variable width and that is the GB control ?
Would you mind suggesting some timings please ?
Thanks