Ground Balance

[Carl-NC]

The second inverted output going to the 4066 has extra gain to compensate for a lower level of signal plus it allow the 10k pot to be adjusted more towards the S1 sample point, and this gives more response to wanted targets.

Normally you want the EF sample to match the main sample, that is, have the same gain and sample width. The idea being that the Earth field response is reasonably identical across the pulse period. Your adjustment method would probably work well for the GB sample, however.

[Mick-GD]

Normally you want the EF sample to match the main sample, that is, have the same gain and sample width. The idea being that the Earth field response is reasonably identical across the pulse period. Your adjustment method would probably work well for the GB sample, however.

Hi Carl, the first circuit I made used a unity gain inverting stage then for the adjustment I tried a lower value variable resistor 500ohms but for some reason couldn't get the EF to balance out, I then put a 10k in and got it to adjust to about mid point on the pot but then lost gain on the main sample channel due to the extra resistance. When I added gain to the inverting stage I was able to turn the pot more to the main sample channel thus having less resistance between it and the input of the integrator.

The main sample at moment is set to 8us and the EF1 sample I pushed out to 200us.

Any other ideas would be grateful



Cheers
Mick-GD

[Carl-NC]

How are you testing for EF cancellation?

[Mick-GD]

How are you testing for EF cancellation?

Coil movement in a horizontal plane.

Could you use a big magnet waved horizontal ?

[Carl-NC]

Yes, I would use a magnet.

[Mick-GD]

No worries I'll go find a few different magnets and do some more testing and report back shortly.

[Mick-GD]

Ok I found a few different magnets and all showed no response using the same circuit.

I then decided to have another look at the balance circuit. I changed the 10k resistor back to 3.9k on the inverting amp and changed the pot to a 500 ohm that I had tried earlier and I was also able to balance the EF1 out. All I can think of is maybe I had a wiring issue when I tried the 500 ohm pot the first time.

Once I have the GB and EF2 circuit built I'll post up a picture of the modified circuit.


Carl, with regards to the output of both the integrator's would it be best if I amplifiy the GB pre the GB/EF2 integrator or after when it becomes a DC value of both GB/EF2 ?

[Carl-NC]

Carl, with regards to the output of both the integrator's would it be best if I amplifiy the GB pre the GB/EF2 integrator or after when it becomes a DC value of both GB/EF2 ?

Usually the signal has been amplified to the point there isn't enough headroom for any more gain before the integrator. So the add'l gain is applied after the integrator. Or, you can apply the extra gain within the integrator itself.

[Mick-GD]

Usually the signal has been amplified to the point there isn't enough headroom for any more gain before the integrator. So the add'l gain is applied after the integrator. Or, you can apply the extra gain within the integrator itself.

I actually had the frontend rail today with a big target so I'm going to reduce the frontend gain a little and add the gain after the integrator.

Something else I noticed today when working on the target and EF1 sample circuit at one stage I had the circuit setup with only the EF1 sample connected directly to the integrator and notice it still picked up some bigger targets one was a big screwdriver and the other was a five ounce lead weight. I'm guessing we wan't to minimize target response in the EF samples because anything thats out that far should be rubbish only. The EF1 sample is set at about 200us at the moment so i'll adjust it out to 300 or 350 tomorrow and see what happens.

I should also mention at the moment I'm still running 100us TX and a receive time of 450us till next TX.



Cheers
Mick-DG

[Orbit]

Hello Mick ,I also tried something like with magnet
really function , but I tried to brick red with much particle iron ,and will not it has response .I think talk only about one channel ,if I understand your ,,

[Orbit]

provided you follow stability of output whether there chatter

[Mick-GD]

Hello Mick ,I also tried something like with magnet
really function , but I tried to brick red with much particle iron ,and will not it has response .I think talk only about one channel ,if I understand your ,,

Hi Orbit, the magnets worked well I was able to null out the EF (earth field) signal at the output of the integrator.

With the red brick test I could see an output being produced when the brick was waved over the coil I should also mention the red brick produced less responce than a one gram lead weight at the same distance, Once I have the GB and EF2 samples set up It should adjust out easily. Fingers crossed.



Mick

[Orbit]

That , I used a brick of 400gr ,of course with f2 channel you will get what expected ! a thank you Carl about clarification solutions about VCO and Threshold na TDI ,thank you once more Karll ...

[Mick-GD]

Hi Carl, back in post 24 I wrote that when looking at the EF1 sample separately and the delay set to about 200us larger targets lead and steel still get detected at a much lower level. I tried at 300us and the lead and steel targets still get detected but once the EF1 sample was set to 500us the lead is no longer detected but the steel target is still detected.


What I'm wanting to know is does the TDI have it's EF sample points out this far and or does it really matter if a target is being detected in the EF samples ?



Cheers
Mick

[Carl-NC]

TDI EF=75us. Long TC targets are still detected in the EF sample, but should always be substantially weaker than the main sample. The result is it doesn't matter much.