Gradiometer VCO problem.

[Geo]

Hi.
The VCO of my Gradiometer do not work good
I connect a digital voltmeter at the feedback of the AD557 (pin14) and i check the out of gradiometer. The balance is not perfect (i check it in the underground place beside the Lab (200 square meter free of big metallic objects). So when i have the Trshold at Max resistance (R6), the min out is 140mV. When i rotate it a little, and the voltmeter reads 148mv then the signal Led is going "On" but not audio . The beeps start at 270mV and stop at 770 mV. After it (out>770mV) no beeps, only the Signal Led is On.
When i adjust the TRShold at zero resistance, the out at pin14 is min 220mV and the signal led begin to light at 225mV. The beeps start at 760mV and stop at 2V. After it (out>2V) very-very low volume audio, only the Signal Led is On.
So i have 2 problems... no audio at small signals and no audio at very strong signals.
Can somebody to tell me how his Gradiometer works???
Regards

[damiafix]

Wonderful words! I've the same problem when I use it in the field!

All the best, D.

[Krzysztof]

Hi Damiafix, Geo.
Dear users.
I think that is normal.
FMX-1 has a narrow scale due to simple design.
I proposed a reduction of R6 and attach additional 2.2 Kom to GND to increase the charge current C5 at the beginning of the measuring scale.
If you work on large signals are continuously increasing R6 range of audible frequencies move down, while an increase in signal level.
I think that after proper calibration in the field, in an upright position without leaving MAG get a signal in the area "0".
Then the low value of R6 can distinguish small signals.
In the actual signal measured after calibration with the official turning points MAG approximately 70% (because 100% is in position 57deg angle to the line N-S), (in a horizontal position on the line N-S is about 35%).
The specificity of the measurement FMX-1 is a close position vertical, the same height above the ground, the measurement set in one geographical direction.
It really gives the results despite the resolution of 8bit (256).
Good luck. Chris.

[damiafix]

Dear Chirs,
I'm back today morning from another test in the field, the first problem is that the sensors must to be aligned every time, and every time for to get a good aligment, is necessary to move or rotate the sensors, for to hear a low frequency or to get a low voltage (ideally , 0v) on the DAC PIN14/15 and GND.
Certainly, this results depends from my errors, but I've noted that with the resistor of 2.2kohm inserted between the DAC Pin14/15 and GND, is very hard to understand, through the speaker, when the two sensors are perfectly aligned, becouse the frequency is most hight than without, and with a good aligment I should to get a low frequency, close the silence (in the ideal case); so I'm back in the "laboratory" I've removed the resistor, and I went again to the field.
In the field, without this resistor, I get finally a partial aligment of the sensors, but the performances of the gradiometer are very low, so I know that with the 2.2kohm resistor the performances would be better, but I think that should to benecessary to do some fix to the original design!
A solution could to be, insert the resistor with in serial a switch, for use it, after the calibration phase, in the field, during the research.
Or with the resistor, insert a meter, for to read the voltage, and understand that the alignment is ok, when the value is most close the 0v.
What do you think?


Best regards, Dam.

[Krzysztof]

Hi Damiafix.
Be sure to adjust the FOT3, 4 and 5 from a post on the calibration.
Bow down to compensate for sensor will result in increasing sensitivity.
So step by step several times, checking the "0" on lines indicate the normal W-E and vertically.
If you have a R6 Simply add 2.2 Kom and you have full adjustment to the test.
There must be a switch, it covers practically everything.
Best regards Chris.

[damiafix]

Thanks, I'll try and I'll let you know the results, I hope in improvements!

Have a good day, D.

P.S. In every case, after closed this project I would to start with the EPE Magnetometer, I've read about many users that say well of it.

[damiafix]

I've resolved the VCO problem, with the calibration and a better aligment, and with the insert of a 2.2kohm resistence between the DAC PIN14/15 and GND (after the suggestion of Chris). Read here for the details.
Better performances, but still low sensitive for a real research usage in field.
Are still necessary other test, per understand if this tool could still to be improved, or if his limit is reached.

All the best, D.