Tweet
4. Enable TX
Balance AIR signal.
Set GAIN and TX POWER to minimum and enable TX to operate.
Now you are receiving the AIR signal in addition to both signals EMI and Noise.
Since you are making the experiment in laboratory, where search coil is far from earth, there is no GND signal. In practice all metal objects in your laboratory generate something like GND signal, but it is stabile because search coil not moves.
Let suppose that metal objects are so far from coil, that when you enable TX, it creates only AIR signal. To avoid saturation in output of RFA, start with low TX POWER and low GAIN settings. However to avoid saturation you have not only these two buttons. All buttons shown in posting #1 can help you to avoid saturation and distortion, ie you can use BALANCE X and BALANCE Y.
The purpose of adjustment described is this thread is to set RFA to operate at the threshold of saturation with minimal power of TX. This is executed perfectly by the AGC described in thread
http://www.geotech1.com/forums/showt...eferrerid=2910
However, since you have manual GAIN control instead AGC, you are forced to rotate the GAIN button until "saturation alert circuit" shows that gain is set too high. Then you must rotate X and Y BALANCE buttons to stop saturation alert.
The oscope shows combination of three signals in output:
- AIR, caused a) by mutual inductance and capacitance between TX and RX coils and b) by eddy currents environment (mainly in coil shielding),
- EMI, caused by power mains, by nearby metal detectors and by radio stations, and
- N, internal noise generated mainly by components of RX front end.
The X and Y BALANCE buttons can balance only AIR signal. When you balance AIR signal to zero, EMI and N signals remain. That's why to operate RFA at maximal high GAIN, you have make improvements described in posting #5.
How seems AIR signal in complex plane?
In the attached diagram are shown two AIR signals caused by mutual inductance M only. Signal OM+ shown in left figure is generated by positive mutual inductance M. It has phase lead 90 deg relative to TX coil current. Self-inductance of TX coil also generates as OM+ signal.
Signal OM- shown in right figure is caused by negative M. It has phase lag 90 deg relative to TX coil current.
Note that phase of AIR M signal is independent on frequency. The phase is always in quadrature to TX coil current.
You can change the sign of mutual inductance M by several ways:
- swaping leads of RX or TX coil (one coil only),
- relative shifting between coils,
- coil shape deforming and
- placing a ferrite core in suitable position between RX and TX coils.
Not only mutual inductance generates signals in quadrature to TX current; ferrite core, "hot rock" and nonconductive ferromagnetic (mineralized) soil also generate signals in quadrature, but these are TGT and GND signals.
Now try to answer a question:
"How to choose minimal TX power in the case when we have small AIR signal but GND signal is zero?"
HINT: In this case we have in the input three signals: AIR, EMI and N (noise). With button TX POWER we can change only AIR signal. The EMI and N remain unchanged.
Balance AIR signal.
Set GAIN and TX POWER to minimum and enable TX to operate.
Now you are receiving the AIR signal in addition to both signals EMI and Noise.
Since you are making the experiment in laboratory, where search coil is far from earth, there is no GND signal. In practice all metal objects in your laboratory generate something like GND signal, but it is stabile because search coil not moves.
Let suppose that metal objects are so far from coil, that when you enable TX, it creates only AIR signal. To avoid saturation in output of RFA, start with low TX POWER and low GAIN settings. However to avoid saturation you have not only these two buttons. All buttons shown in posting #1 can help you to avoid saturation and distortion, ie you can use BALANCE X and BALANCE Y.
The purpose of adjustment described is this thread is to set RFA to operate at the threshold of saturation with minimal power of TX. This is executed perfectly by the AGC described in thread
http://www.geotech1.com/forums/showt...eferrerid=2910
However, since you have manual GAIN control instead AGC, you are forced to rotate the GAIN button until "saturation alert circuit" shows that gain is set too high. Then you must rotate X and Y BALANCE buttons to stop saturation alert.
The oscope shows combination of three signals in output:
- AIR, caused a) by mutual inductance and capacitance between TX and RX coils and b) by eddy currents environment (mainly in coil shielding),
- EMI, caused by power mains, by nearby metal detectors and by radio stations, and
- N, internal noise generated mainly by components of RX front end.
The X and Y BALANCE buttons can balance only AIR signal. When you balance AIR signal to zero, EMI and N signals remain. That's why to operate RFA at maximal high GAIN, you have make improvements described in posting #5.
How seems AIR signal in complex plane?
In the attached diagram are shown two AIR signals caused by mutual inductance M only. Signal OM+ shown in left figure is generated by positive mutual inductance M. It has phase lead 90 deg relative to TX coil current. Self-inductance of TX coil also generates as OM+ signal.
Signal OM- shown in right figure is caused by negative M. It has phase lag 90 deg relative to TX coil current.
Note that phase of AIR M signal is independent on frequency. The phase is always in quadrature to TX coil current.
You can change the sign of mutual inductance M by several ways:
- swaping leads of RX or TX coil (one coil only),
- relative shifting between coils,
- coil shape deforming and
- placing a ferrite core in suitable position between RX and TX coils.
Not only mutual inductance generates signals in quadrature to TX current; ferrite core, "hot rock" and nonconductive ferromagnetic (mineralized) soil also generate signals in quadrature, but these are TGT and GND signals.
Now try to answer a question:
"How to choose minimal TX power in the case when we have small AIR signal but GND signal is zero?"
HINT: In this case we have in the input three signals: AIR, EMI and N (noise). With button TX POWER we can change only AIR signal. The EMI and N remain unchanged.
Comment