I recently had to make a number of extremely low level measurements, to evaluate the performance of the Nonoiser, and to test the X10 transformers (one physical and the other virtual) to enhance the sensitivity of my LNA.
One of the most difficult issue was the pervasive presence of 50Hz magnetic field + harmonics.
I had to resort to notch filters and digital filtering techniques to arrive at a not too polluted level.
I tried magnetic shields, but the field induces parasitic emf's in the test cabling, in particular the grounds, meaning there is a huge area to cover.
Now, I have experimented with an active shield, that counteracts the stray AC fields with an opposite drive.
The principle is well-known, but here it is a bit peculiar: both the sensor and actuator are simple coils, which is easy and cheap, but presents a number of challenges.
The setup necessarily operates in a closed-loop fashion, and stabilizing the whole thing for a useful frequency range is not an easy task: there are multiple poles to take into account, not only at the top of the spectrum, but also in the VLF region because the system does not pass DC, obviously.
As I hate to wind my magnetic components myself, I reused ready-made coils: the sensor is from the actuator of a HDD, and the drive coil is a degaussing harness from an old CRT monitor:
I measured the electrical and magnetic parameters of both: the sensor has an inductance of 950µH, a DC resistance of 8.2 ohm and a // capacitance equivalent to 18.5pF.
The degaussing loop has an inductance of 770µH and a resistance of 2.5 ohm.
The coupling coefficient is ~0.0056.
Here is the concept:
L1 is the drive coil, L2 is the sensor, and L3 generates the perturbating field.
The conditioning circuit is an integrating error amplifier, driving a transamp symbolized by G1.
Of course, this is only 1/3rd of a complete shielding system: you need two identical channels for the Y and Z axis, and if you want to do things really properly, a total of 6 drive coils in a Helmotz configuration is required
The loop gain reaches ~60dB @ 100Hz, which is very substantial.
One of the most difficult issue was the pervasive presence of 50Hz magnetic field + harmonics.
I had to resort to notch filters and digital filtering techniques to arrive at a not too polluted level.
I tried magnetic shields, but the field induces parasitic emf's in the test cabling, in particular the grounds, meaning there is a huge area to cover.
Now, I have experimented with an active shield, that counteracts the stray AC fields with an opposite drive.
The principle is well-known, but here it is a bit peculiar: both the sensor and actuator are simple coils, which is easy and cheap, but presents a number of challenges.
The setup necessarily operates in a closed-loop fashion, and stabilizing the whole thing for a useful frequency range is not an easy task: there are multiple poles to take into account, not only at the top of the spectrum, but also in the VLF region because the system does not pass DC, obviously.
As I hate to wind my magnetic components myself, I reused ready-made coils: the sensor is from the actuator of a HDD, and the drive coil is a degaussing harness from an old CRT monitor:
I measured the electrical and magnetic parameters of both: the sensor has an inductance of 950µH, a DC resistance of 8.2 ohm and a // capacitance equivalent to 18.5pF.
The degaussing loop has an inductance of 770µH and a resistance of 2.5 ohm.
The coupling coefficient is ~0.0056.
Here is the concept:
L1 is the drive coil, L2 is the sensor, and L3 generates the perturbating field.
The conditioning circuit is an integrating error amplifier, driving a transamp symbolized by G1.
Of course, this is only 1/3rd of a complete shielding system: you need two identical channels for the Y and Z axis, and if you want to do things really properly, a total of 6 drive coils in a Helmotz configuration is required
The loop gain reaches ~60dB @ 100Hz, which is very substantial.