Hi
Today I did some investigation and experimentation trying to figure out why the 3dB bandwidth is down to 60MHz instead of the ~160MHz expected. I don't even need 60MHz but I'd still like to know why.
I checked the current fixture. Even though the current probe is a tight fit within the fixture, it still works very well. I figured out that if I connected a 10dB attenuator where the 50ohm load is normally connected, I could normalise the injected current with the spectrum analyzer. I could then remove the small error of the fixture (<0.5dB).
Analysis of previous measurements isolates the reduced performance to the outer shield/enclosure. There is something about the outer enclosure that is causing a significant loss, rising with frequency. It can't be the solid metal sections because at 100HHz, the skin depth is only ~7um. I was able to machine the outer enclosure to increase the gap to the inner shield. That had no effect.
I checked that the inner and outer shields were not shorted together. No short and no change.
The design offset the radial flux breaking grooves between the inner and outer shields. This offset improves e-shielding and eliminates any e-field break through.
I tried aligning the two grooves but this only allowed a small and visible break through.
I don't know what is causing the significant increase in losses as the frequency rises. Based on development with the 3D printed prototypes, I was expecting a flat response out to about >150MHz.
The first image shows the frequency response of the fixture with the current sensor fitted in place. The fixture is good out to 600MHz, and usable well beyond that.
The other plots show the frequency response of the current probe. The 3dB point is at 60MHz, well above my initial requirements for EMC work, but still not as good as the 3D printed prototypes indicate should be achievable.
The current probe is entirely usable out to 100MHz to make relative current measurements. Absolute measurements will require error correction.
The photograph shows the setup to measure the fixture error with the current probe fitted in place. The current probe is terminated with a 50ohm load to simulate connection to the spectrum analyser. This load will appear as a parallel load of about 450ohm to the current going through the fixture. The 10dB attenuator terminates the fixture current to 50ohm. The output of the attenuator measures the actual current passing through the current probe, which includes the effects of the current probe in the fixture. This is a good way of measuring the performance of the fixture and also the influence of the current probe on measurements. Having a big chunk of metal in the middle of the fixture has little effect on performance. It is also simple, easy and cheap to make.
So in conclusion, the current sensor exceeded the original target of 30MHz bandwidth for EMC work but the 60MHz achieved on the final version is substantially less than the +150MHz bandwidth indicated by the 3D printed and foil wrapped prototypes. There is too much attenuation as frequency increases. Resonance is heavily suppressed, which is good, but sensitivity also suffers as frequency rises.
The double skinned design completely eliminates e-field break through. There is no pickup of stray e-fields. This is good because the current sensor is only measuring the current through the probe, and not the voltage.
So I now have a nice shiny paper weight that can measure currents out to >60MHz. More R&D would be required to figure out why the outer enclosure is reducing the bandwidth, but I am not going to go down that path.