The MC90R sensor provides a very strong response from Extremely Low Frequency (ELF) through Very Low Frequency (VLF) bands, with a very sharp maximum near 1.8 kHz. The resonant frequency may vary due to assembly variations and the complex output impedance that the sensor output is connected to.  

Compared with the other sensors, Models MC90R and MC90-110 can be more easily damaged by suddenly turning on or off a strong field, or by a fast rise or fall time, or other sharp discontinuities in a strong field like sudden frequency or phase changes. These may produce a transient voltage spike in the large coil and output of these B-dot sensors, which could damage the sensor. If unsure, better to gradually ramp-up and down the field strength.