How Can I Eliminate 60Hz Magnetic Field Interference on a 1 MHz Transmitter?

[Michael Lin]

Dear all,
If I have a strong magnetic field present (an oscillating field at say 60Hz), how can I eliminate its interference effect on a neighboring circuitry that has a transmitter operating at say 1 MHz? Would simple filtering at the receiver be enough? I just want to check the feasibility of a functional transmitter without getting into using magnetic shielding.

Thanks,
Mike

 

[xez]

Well because 60Hz is so different than 1MHz you could
often use simple signal frequency based discrimination
to reduce the interference of the 60Hz on the 1MHz.

the frequency ratio is 1.66*10^4, or 14 octaves,
so even a 6dB/octave roll-off filter would give
6*14 = 84dB of rejection of 60Hz relative to a cut-off
frequency of 1MHz.

The interesting question is to ask HOW AND WHY the
60Hz couples into the transmitter circuitry.

One way would involve the 60Hz ripple appearing as
a differential signal on the power supply leads of
circuitry in the transmitter. In that case you'd look at
how well the differential 60Hz power supply signal variance
is amplified or attenuated by your transmitter circuitry.
This kind of power supply ripple noise based interference
is commonly due to inadequate filtration of 60Hz variance
in a linear 60 Hz mains operated power supply.

Another way 60Hz interference can form is magnetic
field coupling from the magnetic field of a wire or
coil / transformer carrying 60Hz signals through mutual
inductance with wires, coils, transformers that carry
other signals. Using geometric means of distancing and
orienting the circuits to minimize their mutual inductance
can be helpful. Using thick high permeability magnetic
materials to enclose the 60Hz noise inductors and/or
the susceptable inductors that can pick up the noise
can help.

Electric field pickup can happen if there were 60Hz
voltage sources that capacitively coupled to parts of
the sensitive circuit, but this is usually less dominant of
a mechanism than others.

Finally, common mode coupling can occur where a
common mode 60Hz noise voltage can appear on
your equipment or various cables running into the
equipment. Generally devices are insensitive to common
mode voltages, but the common mode noise present can
be turned into differential (unbalanced) noise by any
unbalance of part of the circuit so that a differential
noise voltage or current can be present between the
common mode potential and Earth or whatever.

Usually it's best to design the system so that it has both
low EMI generation capability as well as low EMI
noise susceptability for whatever EMI is present in
the environmental space, in the power supply, coupled
in through cables/wires/signals, et. al.