Editor’s note: This question was asked in response to Interference Technology’s recent webinar by Keith Armstrong. To view the webinar, click here.
Question: How do we identify which is the dominant: E-factor or H-factor?
Answer: In any circuit, if we measure the voltage of the noise it has picked up from its electromagnetic environment, we can then load the circuit with a low value of shunt resistance (e.g. 1kilohm or less, as low as the circuit will stand and still operate correctly) and re-measure the noise.
If loading the circuit reduces the noise voltage, it generally means E-field (stray capacitance) noise coupling is the dominant mode.
But if the noise voltage stays the same despite the circuit loading, then it generally means H-field (stray mutual inductance) noise coupling is the dominant mode.
It is not unusual to find that E-field coupling is dominant for some frequencies, and H-field coupling is dominant for others.
Series CM chokes work best at suppressing H-field coupled noise, whereas shunt capacitors work best at suppressing E-field coupled noise. So we sometimes find that types of suppression are needed – but beware of adding a shunt capacitor suppressor to any output signal that has DM RF content (e.g. digital waveforms with fast edges). Such capacitors generally need to add a series resistor or choke as well, to limit the RF current that the output can drive into the capacitor.
It should be possible to use a shunt capacitor that has low impedance at the frequency of the noise (instead of a shunt resistor) but I’ve never tried this. It might have some advantages for some circuits which don’t like being loaded heavily by resistors, when the noise frequency is much higher than the signal frequencies in the circuit.