In the set-up described, the equipment under test comprises a circuit board with a power supply unit, micro-controller, PWM and a direct-current motor. The direct-current motor is connected to the circuit board's PWM output. The direct-current motor usually comprises an armature, a stator tube and a commutator with carbon brushes.
Equipment under test and the various emission sources
There are three distinctive emission sources:
1. Power supply unit (interference suppression is difficult if the power supply unit is galvanically isolated)
2. PWM circuit
3. Switching operation of the commutator in connection with the armature inductance of the direct-current motor.
A line impedance stabilization network can be used to measure the emissions at the power supply input of the circuit board. Interference suppression is a complex task because the three emission sources overlap. It is usually rather difficult to trace the emissions back to their source on the basis of a spectrum.
In addition, there is a resonance between the PWM signal and the armature inductance that is produced by commutation if the direct-current motor is controlled by PWM. This resonance significantly increases the emissions. Operating the motor in the direct-current mode makes things easier.
It is helpful to suppress interference in each of the three emission sources separately; asnd it is best to start with the power supply unit.
A load resistor is used instead of the direct-current motor. The direct-current motor can be integrated into the test set-up as a passive component and is only connected via the anti-interference capacitors C3. The PWM is switched off in this case.
Galvanically isolated power supply units generate a very high capacitive current that flows through the switching power-supply transformer. This current can be reduced through the du/dt limitation of the switching and kickback voltage. However, this is usually not sufficient. This current can be fed back to the source side if a capacitor C1 is