Will external cables connected to the edge box exacerbate excessive radiated interference?

External cables to the edge box, such as power cords, network cables, and serial cables, are highly efficient monopole antennas. Their radiation efficiency is extremely high when the length is a multiple of the wavelength. For example, a 10-meter cable corresponding to a wavelength of 30MHz will directly transmit PCB common-mode noise if not filtered, exacerbating the problem.

Countermeasure 1: Connect a common-mode choke in series at the interface. Use a CMZ2012A-900T for the network port and a CML3225A-510T for the serial port. The CML common-mode inductor is 3225 (3.2×2.5mm), and the 510T has an inductance of 51μH, capable of suppressing 10-100MHz common-mode current.

Countermeasure 2: Shield the cable and ground the shielding layer 360°. Connect the metal connector to the chassis via conductive foam.

Countermeasure 3: Add ferrite cores to the power cords. Audiotech provides a CMZ7060A-701T common-mode core with an impedance of 700Ω.

Tests conducted by directly attaching a 100MHz amplifier to the cable end showed that without intervention, the cable radiation exceeded the standard by 9dB. After adding a common-mode choke and shielding, the margin reached 8dB. Therefore, external cables are the key factor causing excessive radiation in amplifiers, and strict control must be exercised.