An input N-plexer filter stage is susceptible to receive surge energy via an input conductor, when the surge energy occurs. A high-pass filter included in a diplexer filter stage applies a radio frequency (RF) signal in a signal path between the input N-plexer filter stage and an output of a Data Over Cable Service Interface Specification (DOCSIS) transmitter stage. A low-pass filter included in the diplexer filter stage couples a surge suppressing threshold device to the signal path to dissipate the surge energy in the surge suppressing threshold device, when the surge energy occurs. The low-pass filter has a cut-off frequency that is lower than a normal operation frequency range of the RF signal. The low-pass filter isolates the surge suppressing threshold device from the signal path in normal operation frequency range of the RF signal, when no surge energy is applied.