Communication system and electronic choke circuit

A communication system comprising:

• a management device and a terminal device designed to communicate with each other via a transmission line;
• and
• a power supply unit configured to supply electric power to said terminal device via the transmission line,
• wherein said terminal device comprises:
• an electronic choke circuit configured to separate the electric power supplied from said power supply unit and a communication signal from each other;
• and
• a power receiving unit configured to receive the electric power separated from the communication signal by said electronic choke circuit, and
• said electronic choke circuit comprises:
• a first terminal and a second terminal constituting an input terminal adapted in use to be connected the transmission line;
• a third terminal and a fourth terminal constituting an output terminal adapted in use to be connected said power receiving unit;
• a variable impedance element placed in at least one of a position between said first terminal and said third terminal and a position between said second terminal and said fourth terminal, said variable impedance element being configured to vary its impedance in accordance with a voltage applied to a control terminal;
• an inductor interposed between said variable impedance element and said output terminal;
• a first capacitor connected between said first terminal and said second terminal via a first resistor so as to apply its terminal voltage to said control terminal of said variable impedance element;
• a second capacitor interposed between said third terminal and said fourth terminal;
• and
• a second resistor connected between said variable impedance element and said output terminal, said second resistor configured to suppress a phenomenon that resistance characteristics of a circuit between said input terminal and said output terminal have a negative resistance region within a frequency band including at least a frequency of the communication signal.