Broadcast channel testing

Testing a broadcast channel in which information is transmitted within a synchronous area of an electric power grid (100), whereby a characteristic relating to a frequency of power flowing in the electric power grid (100) is measured at a test receiver (121) and a signal based on a frequency modulation of the grid frequency is extracted from the electric power grid, and data indicative of a characteristic of the signal is sent to a control node (122) via a channel separate from the broadcast channel. Within the electrical power grid (100) there are transmitters (118) for transmitting information under control of a controller (122), and receivers (120) for receiving the information, such that the electric power grid (100) acts as a broadcast channel, and information is encoded by frequency modulation superimposed on the grid power frequency. Across the synchronous area of the electric power grid (100) the power frequency may be assumed to be essentially constant, such that the transmission characteristics of the broadcast channel measured at one position (i.e. at the test receiver (121)) within the electric power grid (100) may be considered representative of the transmission characteristics throughout the electric power grid (100) (however, in some embodiments more than one test receiver (121) may be used).