Built-in linearity testing arrangement for an FM radio system

In a radio system, including a transmitter and a receiver, which employs frequency modulation of information signals, apparatus for providing a bulit-in system linearity test arrangement comprising:

• a transmitter having a baseband input terminal for accepting signals to be conditioned for radio transmission to said receiver;
• a pilot frequency oscillator having an output terminal connected to an input terminal of said transmitter baseband;
• square-wave generation means for providing a predetermined deviation of said frequency modulated transmitter at an output terminal when said output terminal is connected to the baseband input of said transmitter;
• switch means having first and second input terminals adapted respectively, to receive the information signals and the square-wave test signals and to selectively apply either one or the other of the said signals to said baseband input of said transmitter;
• a frequency modulation receiver adapted to receive the frequency modulated signals from said transmitter and to process said received signals so as to substantially recover and provide at an output port the transmitted pilot and square-wave test signals transmitted, said receiver also providing at said output port the recovered pilot frequency which, as a result of interaction between the pilot and square-wave test signals, because of system non-linearity, may have superimposed thereon and AC signal, thus providing at an output an altered pilot frequency signal;
• a product detector for deriving a DC signal which is representative of the amplitude of said altered pilot frequency, said DC signal including said AC signal superimposed thereon, the amplitude of said AC signal being proportional to the slope in the system non-linearity;
• means for detecting said AC signal;
• and
• means for measuring the amplitude of said detected AC signal.