Apparatus and method for monitoring a communications system

Apparatus for monitoring a communication system in which information signals are transmitted within a particular frequency band, comprising, in combination:

• means generating test signals falling within said particular frequency band for transmission over the communication system simultaneously with the information signals, said test signal generating means including first oscillator means generating as a first output a carrier signal at a first fixed frequency within said frequency band, second oscillator means generating as a second output a signal swept across at least a portion of said frequency band, and control means coupled to said first and second oscillator means for determining the particular output of said test signal generating means;
• a plurality of monitor circuits each in communicative connection with said test signal generating means and each disposed at different preselected points in the system, each said monitor circuit including:
• detector means for detecting all signals in said frequency band as received at the respective point of said monitor circuit,
• local oscillator means for generating a local carrier signal at a discrete fixed frequency falling outside said frequency band, the carrier frequency of the oscillator of each monitor circuit being different so that the identity and location of each said monitor circuit can be recognized, and
• modulator means coupled to said detector means and said local oscillator means for modulating said local carrier with said detected signals;
• and
• signal analysis means in communicative connection with said monitor circuits and disposed at a position remote therefrom to receive and analyze said modulated local carriers, said signal analysis means including means for selectively receiving the individual modulated local carriers of each of said monitor circuits, means for detecting the percent of modulation of each of said received local carrier signals, means for storing the output of said percent of modulation detector, and oscilloscope means for displaying the frequency response of each received local carrier signal;
• the amplitudes of said test signals generated by said first and second oscillator means being greater than said information signals such that modulated local carrier signals received by said signal analysis means and resulting from the transmission of either of said test signals may be differentiated from received modulated local carrier signals resulting from the transmission of information signals, whereby the operative status of the entire system may be monitored at a central location.