Testing system for measuring the characteristics of a network over a variable frequency range

Claim 1

A testing and measuring system comprising a test oscillator capable of producing a frequency modulated output signal, an evaluation system for comparing a pair of input signals to produce indications indicative of differences between said input signals, a test object for which the transmission properties are desired, a first signal path including said object connected between said test oscillator and said evaluation system, and a second signal path between said test oscillator and said evaluation system which does not include said test object, said input signals being continuously supplied to said evaluation system through said first and second signal paths, a first modulator in said first signal path for producing a first side frequency signal from said frequency modulated output signal, a second modulator in said second signal path for producing a second side frequency signal from said frequency modulated output signal, and a pair of auxiliary low frequency sources respectively connected to said first and second modulators, a common portion of said first and second signal paths comprising an adder receiving the outputs of said first and second modulators, a frequency converter receiving the output of said adder, said first and second signal paths separating after said frequency converter, a first band pass filter tuned to the converted first side frequency signal and connected between said frequency converter and said evaluation system in said first signal path, a second band pass filter in said second signal path tuned to the converted second side frequency signal and receiving an output of said frequency converter, a second frequency converter connected between one of said band pass filters and said evaluation system in the signal path allocated to that filter, and a mixer supplying an input to said second frequency converter, the output signal of said second frequency converter constituting one of said input signals and the output signal of the band pass filter in the signal path without said second frequency converter constituting the other of said input signals.