An apparatus for testing the phase linearity of a network comprises a first local oscillator, a first mixer, and a first filter, a second local oscillator, and a second mixer for mixing the first IF to the desired output frequency. The output of the second mixer is coupled to a second filter which passes the desired frequency band, but rejects the high frequency mixing harmonics produced in the second mixer. The output of the second filter is coupled to a variable gain amplifier capable of amplifying the frequency band of interest to the desired level and a third filter which further rejects unwanted high frequencies in the output.