Abstract: An interferometric apparatus (10) for producing an output signal characteristic of phase and/or amplitude noise of a device under test (22), an input signal being provided to the interferometric apparatus (10), comprising a signal generation means (18) arranged to produce a third signal having a carrier frequency offset from that of the input signal; a first bridge (12) having first (14) and second (16) arms, the first (14) and second (16) arms having input thereto first and second signals, respectively, produced from one of the input signal or the third signal; the device under test (22) being provided in one of the first (14) or second (16) arms of the first bridge (12); a carrier suppression means (24) connected to the first (14) and second (16) arms of the first bridge (12) to produce a carrier suppressed signal; a first amplifier (32) arranged to amplify the carrier suppressed signal; first mixing means (34) responsive to the third signal and to the carrier suppressed signal to produce a signal character