Interferometric signal processing apparatus

An interferometric signal processing apparatus (10) producing an output signal from a first input signal (34) and a second input signal (34), said input signals (34) having substantially equal carrier frequencies, comprising: a bridge (12) having a first arm (26) and a second arm (28), each arm having a first end (30) and a second end (32), the first and second input signals (34) being input to the first end (30) of the first and second arms (26, 28), respectively; a device-under-test (36) provided the first arm (26); a carrier suppression means (14) connected to the second ends (32) of the first and second arms (26, 28) to produce a carrier-suppressed signal at its output (A); an amplifier (16) arranged to amplify said carrier-suppressed signal; and a mixing means (22, 24) responsive to the amplified carrier-suppressed signal and a carrier-dominated signal to produce the output signal; wherein the differential group delay between: the first end (30) of the first arm (26) and the output (A) of the carrier suppression means (14); and the first end (30) of the second arm (28) and the output (A) of the carrier suppression means (14) is less than or equal to 1000/fo seconds, where fo is the time-averaged mean value of the carrier frequencies of the input signals (34).