phase-switching interferometerIn its simplest form, a correlating interferometer (see correlation receiver) in which the signals from two antennas, V 1 and V 2, are alternately added and subtracted before being applied to the input of a square-law detector to give outputs (V 1 + V 2 )2 and (V 1 – V 2 )2. The sign changing is carried out at a rate of between 10 and 1000 Hz by means of a phase switch, which adds an extra half wavelength to the path of the signal from the second antenna every alternate half cycle of the switching frequency. A phase-sensitive detector, fed also by the switching signal, takes the difference between the two terms, equaling 4V 1 V 2. The time average of this is therefore proportional to the product of the two antenna signals, and is a measure of the correlated signal between the two antennas. Uncorrelated noise is reduced by the averaging process.
The strength of the correlated output depends on both the flux density of the radio source and the relative phase of the two signals at the antennas. As a radio source moves across the sky, interference fringes appear in the output of the phase-sensitive detector. The fringes are similar to those from an ordinary adding radio interferometer, except that their average is zero rather than being superimposed on a steady background deflection. Phase-switching makes the interferometer far less sensitive to interference and reduces the effects of changes in the gain and other parameters of the system.
Phase switches are sometimes used to modulate a weak, but relatively constant, signal before it is amplified and then to demodulate it afterwards. In this way any offsets introduced by the amplification process appear modulated at the output and can be removed with a filter.