2.2 The Heterodyne Interferometer

Figure 2.2 is a schematic illustration of a 2-antenna heterodyne interferometer.

  

Figure 2.2: Schematic Diagram of a two-element interferometer

Let us forget the frequency conversion for some time, i.e. assume $\nu_{IF} = \nu_{RF}$...

The input (amplified) signals from 2 elements of the interferometer are processed by a correlator, which is just a voltage multiplier followed by a time integrator. With one incident plane wave, the output r(t) is

$$r(t) = < v_1 \cos(2\pi\nu(t-\tau_g(t))) v_2 \cos(2\pi\nu t) > = v_1 v_2 \cos(2 \pi\nu \tau_g(t))$$ (2.11)

where $\tau_g$ is obviously the geometrical delay,

$$\tau_g(t) = (\ensuremath{\mathbf b} . \ensuremath{\mathbf s} )/c$$ (2.12)

As $\tau_g$ varies slowly because of Earth rotation, r(t) oscillates as a cosine function, and is thus called the fringe pattern. As we had shown before that v₁ and v₂ were proportional to the electric field of the incident wave, the correlator output (fringe pattern) is thus proportional to the power of the wave.