5.4.3 Further signal processing

In each correlator a variable section of the IF2 band is down-converted to baseband by means of two frequency changes, with a fixed third LO and a tunable fourth LO. It is on that LO4 that the phase rotations needed to compensate for residual phase drifts due to the geometrical delay change are applied (in fact that LO4 plays the role of the second frequency conversion in the above analysis). No phase rotation is applied on the second and third local oscillators.

The phase rotation applied on the fourth LO's is:

$$\ensuremath{\varphi_\mathrm{\scriptscriptstyle 4}} = (\ensure... ...riptstyle 4}} ) \ensuremath{\tau_\mathrm{\scriptscriptstyle G}}$$ (5.21)

since the second and third conversions are LSB while the fourth is USB. It is different in the different correlator units since the frequencies are different.