The three-level correlator, in which both the data and the fringe rotation function are quantized to three levels, is another scheme in use. The three-level correlator can be considered as a special case of the four-level correlator where both the inner levels are reassigned a weight of zero. As with the four-level correlator, the conversion function cannot be solved for explicitly. The numerically calculated conversion function is shown in Fig. 3 (click here).

The efficiency of the three-level correlator can be derived from the expression for the four-level
correlator efficiency by retaining only the terms in *n*^{2}. The result is

The efficiency is maximised for () where it has a value of 0.389, increasing to 0.778 by virtue of the complex correlator architecture. This compares to

0.81 for a three-level correlator without digital fringe rotation. In practice () is used with virtually no loss in efficiency.

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