Error correction system for digital length or angle measuring instrument

In a digital position measuring instrument of the type comprising means for generating a plurality of analog, periodic primary signals indicative of the relative position of a first object with respect to a second object, the plurality of primary signals defining a set of primary signals, the improvement comprising:

• signal generator means for generating a plurality of reference signals, the plurality of reference signals defining a set of reference signals, each reference signal associated with a respective one of the primary signals;
• comparator means, responsive to the plurality of primary signals and the plurality of reference signals, for comparing the primary and the respective reference signals with respect to phase and amplitude;
• correction means, coupled to the generator means and the comparator means, for determining correction data for symmetry, amplitude ratio, phase angle, and signal form of the reference signals;
• and
• means, responsive to the correction means, for adapting one of (1) the set of reference signals and (2) the set of primary signals to correspond to the other of (1) the set of reference signals and (2) the set of primary signals in response to the correction data.