A System for the Precise Calibration of Air Navigational Receivers

A new RF signal generator for the calibration of very high frequency omnidirectional radio (VOR) and instrument landing system (ILS) navigational receivers is presented. This new instrument improves the calibration accuracy of VOR angle and glide slope and localizer angle deviation by an order of magnitude over the calibration systems used up to now. The same order of magnitude improvement applies to the precision of AM level, phase angle settings, RF level, etc., and the stability of the simulated RF carrier in order to make the improved accuracy realizable. A new frequency lock technique allows to cover the RF frequency ranges continuously and still obtain a stability close to frequency synthesizer performance. The two major contributions to the improved performance are a linear and flat carrier amplitude demodulator using a two-diode voltage comparator and a new digital frequency lock technique. The carrier and modulation specifications are discussed which are necessary to achieve the improved accuracies. Measurement techniques are shown which are used to verify these accuracies along with the measurement results.