An analyzer device for VOR navigational receivers utilizes a first receiver section in which the reference phase, transmitted in the form of a frequency modulation, is available in demodulated form as reference signal, and a second receiver section in which the rotational phase, depending upon the particular direction of the antenna pattern, occurs in the form of an amplitude modulated rotating signal. Angular information in the azimuthal plane is obtained from a comparison of the reference phase with the rotational phase. In one of the two receiver sections a demodulated signal is applied to a frequency multiplier having a multiplication factor k, which is selected as a whole number multiple (k = 360.n) or as a fraction (k = 360 : n) of 360. The multiplied received signal is fed to a counter which converts the phase difference between the reference signal and the rotational into a corresponding number of counting increments which are displayed by a display device as the angular deviation from a reference direction.
A very high frequency omni-range (VOR) receiver for use in aircraft radio-navigation, which provides an indication of course deviation that is substantially immune to effects such as reflections in transmitted VOR signals, transients occurring in an aircraft's electrical system and propeller and/or rotor modulations of an incoming VOR signal and the like which are all unrelated to actual course deviation, is disclosed. The receiver phase synchronizes a signal from a 1.08 MHz crystal oscillator to a 30 Hz reference signal and a 30 Hz variable signal; both the reference and variable signals are received from a VOR ground station. As a result of the phase synchronization, the receiver produces timing pulses which are accurately phase synchronized to a synchronized reference pulse stream derived from the 30 Hz reference signal. Additionally, the receiver produces a synchronized variable pulse stream that is derived from the 30 Hz variable signal. The receiver produces, in response to the synchronized reference pulse stream and the timing pulses, a signal (henceforth referred to as the "OBS" signal) which is phase shifted by an amount representative of a selected radial emitted by that station. To facilitate determining deviations from the selected radial, the receiver determines any phase difference between the "OBS" signal and the synchronized variable pulse stream. This phase difference is used to produce a deviation signal having a value that represents any course deviations existing between the selected radial and a present magnetic course of the aircraft with respect to the VOR station.
A VOR monitor employs a microprocessor-based system in which, after initial analog detection of a transmitted VOR signal, the critical parameters of the signal are extracted by digital signal processing to ensure that the transmitted signal in space remains within tolerance. Initial processing to isolate the 30 Hz AM (variable) modulation component and the 30 Hz reference component is conducted in the time domain. The 30 Hz reference is recovered by FM quadrature demodulation which, as a by-product, yields data from which an rms calculation is made of the amplitude of the 9960 Hz subcarrier. Further digital processing is then conducted in the frequency domain to yield spectra from which critical parameters of the VOR signal are extracted, e.g. the phases of the 30 Hz components (determining azimuth), the modulation percentages of the 30 Hz variable and the 9960 Hz subcarrier, and the 9960 Hz FM deviation ratio. The identification tone of the transmitted VOR signal is recovered separately from the digital data by application of the Goertzel algorithm and verified by an autocorrelation analysis.
