VHF Omni-Range (VOR) is the internationally recognised short range navigational aid.
The principle of operation is that two independent 30 Hz modulations are impressed on a VHF ground station radio transmission in the 112.0 MHz to 117.9 MHz band.
These two modulations are known as the reference and variable phases and their difference in phase, measured in degrees, as received at any remote station, corresponds to the bearing of that station with respect to magnetic north.
The variable phase is a 30 Hz amplitude modulation whilst the reference phase consists of a 30 Hz frequency modulation impressed on a 9960 Hz amplitude modulated sub-carrier.
How is the 9960 Hz Signal Produced ?
The most basic form of reference phase generator consists of a metal wheel mounted on a shaft and which has 332 teeth cut around its circumference. Mounted closely adjacent to these teeth is an electromagnetic pick-up which gives an electrical output each time a tooth passes by its head. If the wheel is rotated at 1800 rpm the output will therefore be at 9960 Hz, i.e. the reference sub-carrier frequency.
The teeth, however, are arranged in a somewhat staggered manner and this irregularity imparts a cyclic variation of between 9480 Hz and 10440 Hz on the output frequency, the actual frequency being dependent on the instantaneous position of the toothed wheel. This circuit is commonly called a tone wheel generator.
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