Patent GB-1339245-A (GB)
Title: Twochannel direction finders
IPC: H03L7/14
Filed: 1972-05-04
Abstract: 1339245 Radio direction-finders; AFC systems INTERNATIONAL STANDARD ELECTRIC CORP 4 May 1972 [7 May 1971] 20834/72 Headings H4D and H3A The invention is concerned with a frequency locking system for a two-channel directionfinder which comprises loop aerials and a sense aerial, means for setting the direction-finder to a "frequency" condition during which it may, by means of a local oscillator, be tuned to a new frequency, or to a "bearing" condition during which bearings of stations operating on said new frequency may be taken, and a digital display device for showing either frequency or bearing. The system also comprises a locking device for the local oscillator, operative during "bearing" condition periods to lock the frequency of said local oscillator to a predetermined value at instants separated by predetermined time periods, and means responsive to variations exceeding a predetermined amount in the frequency (unlocked) of said oscillator during said predetermined time periods to release said locking device and to cause the setting means to change form the "bearing" condition to the "frequency" condition. As described, signals from a loop aerial and sense aerial assembly 1, Fig. 1, are evaluated in a bearing and sense treating circuit 2. The direction-finder may be set either to a "bearing" condition (B) or to a frequency condition (F) by a setting means 3, and bearing or frequency information is shown on a digital display unit 4. A local oscillator 5 is used for controlling the receiving circuits of the direction-finder, and its frequency is mainly adjusted by a tuning device 6. Its frequency is also adjustable by a signal from a comparison circuit 10. With the direction-finder set to (F), the device 6 is adjusted manually until the desired frequency is shown on the display 4. A locking/unlocking circuit 9 is then operated whereby the four least significant binary digits of the said desired frequency as counted by a sampling circuit 8 are stored in a memory 7, and the direction-finder is set to (B). At regular time intervals thereafter (e.g. every 0À1 second) the frequency of the local oscillator 5 is sampled, and is compared by the circuit 10 with the value stored in the memory 7. So long as the frequency drift detected is within certain limits, a compensating signal is applied by the circuit 10 to the local oscillator 5. However, if a drift exceeding said limits is detected, the circuit 10 controls the circuit 9 to release the locking of the memory 7, and to set the direction-finder to the (F) condition. A detailed circuit arrangement of the above embodiment is described with reference to Fig. 2. It comprises first and second division means 20, 24 which are arranged respectively to divide the desired frequency and the instantaneous oscillator frequency so as to provide remainders comprising a predetermined number of least significant digits, the first of which is stored and is compared with the second in the comparison circuit. Any difference is converted to a phase shift, which causes a phase detector to send a control signal to adjust the frequency of the local oscillator. The circuit arrangement comprises means for accumulating local oscillator frequency drift over a succession of time periods whereby, when said drift exceeds a predetermined amount, the direction-finder is caused to revert to the "frequency" condition.