December 1980 | NTIA Technical Report TR-80-56
John R. Juroshek; Douglass D. Crombie; G. E. Wasson
Abstract: Interference caused by local oscillator radiation from AM radios is a known problem. It is particularly troublesome when the local oscillator frequency of one radio coincides with the tuned frequency of another, nearby AM radio. This condition produces an annoying tone in the victim receiver whose frequency is dependent on the frequency difference between the local oscillator and tuned frequency of the victim radio. It has been suggested that this source of interference would be more troublesome if AM radio stations were assigned frequencies in 9 kHz channel increments instead of the current 10 kHz increments. These arguments, however, are largely based on idealized assumptions for AM radios. This report describes the measurement of local oscillator radiation from a sample of test radios. Measurements were made of local oscillator frequency for various tuning conditions~ Also described in the report are measurements of the separation distance required to produce a given amount of interference in a victim receiver and the image rejection ratio with 9 kHz and 10 kHz spacings. The report concludes that there are no significant differences in interference between 9 kHz and 10 kHz assignments insofar as receiver local oscillator radiation and image frequency susceptibility are concerned.
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