Developmental trends in radio receiver design

A historical review of the development of shortwave receiver design is followed by an enumeration of the characteristics of the state of the art in shortwave reception equipment: (1) the problem of band coverage, for example in marine receivers from 10 KHz to 30 MHz, has been resolved by the use of infradyne circuits with an IF of 40 to 80 MHz and a broadband preselector; such designs also make it possible to base the local oscillator on a phase-locked loop; (20 suppression of spurious frequencies is improved by placing them outside the working band through the use of a high first IF; (3) enhanced linearity of the broadband RF amplifier and converter has extended the dynamic range of receivers, and eliminated the need for a narrow band preselector with the use of simple suboctave filters and low pass filters at the input; and (4) the use of a decade frequency synthesizer based on a variable divisor frequency divider, highly stable master crystal oscillators, electronic varactor tuning of the local oscillator and Phase-locked loops provide for precise tuning to the requisite frequency, which is also improved by digital tuning and displays. A major aspect of present design work is the application of microprocessors to automatic tuning, programmed frequency setting and the performance of a variety of diagnostic functions in the receiver.