Scanning radio receiver including detector circuit for AM and FM reception

摘要

There is disclosed a scanning radio receiver for use on the frequencies assigned to the Public Safety Radio Services in which frequency modulation is used and which also will receive the aircraft band from 118 to 136 MHz in which amplitude modulation is used. A detector circuit is provided including separate AM and FM demodulators with there being a squelch circuit associated with the FM demodulator, the squelch circuit being operative in AM as well as FM reception. A window detector circuit for minimizing the effect of strong signals outside a desired band is also associated with the FM demodulator and is also operative during AM reception. Common circuits are used for both AM and FM reception, including common RF portions, a common first IF portion operative at 10.8 MHz, a common second mixer and stages of a second IF amplifier operative at 400 KHz.