Abstract: Circuitry for attachment to a radio receiver to monitor and test a detected intermediate frequency signal and an audio signal from a demodulator portion of the receiver to determine whether a transmitted radio frequency signal is within frequency tolerances and modulation limits and to provide an alarm, either visual or audio, when the limits or tolerances are exceeded. An off frequency monitor tests the DC component of the radio demodulator detected IF signal and includes input signal conditioning circuitry for filtering the audio signal, high impedance for minimal loading of the radio, and level shifting. Upper and lower level voltage references indicate maximum permissible frequency deviation from the nominal transmitting frequency. Upper and lower level comparators compare the conditioned direct current (DC) signal to the respective voltage references and provide a changed output state upon an out of tolerance transmission.

Abstract: Circuitry for attachment to a radio receiver to monitor and test a detected intermediate frequency signal and an audio signal from a demodulator portion of the receiver to determine whether a transmitted radio frequency signal is within frequency tolerances and modulation limits and to provide an alarm, either visual or audio, when the limits or tolerances are exceeded. An off frequency monitor tests the DC component of the radio demodulator detected IF signal and includes input signal conditioning circuitry for filtering the audio signal, high impedance for minimal loading of the radio, and level shifting. Upper and lower level voltage references indicate maximum permissible frequency deviation from the nominal transmitting frequency. Upper and lower level comparators compare the conditioned direct current (DC) signal to the respective voltage references and provide a changed output state upon an out of tolerance transmission.