An alarm system receiver that is capable of receiving signals transmitted from wireless intrusion detectors at more than one frequency and automatically switching from receiving at one frequency to receiving at a different frequency based on the detection of an interference signal. The alarm system receiver senses a first signal, filters the sensed first signal at a first frequency, demodulates the filtered first signal at the first frequency, and determines if a first interference signal is present in the demodulated first signal for a predetermined time. If the first interference signal is not present in the demodulated first signal, it processes the demodulated first signal as an alarm system signal. If the first interference signal is present in the demodulated signal, the alarm system receiver senses a second signal, filters the sensed second signal at a second frequency, demodulates the filtered second signal at the second frequency, and processes the demodulated second signal as an alarm signal. The alarm system receiver consists of an antenna, more than one filter, more than one local oscillator for providing oscillator signals at the predetermined frequencies, a demodulator for inputting one of the oscillator signals and a corresponding one of the filtered signals to provide a demodulated signal, and an interference detection circuit for inputting the demodulated signal, determining if an interference signal is present in the demodulated signal, and providing an interference detection signal.