a system for remote convenience control of performance of a function includes a portable transmitter unit and a receiver unit . in one embodiment , the system is a remote convenience vehicle system . the transmitter unit transmits a signal . the signal has a rf frequency at an unknown value within a range of values and conveys a message that contains a function request . within the receiver unit is superheterodyne circuitry that has a mixer . the mixer mixes a rf signal coming from an antenna and a reference frequency signal from a reference oscillator , and outputs an intermediate frequency signal conveying the message . a bandpass filter filters the intermediate frequency signal to pass a range of frequency values smaller than a known range of intermediate frequency values . tuning control circuitry automatically adjusting the bandpass range of the bandpass filter to vary the bandpass range over the intermediate frequency range . passage detector circuitry senses the intermediate frequency signal passing through the bandpass filter and automatically causes the adjustment of the bandpass range to stop .