A system and method of scanning machine-readable symbols with a signal source modulated by a local oscillator to create by heterodyne mixing upconverted signals at the sum and difference frequencies of the local oscillator frequency and the frequency of the time-varying reflectance off the scanned symbols is disclosed. The reflected heterodyned signal is detected by a receiver. Ambient noise is filtered out from the reflected signal, and the received signal at the sum and difference frequencies is processed.