Question 6A470

In a superheterodyne receiver, a local oscillator (LO) mixes with the incoming radio frequency (RF) signal to produce a stable intermediate frequency (IF). The IF is then amplified and filtered. For this process to work effectively and maintain the desired signal within the narrow bandwidth of the IF filter, the LO frequency must be extremely stable. Crystal-controlled oscillators use the piezoelectric properties of a quartz crystal to provide a highly precise and stable resonant frequency. This inherent stability ensures that the LO frequency does not drift, which would cause the IF to also drift and the desired signal to move out of the IF filter's passband, leading to poor reception or loss of the signal. Options B, C, and D are incorrect. While a well-designed crystal oscillator can be low-noise, its primary advantage is frequency stability, not noise reduction (B). Crystal oscillators, like other RF circuits, still require shielding (C) to prevent interference. Crystals are part of the resonant circuit that determines the oscillation frequency; they don't directly "control" feedback in the general sense, which is a fundamental requirement for any oscillator (D).