Question 3-50F1

Low-pass filters (LPFs) are crucial in radio receivers for several reasons: * **In the AVC (Automatic Volume Control) circuit:** An LPF is essential here to smooth out the varying RF and audio components of the detected signal. The AVC circuit needs to generate a stable DC voltage proportional to the *average* strength of the incoming signal, not its instantaneous fluctuations. The LPF filters out these higher-frequency components, providing a steady DC control voltage to adjust the gain of earlier RF/IF stages, preventing distortion and ensuring stable audio output levels. * **In the Power Supply:** LPFs are vital in the power supply section to filter out unwanted AC ripple from the rectified DC voltage. The power supply converts AC into DC, but often some residual AC "ripple" remains. An LPF removes these higher-frequency ripple components, providing a clean, stable DC voltage for the sensitive receiver circuits, which prevents hum and noise from being introduced into the receiver's audio output. While filters are used in the oscillator stage (e.g., to ensure spectral purity), these are more commonly band-pass filters to select the desired oscillating frequency and reject harmonics, rather than general low-pass filters in the sense of blocking all frequencies above a certain cutoff.