What is the typical purpose of using a low-pass filter in a transmitter or receiver chain?

• A. Increase the noise figure
• B. Remove out-of-band RF components, suppress harmonics, and limit bandwidth
• C. Improve antenna gain
• D. Reduce receiver temperature

Answer: (B)

Low-pass filtering shapes the spectrum that the system processes by passing frequencies up to a chosen cutoff and attenuating those above it. This is crucial in a transmitter or receiver chain because it keeps out of band energy, especially harmonics and spurious signals produced by nonlinear stages, from propagating into the desired signal path or into neighboring channels. In transmission, the low-pass filter suppresses harmonics created by mixers and amplifiers so the emitted spectrum stays within allowed limits and reduces interference to other users. In reception, a low-pass filter before the mixer or ADC reduces out-of-band RF energy that could fold into the passband (or be aliased) and confines the processed bandwidth to what’s actually needed, improving selectivity and reducing interference. It’s about ensuring only the wanted portion of the spectrum is processed, not about boosting antenna gain, lowering temperature, or increasing noise figure.