Harmonic frequency rejection is a core performance metric for modern radio frequency (RF) duplexers, directly determining the spectral purity of wireless communication systems and preventing adjacent-channel and spurious signal interference. In RF transceivers, power amplifiers (PAs) generate fundamental transmission signals along with integer multiple harmonic frequencies during signal amplification, while local oscillators and switching circuits also produce unintended harmonic spurs. Without effective duplexer harmonic rejection, these out-of-band harmonic signals will leak into the receiving channel, mask weak target signals, and cause system desensitization. This issue is particularly prominent in high-power transmission scenarios, where harmonic power levels can reach dozens of decibels above the noise floor, severely degrading communication reliability.

Duplexers achieve harmonic suppression through optimized resonant circuit design and frequency selective filtering structures. Traditional ladder-type surface acoustic wave (SAW) and bulk acoustic wave (BAW) duplexers adopt multi-stage ladder resonator networks, which create deep stop-band notches at twice, three times, and four times the fundamental operating frequency. Engineers adjust the resonant and anti-resonant frequencies of series and parallel resonators to precisely align stop-band attenuation peaks with common harmonic frequency points, ensuring minimum rejection of 40dB to 60dB at targeted harmonic bands. For high-frequency wireless systems such as 5G and microwave communication, acoustic wave duplexers are combined with low-pass filter topologies to further suppress high-order harmonics that exceed the operating bandwidth of acoustic resonators.