Wideband characterization of LCD baseband noise modulation for RF interference in mobile phones

A wideband measurement technique to characterize radio frequency (RF) interference caused by a liquid crystal display (LCD) baseband noise modulation is proposed. The LCD noise in baseband frequency range modulated with the Tx carrier signal of cellular communication interferes with the Rx signal, resulting in a RF receiver desensitization problem. Since the frequency offset between the Tx and Rx bands is small (several tens of MHz), a high Tx band rejection filter is essential to characterize the relatively small modulation noise with small frequency offset along with the strong Tx carrier signal. A typical solution is to use a duplexer, which only works for the dedicated communication band and, thus, a narrow band. The proposed measurement technique utilizes a 180° hybrid coupler and reflection cancellation to obtain high Tx band rejection without any frequency limited components. In addition, another reflection cancellation method is demonstrated for a large offset frequency case.