A wideband noise measurement technique with improved isolation is demonstrated for a practical mobile phone. The liquid crystal display (LCD) baseband noise modulated with a transmitted radio frequency (RF) signal interferes with the received RF signal, resulting in an RF desensitization problem. A wideband and simple measurement setup based on a 180° hybrid coupler is proposed and demonstrated to characterize the modulation and its coupling behaviors in duplex systems. This setup overcomes the frequency limitation of a duplexer-based measurement setting. In addition, signal-to-noise ratio improvement in the proposed measurement technique is quantitatively analyzed.