In FDD systems such as WCDMA and LTE, TX and RX operate simultaneously, while the duplexer provides the necessary isolation between them. In order not to desensitize the RX path, the TX noise in WCDMA band 1 has to be lower than −155dBc/Hz at 190MHz offset (RX band), given a 45dB TX-to-RX isolation from the duplexer . Furthermore, when transmitting at 1920MHz, the nearest frequency offset to the DCS band (1805 to 1880MHz) is 40MHz. At this offset typical WCDMA duplexers only offer a few dB of attenuation. To comply with the 3GPP emissions mask, the TX noise at −40MHz offset is expected be lower than −148dBc/Hz (with 3dB margin). Currently there is a growing demand to integrate WCDMA and GPS applications in the same mobile device. In order not to interfere with the GPS receiver, the far-out noise floor of the WCDMA TX is also desired to be very low.
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