A Low-Power WCDMA Transmitter with an Integrated Notch Filter

In a WCDMA FDD system, since the transmitter and receiver operate concurrently, the receiver is plagued by the transmitter leakage due to finite isolation of antenna duplexer. As a consequence, the transmitter noise at the receive band degrades the sensitivity, unless the noise is further attenuated before reaching the receiver. Due to the modest Q of on-chip inductors, typically the use of a costly SAW filter placed before the PA is inevitable. On the other hand, the external filter may be eliminated through sufficiently lowering the transmitter noise for any given duplexer. However, given the power-noise trade off, in all previous designs due to the lack of an on-chip filtering mechanism, meeting the stringent noise requirement has resulted in an excessive transmitter power dissipation. To address these issues, a feedback-based filtering technique to suppress the receive-band noise with negligible area or power penalty is presented. Stability analysis reveals that with practical transmitter gain distribution, optimum component sizes, and for a typical required noise suppression, using a simple first- or second-order RC LPF, the feedback remains stable and no additional compensation is required.