Local Oscillator Leakage

Co-location Deployment Considerations for Direct RF Sampling Transceivers

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Perhaps the most widely known issue with ZIF is the local oscillator (LO) leakage at the baseband output. The LO leakage includes a static component and a dynamic component. The static component does not change much during runtime but can vary from unit to unit. The dynamic leakage component is a function of many factors such as temperature, input signals, non-linear distortions, receiver component lineup variation, analog mixer, LO isolation, etc. In practice, the receiver must incorporate a non-trivial DC nulling algorithm for real-time reduction of the LO leakage power so that it does not degrade the signal to noise ratio (SNR) of the wanted signal. Due to the dynamic nature of the leakage power, the nulling algorithm must trade off tracking speed, nulling accuracy, and implementation cost, which makes it very difficult to remove the LO leakage to a level that does not degrade the receiver sensitivity. Reducing the LO leakage power in the vicinity of –109 dBm (reference to the antenna input) can be very difficult to achieve in practice.

This issue is less impacting in 4G LTE because LTE carriers do not have a sub-carrier located at the center of the carrier. As such, the radio designer can use a sub-par algorithm to achieve an acceptable level of performance. Component carriers for 5GNR do not have this characteristic. The entire occupied bandwidth of the carrier can have valid sub-carriers present. This further complicates the LO nulling algorithm as it is difficult to distinguish between the wanted signal and the LO leakage. Any non-zero integration loop bandwidth of the algorithm will be detrimental to the sub-carrier of the wanted signal centered around DC.

Another well understood artifact that impacts the 5G signal is the inherent 1/f flicker noise at DC generated by the LO and mixer. This noise cannot be reduced with the LO-nulling algorithm and will have meaningful impact on the sensitivity of the receiver in that region of the band.